Youyi Lu
email: y.lu@dcs.shef.ac.uk

Martin Cooke

The contribution of changes in F0 and spectral tilt to increased intelligibility of speech produced in noise

Keywords: intelligibility, noise, speech production, spectral tilt

Talkers modify the way they speak in the presence of noise. As well as increases in voice level and fundamental frequency (F0), a flattening of spectral tilt is observed.

The resulting "Lombard speech" is typically more intelligible than speech produced in quiet, even when level differences are removed. What is the cause of the enhanced intelligibility of Lombard speech? The current study explored the relative contributions to intelligibility of changes in mean F0 and spectral tilt. The roles of F0 and spectral tilt were assessed by measuring the intelligibility gain of non-Lombard speech whose mean F0 and spectrum were manipulated, both independently and in concert, to simulate those of natural Lombard speech. In the presence of speechshaped noise, flattening of spectral tilt contributed greatly to the intelligibility gain of noise-induced speech over speech produced in quiet while an increase in F0 did not have a significant influence. The perceptual effects of spectrum flattening was attributed to its ability of increasing the amount of speech time-frequency plane "glimpsed" in the presence of noise. However, spectral tilt changes alone could not fully account for the intelligibility of Lombard speech. Other changes observed in Lombard speech such as durational modifications may well contribute to intelligibility.

Introduction

Speech intelligibility degrades in the presence of moderate and intense noise. Many studies have attempted to determine acoustic and acoustic-phonetic correlates of speech intelligibility, the discovery of which has important implications for the development of speech enhancement algorithms, particularly for listeners with hearing impairment. While factors such as an increase in speech output level can, to some extent, boost intelligibility by raising signal-to-noise ratio (SNR), level increases alone are undesirable due to their unpleasant and fatiguing effect on the listener. Fortunately, other acoustic and acoustic-phonetic properties have been shown to affect how well speech is understood in noise. [START_REF] Hazan | Acoustic-phonetic correlates of talker intelligibility for adults and children[END_REF] and [START_REF] Barker | Modelling speaker intelligibility in noise[END_REF] reported higher intelligibility of female talkers compared to males, which might have been due to the differences in acoustic consequences resulting from the differing gender-based vocal tract characteristics. [START_REF] Laures | Perceptual effects of a flattened fundamental frequency at the sentence level under different listening conditions[END_REF] and [START_REF] Watson | The effect of fundamental frequency on the intelligibility of speech with flattened intonation contours[END_REF] found a flattened fundamental frequency (F0) contour within individual utterance negatively influences sentence recognition accuracy in noise. Vowel formant space expansion (i.e. greater discrimination between vowel categories) has also been shown to benefit speech intelligibility [START_REF] Bond | A note on the acoustic-phonetic characteristics of inadvertently clear speech[END_REF][START_REF] Ferguson | Vowel intelligibility in clear and conversational speech for normal-hearing and hearing-impaired listeners[END_REF]. In the presence of noise, [START_REF] Gordon-Salant | Recognition of natural and time/intensity altered CVs by young and elderly subjects with normal hearing[END_REF] and [START_REF] Hazan | The effect of cue-enhancement on the intelligibility of nonsense word and sentence materials presented in noise[END_REF] found that enhancement of consonant-to-vowel (C/V) amplitude ratio by 10 dB increased intelligibility by up to 10 percentage points. It has also been reported that the fine-grained acoustic-phonetic consequences of precision of articulation are able to affect speech intelligibility in noise [START_REF] Bond | A note on the acoustic-phonetic characteristics of inadvertently clear speech[END_REF][START_REF] Hazan | The effect of cue-enhancement on the intelligibility of nonsense word and sentence materials presented in noise[END_REF]. In addition, the intelligibility advantage of clear speech over normal conversational speech in the presence of noise is found to be associated with dynamic formant movement [START_REF] Ferguson | Vowel intelligibility in clear and conversational speech for normal-hearing and hearing-impaired listeners[END_REF] and higher temporal amplitude modulation [START_REF] Krause | Acoustic properties of naturally produced clear speech at normal speaking rates[END_REF].

Further insights into the acoustic-phonetic correlates of intelligibility come from studies on speech produced in noise, so called "Lombard speech". Lombard speech has been found to be more intelligible than speech produced in quiet when both are mixed with noise at the same SNR [START_REF] Dreher | Effects of ambient noise on speaker intelligibility for words and phrases[END_REF][START_REF] Summers | Effects of noise on speech production: Acoustic and perceptual analysis[END_REF][START_REF] Junqua | The Lombard reflex and its role on human listeners and automatic speech recognizers[END_REF][START_REF] Pittman | Recognition of speech produced in noise[END_REF][START_REF] Lu | Speech production modifications produced by competing talkers, babble and stationary noise[END_REF]. Apart from an increase in speech level, these and many other studies (e.g. [START_REF] Tartter | Some acoustic effects of listening to noise on speech production[END_REF][START_REF] Hansen | Analysis and compensation of speech under stress and noise for environmental robustness in speech recognition[END_REF][START_REF] Steeneken | Speech under stress conditions: overview of the effect on speech production and on system performance[END_REF][START_REF] Garnier | An acoustic and articulatory study of Lombard speech: Global effects on the utterance[END_REF] have converged on a set of primary acoustic changes seen in Lombard speech relative to speech produced in quiet. Specifically, Lombard speech demonstrates an overall increase in duration (although vowels and consonants are differentially affected), and increase in F0 and a flattening of spectral tilt (more energy at higher frequencies). The scale of these changes varies with background noise level [START_REF] Dreher | Effects of ambient noise on speaker intelligibility for words and phrases[END_REF][START_REF] Summers | Effects of noise on speech production: Acoustic and perceptual analysis[END_REF][START_REF] Tartter | Some acoustic effects of listening to noise on speech production[END_REF][START_REF] Steeneken | Speech under stress conditions: overview of the effect on speech production and on system performance[END_REF][START_REF] Lu | Speech production modifications produced by competing talkers, babble and stationary noise[END_REF]. In addition to these primary changes, acoustic-phonetic modifications in consonant-vowel energy ratio and formant frequencies have been reported. [START_REF] Junqua | The Lombard reflex and its role on human listeners and automatic speech recognizers[END_REF] and Womack and Hansen (1996) reported a shift of energy from consonant to vowel while [START_REF] Hansen | Analysis and compensation of speech under stress and noise for environmental robustness in speech recognition[END_REF] observed energy shifts from semivowel to vowel and consonant. The first and second formant frequency (F1 and F2) also shift, with the consensus that F1 tends to increase [START_REF] Summers | Effects of noise on speech production: Acoustic and perceptual analysis[END_REF][START_REF] Lu | Speech production modifications produced by competing talkers, babble and stationary noise[END_REF] while F2 has been reported to increase [START_REF] Junqua | The Lombard reflex and its role on human listeners and automatic speech recognizers[END_REF] or decrease (Pisoni et al., 1985).

The issue of how noise-induced speech production changes might contribute to the intelligibility advantage of Lombard speech in the presence of noise has also been addressed [START_REF] Pittman | Recognition of speech produced in noise[END_REF][START_REF] Lu | Speech production modifications produced by competing talkers, babble and stationary noise[END_REF]. [START_REF] Pittman | Recognition of speech produced in noise[END_REF] suggested that the intelligibility gain of Lombard speech is likely to result from complex interactions between vocal level, spectral composition and other acoustic characteristics, rather than a simple relation between each of these parameters and intelligibility. [START_REF] Lu | Speech production modifications produced by competing talkers, babble and stationary noise[END_REF] found that Lombard speech was more intelligible than speech produced in quiet when both were mixed with stationary speech-shaped noise at -9 dB SNR. Using a model of energetic masking [START_REF] Cooke | A glimpsing model of speech perception in noise[END_REF], they found a strong positive correlation between speech intelligibility and the availability of spectro-temporal glimpses of the speech in the presence of noise. The intelligibility gain of Lombard speech over speech produced in quiet was thus attributed to durational increases (i.e. slow speaking rate) and more spectral energy in higher frequencies: an increase in duration provides more opportunities to glimpse acoustic information useful for phonetic distinctions and more spectral energy in higher frequencies leads to more glimpses in the presence of a speech-shaped masker. The pattern of an increased amount of the time-frequency plane glimpsed, as a result of spectral energy shift to higher frequencies, together with durational lengthening, is illustrated in figure 1 using a simple 6-word sentence, drawn from [START_REF] Lu | Speech production modifications produced by competing talkers, babble and stationary noise[END_REF], produced in quiet and in 3 Lombard conditions in which the utterance was produced in noise backgrounds of 82, 89 and 96 dB SPL.

Although an increase in the F0 of speech produced in noise has been widely reported, it is still not clear whether F0 is an attribute that affects Lombard speech intelligibility. In addition, while [START_REF] Lu | Speech production modifications produced by competing talkers, babble and stationary noise[END_REF] suggested that the intelligibility advantage of Lombard speech over speech produced in quiet results from the increase in duration and the flattening of spectral tilt, the individual contribution of a flattened spectral tilt to the intelligibility gain of Lombard speech is unresolved. The primary purpose of the current study was to investigate the absolute and relative contributions, if any, of F0 increase and spectral tilt flattening to speech intelligibility in the presence of noise. Further, the quantitative effect of these parameters on intelligibility was studied using changes observed in Lombard speech induced by different levels of noise. The mean F0 and spectrum of speech produced in quiet were artificially manipulated either separately or together to simulate those of "natural" Lombard speech. Thus, speech intelligibility was measured as a function of parameter type and degree of manipulation. Intelligibility was also compared to that of "natural" Lombard speech to investigate the role of any secondary acoustic modifications in addition to those in F0 and spectrum (such as change in duration). Finally, in order to explore the origin of any difference in intelligibility resulting from different acoustic modifications, the current study used a glimpsing model to determine whether the resulting intelligibility difference of artificial and natural Lombard speech relative to normal speech can be explained by a change in the quantity of speech "glimpses" available in the noise. The glimpsing model is based on the idea that listeners utilise spectro-temporal regions of favourable local signal-to-noise ratio to identify speech.

Glimpsing has been used to predict the intelligibility of intervocalic consonants across a range of masker types [START_REF] Cooke | A glimpsing model of speech perception in noise[END_REF], the relative intelligibility of sentence material from different speakers [START_REF] Barker | Modelling speaker intelligibility in noise[END_REF] as well as the intelligibility of Lombard speech [START_REF] Lu | Speech production modifications produced by competing talkers, babble and stationary noise[END_REF].

Intelligibility of manipulated speech

Speech stimuli and masker

Speech stimuli produced in quiet and in the presence of noise at a number of levels were drawn from the corpus collected by [START_REF] Lu | Speech production modifications produced by competing talkers, babble and stationary noise[END_REF]. In their study, 8 talkers were asked to read out 400 sentences in each of quiet and 3 speech-shaped noise conditions (presentation levels of 82, 89 and 96 dB SPL). Sentence structure was defined by the Grid multitalker speech corpus [START_REF] Cooke | An audio-visual corpus for speech perception and automatic speech recognition[END_REF], which specifies simple 6-word sentence-like materials such as "bin green at K 4 now" or "place red by E 7 please". Four identical sets of 100 Grid sentences, one set from each of the quiet and 3 noise conditions and balanced across the 8 talkers, were used to create the stimuli of the current study. All sentences were endpointed (i.e., leading and trailing silent intervals removed). These 4 conditions are denoted "Quiet", "Lomb_82", "Lomb_89" and "Lomb_96" respectively. An effect of the rise of noise level on the increase in F0 and flattening of spectral tilt is clearly demonstrated by the computations of mean F0 and spectral tilt of long-term average spectrum over the sentences in each of the 4 conditions (F0=148Hz, tilt=-1.62dB/octave for "Quiet"; F0=162Hz, tilt=-1.35dB/octave for "Lomb_82"; F0=166Hz, tilt=-1.29dB/octave for "Lomb_89"; F0=171Hz, tilt=-1.1dB/octave for "Lomb_96"). Mean F0 was obtained by averaging all the valid F0 estimates provided at 10 ms intervals using an autocorrelation-based method [START_REF] Boersma | Accurate short-term analysis of the fundamental frequency and the harmonics-to-noise ratio of a samples sound[END_REF]. Spectral tilt was computed via a linear regression of energies at each 1/3-octave frequency.

To investigate the role of changes in mean F0 and spectral tilt on the intelligibility of Lombard speech, utterances collected in quiet were subjected to 3 types of manipulation on a sentence-by-sentence basis. To evaluate the contribution of increases in F0, each quiet sentence was artificially manipulated using a high-quality source-filter vocoder (STRAIGHT v40 i ) to add a constant amount to the F0 across the utterance to obtain a signal having the same mean F0 as that of the corresponding Lombard sentence. Thus, corresponding to the 3 Lombard speech conditions, there were 3 sets of F0-manipulated sentences, denoted "F0_82", "F0_89" and "F0_96".

Similarly, to examine the effect of spectral tilt flattening, each quiet sentence was passed through an infinite impulse response filter of order 100 whose magnitude response was designed in such a way that the overall spectrum of the filtered signal was the same as that of the corresponding Lombard sentence, resulting in 3 sets of spectrum-manipulated sentences derived from the quiet speech (denoted "Spec_82", "Spec_89" and "Spec_96" respectively). Finally, to obtain stimuli having the same mean F0 and spectral tilt of the Lombard sentences, both F0 and spectrum manipulation were applied to each quiet sentence. F0 shift was applied before spectral manipulation. These 3 conditions are denoted "F0_Spec_82", "F0_Spec_89" and "F0_Spec_96".

To illustrate the processing of F0 and spectral tilt, the mean F0 and spectral tilt of a processed quiet sentence (from the condition of "F0_Spec_89") and the corresponding Lombard sentence (from the condition of "Lomb_89") together with the original unprocessed quiet signal (from the condition of "Quiet") were measured as shown in figure 2.

In addition to the 9 manipulated speech conditions, the main experiment included 4 natural speech conditions: speech produced with no noise ("Quiet"), and speech produced in the presence of noise ("Lomb_82", "Lomb_89" and "Lomb_96"). The quiet condition provides a baseline against which the contribution to intelligibility of the various speech manipulations can be measured, while the natural Lombard speech presumably represents a performance ceiling since it contains not only the manipulations represented in the artificial conditions but other changes, such as alterations to formant frequencies and bandwidths, some of which might conceivably contribute to intelligibility.

Since in the current study F0 manipulation was implemented via the tool STRAIGHT, any effect of F0 manipulation on speech intelligibility might also be accompanied by artefacts introduced by the resynthesis algorithm. To check for any such effects, an additional 3 conditions were tested in which the original stimuli from the "Quiet", "Spec_89" and "Lomb_89" conditions were re-synthesized by STRAIGHT without parameter manipulations.

In summary, the experiment contained 16 test conditions: 4 of natural speech, 9 with manipulated speech, and 3 to check any effects of the resynthesis algorithm. The same set of 100 Grid sentences was used for the 16 conditions. In all 16 conditions, each sentence was mixed with a speech-shaped noise masker at an overall SNR of -9 dB, a value chosen to avoid ceiling and floor effects as reported in [START_REF] Lu | Speech production modifications produced by competing talkers, babble and stationary noise[END_REF]. The spectrum of the masker equalled the long-term average speech spectrum of the Grid corpus (figure 3). A masker with a speech-shaped spectrum was chosen because it was found in [START_REF] Lu | Speech production modifications produced by competing talkers, babble and stationary noise[END_REF] to elicit both a strong overall Lombard effect and a flattening of the speech spectrum, which was suggested as a possible basis for intelligibility gains based on release from energetic masking in the presence of a speech-shaped noise masker. Maskers were gated on and off with the stimuli and the mixed signals were scaled to a presentation level of approximately 68 dB SPL.

Listeners

Ten native speakers of British English (7 males and 3 females) took part in the intelligibility experiment. All received a hearing test using a calibrated software audiometer which was used to test each ear at the 6 frequencies: 250, 500, 1000, 2000, 4000 and 8000 Hz. All had normal hearing level. Ages ranged from 20 to 31 years (mean: 26.2). Ethics permission was obtained following the University of Sheffield Ethics Procedure.

Procedure

Listening sessions took place in an IAC single-walled acoustically-isolated booth.

Stimulus presentation and results collection was controlled by a computer program.

Stimuli were presented diotically over Sennheiser HD 250 Linear II headphones.

Listeners were asked to identify in each noisy utterance the letter and digit keywords by entering their results using a conventional computer keyboard. Those keys representing letters were activated immediately following the onset of each utterance.

As soon as a letter key was pressed, the 10 digit keys were enabled. This approach allowed for rapid and accurate data entry. Since the structure of the speech materials provided no contextual information with which to predict the target keywords, the listeners were required to rely on the acoustic information rather than the semantic content of the sentence to identify the target words. Each participant completed the 16 conditions over 2 sessions. Each condition consisted of 100 sentences, and required 4-5 minutes to complete. For each condition, keyword identification rate was computed as the percentage of correctly identified keywords. Condition orders were randomized across listeners. There were 10 additional unscored tokens (5 in quiet and 5 in noise) for practice in the beginning of the first session for each listener.

Results

Effect of resynthesis procedure

Speech intelligibility in the re-synthesized and original conditions of "Quiet", "Spec_89" and "Lomb_89" was compared to determine the effect of any artefacts which might have been introduced by STRAIGHT processing. A 2-way repeatedmeasures ANOVA with factors of type of speech signal (re-synthesized, original) and type of manipulation ("Quiet", "Spec_89", "Lomb_89") demonstrated that the effect of type of speech signal collapsed over the three conditions was not significant (F(1,9)=1.76, p=0.22) and none of the differences in any of the 3 manipulation conditions reached significance (p>0.20).

This finding supports that of [START_REF] Assmann | Synthesis fidelity and vowel identification[END_REF], who reported that when no parametric modifications were introduced, vowels synthesized with STRAIGHT were identified as accurately as the natural version. Kawahara (1998) also found the resynthesized speech using STRAIGHT provided equivalent "naturalness" compared to the original speech, bearing out the claim [START_REF] Kawahara | Restructuring speech representations using a pitch-adaptive time-frequency smoothing and an instantaneous-frequency-based F0 extraction: Possible role of a repetitive structure in sounds[END_REF] that STRAIGHT is capable of high-fidelity speech manipulation. Both subjective impressions and the results of the present listening test suggest that STRAIGHT processing in the current study was unlikely to introduce important artificial timbre or other deleterious effects when manipulating F0.

Effects of manipulated speech on intelligibility

Figure 4 summarizes relative improvements in keyword identification rates in all 12 speech manipulation conditions over quiet, shown as the proportional increase in scores. The baseline performance in quiet was 56%, while intelligibility for both the manipulated and natural Lombard conditions exceeded this score, with up to 30% relative improvement. Using the same SNR and type of noisy stimuli, the baseline score for utterances produced in quiet was somewhat higher than the 42% reported in [START_REF] Lu | Speech production modifications produced by competing talkers, babble and stationary noise[END_REF], and consequently, the average increase of Lombard speech intelligibility over the quiet was somewhat lower in the current study compared to that reported in Lu and Cooke (16 versus 24 percentage points). This difference may be due to the fact that 7 of the 10 listeners recruited for the current study had prior experience of Grid sentences in other speech perception and production experiments.

Paired-samples t-tests were computed between the quiet condition and each of the 12 speech manipulation conditions. Compared to quiet, the three F0-shifted speech conditions did not increase intelligibility (p>0.05) while all the other conditions did (p<0.001). For the 12 conditions, a 2-way repeated-measures ANOVA with factors of manipulation type = {F0, Spec, F0_Spec, Lomb} and manipulation level = {82, 89, 96} was also computed. The analysis showed there was no significant interaction between these two factors (F(3.50,31.53)=0.60, p=0.73) while demonstrating a significant main effect of manipulation type (F(2.30,20.74) Between the 4 types of manipulation collapsed across manipulation level, post-hoc pairwise comparisons (here and elsewhere with Bonferroni-adjustment for multiple comparisons) indicated that the intelligibility of speech with a manipulated spectrum increased significantly compared to that with F0 shifted separately (p<0.001). There was no additional benefit of modifying F0 and spectrum together over changing the spectrum alone (p>0.05). Natural Lombard speech was more intelligible (p<0.01) than all other types of manipulation. Since manipulation type did not interact with manipulation level, a similar overall pattern was further confirmed at each of the 3 manipulation levels using pairwise comparisons between the 4 manipulation types (p<0.05), except that at the smallest manipulation scale (82 dB Lombard speech), the intelligibility gain of natural Lombard speech over spectrum-manipulated speech and speech with spectrum manipulated jointly with F0 failed to reach significance (p>0.11).

In addition, post-hoc pairwise comparisons between the 3 manipulation levels collapsed across manipulation type confirmed that there was a significant difference between the largest and smallest manipulation levels (p<0.05) although the 89 dB case did not differ significantly from the other two (p>0.27). This tendency was also observed in each of the 3 manipulation types ("Spec", "F0_Spec" and "Lomb") although none of these reached significance (p>0.08).

Since listeners were exposed to the same set of 100 Grid sentences across conditions, a check was made for learning effects using a repeated-measures ANOVA with factors of background condition and presentation order. This analysis suggested that condition order was not a significant factor for keyword identification score (F=0.29, p=0.59).

Discussion

The behavioural experiment explored the extent to which an increase in F0 and a flattening of spectral tilt influence speech intelligibility in the presence of speechshaped noise. The two findings that F0-shifted speech was no more intelligible than the baseline "quiet" speech and shifting F0 of spectrum-manipulated speech did not further improve intelligibility suggest that increases in F0 make little contribution.

However, it was found that there were significant intelligibility gains of spectrummanipulated speech over quiet speech and the gain tended to increase with manipulation scale. These findings support the claim [START_REF] Lu | Speech production modifications produced by competing talkers, babble and stationary noise[END_REF]) that a flattening of spectral tilt helps to improve intelligibility in the presence of speechshaped noise.

Spectral modifications alone cannot account for the entire intelligibility increase of Lombard speech, since natural Lombard speech was significantly more intelligible than synthetic Lombard speech. Thus, part of the benefit must derive from factors other than a flattening of spectral tilt. Lombard speech has a number of other acoustic and acoustic-phonetic consequences, such as changes in consonant-vowel energy ratio and formant frequencies. A further difference between the natural and synthetic conditions is the durational lengthening in the former. In essence, the same amount of information is spread out over a longer interval in the natural Lombard case, leading to the possibility of a greater resistance to energetic masking. To investigate a role for durational differences, and to examine whether energetic masking can explain the superior intelligibility of spectrally-manipulated speech, the glimpsing model of speech perception in noise [START_REF] Cooke | A glimpsing model of speech perception in noise[END_REF] was employed.

Does manipulated speech offer more glimpsing opportunities?

3.1. Motivation [START_REF] Cooke | A glimpsing model of speech perception in noise[END_REF] demonstrated that recognition of intervocalic consonants solely from those spectro-temporal regions ("glimpses") of clean speech least affected by background noise predicts listener scores across a range of conditions, ranging from competing speech, through N-talker babble to stationary speech-shaped noise. The glimpsing model has since been shown to make good detailed predictions of the intelligibility of individual spoken letters and different talkers in adverse conditions [START_REF] Barker | Modelling speaker intelligibility in noise[END_REF]. In [START_REF] Cooke | A glimpsing model of speech perception in noise[END_REF], glimpses of a signal are defined as those connected regions in an auditory-inspired spectro-temporal representation greater than a certain minimum "area" calculated from the number of spectro-temporal "pixels" and where each spectro-temporal "pixel" has a local SNR larger than a threshold.

Using the same computational model, [START_REF] Lu | Speech production modifications produced by competing talkers, babble and stationary noise[END_REF] also reported a very high correlation between relative intelligibility gains for listeners against relative increases in the amount of information available through glimpsing (r=0.98, p<0.001). The current study tested the hypothesis that the intelligibility of speech with acoustic modifications is likewise dominated by the availability of glimpses of the speech in the presence of noise. Such glimpses could result from factors such as changes in F0, spectral tilt and duration.

Glimpse measures

Two glimpsing statistics were measured for the signal mixtures used in the intelligibility experiment described in the previous section. One, glimpse "area", is the number of spectro-temporal points where the glimpse criteria described above hold. In addition to glimpse area, which is dependent on signal duration, a second measure, the proportion of spectro-temporal points meeting the glimpse criteria was also computed, namely glimpse "proportion". This latter measure is independent of duration, and helps to distinguish those speech production processes which improve glimpsing opportunities by slowing speech rate from those which reallocate energy in the frequency domain.

Computation of glimpse measures was based on a spectro-temporal excitation pattern representation formed for the target and masker independently. This representation is produced by first passing the time-domain signal through a 64 channel gammatone filterbank, smoothing the Hilbert envelopes at the output of the filters, integrating the energy into 10 ms frames, followed by log compression.

Following [START_REF] Cooke | A glimpsing model of speech perception in noise[END_REF], a minimum "area" of 5 "pixels" and a local SNR of -5 dB were used here.

Results

Figure 5 depicts relative changes in the glimpse measures for each of the 12 speech manipulation conditions over speech produced in quiet, shown as percentage increases.

For each of the utterances produced in quiet, there were on average 1390 spectrotemporal points meeting the glimpse criteria, leading to a glimpse proportion value of 11.4%. Only one measure is plotted for the manipulation types of "F0", "Spec" and "F0_Spec" because the speech in these conditions was derived from the quiet speech and thus had the same duration, which made the duration-dependent (glimpse area) and duration-independent (glimpse proportion) measures identical. Paired-samples ttests showed that compared to speech produced in quiet, there was no significant increase in glimpse area/proportion of F0-shifted speech (p>0.05). For all the other conditions, significant increases of glimpse area and proportion over quiet were reported (p<0.001).

For glimpse area, a 2-way repeated-measures ANOVA with factors of manipulation type = {F0, Spec, F0_Spec, Lomb} and level = {82, 89, 96} demonstrated a significant main effect of manipulation type (F(2.10,14.71)=103.23, p<0.001, η p 2 =0.94) and the absence of an interaction with level (F(1,7)=4.01, p=0.08). To test the differences between the 4 types of manipulation collapsed across level, post-hoc pairwise comparisons showed that speech with its spectrum manipulated separately produced a larger increase than that with F0 shifted separately (p<0.001) while there was no significant change (p>0.05) between speech with spectrum manipulated separately and jointly with F0. Lombard speech produced more glimpses than all the other 3 types of manipulation (p<0.05). This pattern is echoed at each of the 3 manipulation levels as shown in figure 5 although for the 82 dB conditions the glimpse area for Lombard speech did not differ significantly from those for spectrummanipulated (p=0.83) and both-parameter manipulated speech (p=0.68).

Figure 5 also shows that glimpse area tended to increase with manipulation level in all 4 types of manipulation apart from the conditions of F0-shift alone in which it changed little with level. This was confirmed by the significant main effect of level (F(1.19,8.32)=8.67, p<0.05, η p 2 =0.53). When collapsed over manipulation type, the difference between the largest and smallest level was significant (p<0.05). The tendency of glimpse area to increase with manipulation level was also observed in each of the 3 manipulation types.

Glimpse area was highly-correlated with listener intelligibility gain (r=0.988, p<0.001) as shown in figure 6 which plots relative increases in intelligibility for listeners against relative increase in glimpse area.

For glimpse proportion, a 2-way repeated-measures ANOVA with factors of manipulation type = {F0, Spec, F0_Spec, Lomb} and manipulation level = {82, 89, 96} was computed. At each level, there were significant differences (p<0.001) between manipulation type "F0" and each of the other 3 types ("Spec", "F0_Spec" and "Lomb") while the differences between "Spec", "F0_Spec" and "Lomb" were not significant (p>0.05). The increase in glimpse proportion with level for natural Lombard speech failed to reach significance (p>0.50).

Figure 5 also demonstrates that the relative increases of glimpse area ("Lomb_area")

were larger than those of glimpse proportion ("Lomb_prop") for the natural Lombard speech conditions, a difference confirmed by a 2-way repeated-measures ANOVA with factors of glimpse measure = {area, proportion} and level = {82, 89, 96}.

Significantly larger increases in glimpse area over glimpse proportion were obtained (F(1,7)=15.14, p<0.01, η p 2 =0.68), which was further confirmed in the 89 and 96 dB (p<0.01) conditions but not at 82 dB (p=0.14). The difference in relative increase in glimpse area over proportion also tended to increase with manipulation level although the interaction between glimpse measure and level failed to reach significance (F(1,7)=5.50, p=0.052). The larger increases in glimpse area over proportion in the Lombard speech conditions are presumably due to the tendency of noise-induced sentences to increase in duration, since glimpse area increases in proportion to duration, while proportion is independent of duration.

General discussion

The current study estimated the relative contribution of F0 increase and spectral flattening to the improvement of speech intelligibility in the presence of speechshaped noise. Compared to speech collected in quiet, an upward shift in F0 did not lead to an increase in intelligibility, while spectral flattening led to a large gain in intelligibility. However, the gain fell short of that obtained by natural Lombard speech.

Such a pattern was found to be highly-correlated with a measure based on the amount of the time-frequency plane glimpsed, suggesting that the main effect of the speech manipulations examined was to create a release from energetic masking. Spectral flattening in the presence of speech-shaped noise is beneficial since it results in an upwards migration of speech energy to regions less likely to be masked by speechshaped noise (figure 3). The increase in F0 led to a rather small amount of energy migration to higher frequencies compared to the speech in quiet (figure 7), which resulted in a small increase in glimpses and a non-significant improvement in intelligibility over the quiet speech. The presence of such an energy migration in F0increased speech may be due to the wider spacing of harmonics. Since the Lombard speech materials used in the current study were collected in speech-shaped noise conditions drawn from [START_REF] Lu | Speech production modifications produced by competing talkers, babble and stationary noise[END_REF], the F0 increase in Lombard speech could be a by-product of other speech changes such as an increase in vocal intensity, rather than a strategy that helps to improve intelligibility in speech-shaped noise. For other maskers (such as a competing voice) it remains possible that F0 changes could help to distinguish a speaker's output from the background.

While a spectral flattening strategy is beneficial for noises with a falling spectrum typical of many natural noise types (e.g. multitalker babble) used to induce Lombard speech, it is not necessarily helpful for noises with a greater energy concentration in higher frequencies. However, [START_REF] Lu | Speech production modifications produced in the presence of low-pass and high-pass filtered noise[END_REF] demonstrated that speech produced in response to high-pass filtered noise also has a spectral centre of gravity which is shifted upwards into the frequency regions containing the noise. Talkers appear unable to adopt what might be considered the optimal strategy in such situations i.e. to shift spectral energy downwards in frequency to noise-free regions.

Evidence for the perceptual contribution of flattening spectral tilt has been mentioned in other studies. For instance, [START_REF] Krause | Acoustic properties of naturally produced clear speech at normal speaking rates[END_REF] found that a migration of spectral energy to high frequencies contributes to increased intelligibility of clear speech relative to conversational speech in the presence of speech-shaped noise. A significant effect on intelligibility in white noise was reported by [START_REF] Niederjohn | The enhancement of speech intelligibility in high noise levels by high-pass filtering followed by rapid amplitude compression[END_REF] who attempted to suppress the first formant by high-pass filtering to emphasize the energy in high frequencies. The intelligibility gain obtained was considered to be due to enhancement of F2 energy relative to that of F1. F2 is claimed to make a larger contribution to overall speech reception than F1 [START_REF] Thomas | The second formant and speech intelligibility[END_REF][START_REF] Thomas | The influence of first and second formants on the intelligibility of clipped speech[END_REF]). In addition, by moving formants upward in frequency via alteration of line spectral pairs derived from linear prediction parameters, [START_REF] Mcloughlin | LSP-based speech modification for intelligibility enhancement[END_REF] reported an enhancement of vowel intelligibility in the presence of noise, which they attributed to the SNR improvement afforded by the low-frequency bias of the noise. However, [START_REF] Assmann | Modelling the perception of frequency-shifted vowels[END_REF] and [START_REF] Assmann | Identification of frequency-shifted vowles[END_REF] reported that an upward shift as well as a downward movement of formants due to a linear scaling of the frequency axis did not yield an improvement on the intelligibility of vowels in quiet, a finding which they attributed to the deterioration of learned relationships between formant frequencies.

The finding that Lombard speech resulted in more potential glimpses overall (as indicated by the glimpse area metric) compared to the spectrum-manipulated speech in the presence of a masker could be due to the increased duration of Lombard utterances, since the spectrum manipulation conditions were applied to utterances produced in quiet, which were shorter. When the effect of duration was normalised by measuring the proportion of the time-frequency plane glimpsed, Lombard speech led to an equivalent glimpsing density as the spectrum-manipulated speech. Given that there is a high correlation between the availability of overall glimpses and the speech intelligibility, it appears that the greater intelligibility of Lombard speech compared to spectrum-manipulated speech could result from the increase in glimpsing opportunities afforded by a slower speaking rate. This is compatible with the finding that the intelligibility gain was larger for the more intense Lombard speech, which was itself of longer duration than the less intense Lombard speech.

A number of studies have investigated the perceptual effect of duration lengthening (which is equivalent to a reduction in speaking rate if utterances of homogeneous length are used). However, evidence for the effect of durational change on speech intelligibility in noise is mixed. While several researchers (e.g. [START_REF] Cox | Intelligibility of average talkers in typical listening environments[END_REF][START_REF] Jones | Synthesized speech intelligibility and persuasion: Speech rate and non-native listeners[END_REF] have demonstrated that slower speaking rates lead to increased speech intelligibility in noise, [START_REF] Sommers | Stimulus variability and spoken word recognition. II. The effects of age and hearing impairment[END_REF] failed to find a perceptual correlate of speaking rate for young listeners with normal hearing. In addition, [START_REF] Bond | A note on the acoustic-phonetic characteristics of inadvertently clear speech[END_REF] and [START_REF] Hazan | Acoustic-phonetic correlates of talker intelligibility for adults and children[END_REF] observed that words with longer duration led to an increased intelligibility in the presence of noise while no such effect of word duration was found in [START_REF] Uchanski | Intelligibility of modified speech for young listeners with normal and impaired hearing[END_REF]. These findings suggest that while it is clear that duration lengthening can increase the amount of acoustic information available, the extent to which it can improve intelligibility in the presence of noise may depend on the characteristics of the listeners and speech materials employed.

The current study did not find a significant effect of increasing F0 on intelligibility, which echoes studies such as [START_REF] Bond | A note on the acoustic-phonetic characteristics of inadvertently clear speech[END_REF] and [START_REF] Hazan | Acoustic-phonetic correlates of talker intelligibility for adults and children[END_REF], who reported that the intelligibility of speech in noise did not correlate with F0 mean. [START_REF] Barker | Modelling speaker intelligibility in noise[END_REF] found that speech intelligibility was correlated with fundamental frequency (F0) only for female talkers at relative low SNRs. [START_REF] Ryalls | Fundamental frequency and vowel perception[END_REF] and [START_REF] Assmann | Identification of frequency-shifted vowles[END_REF] even found a negative influence of large synthetic F0 increase on vowel intelligibility in quiet, which was attributed to the poorly resolved formant peaks that resulted from a sparsely sampled harmonic spectrum. This suggests that there could be other mechanisms apart from glimpsing that are involved in the way F0 increases affects speech intelligibility.

For the current study, the spectral energy reassignment due to spectral flattening contributed approximately 70% of the intelligibility gain, with the residual possibly due to temporal reassignment (slower speaking rate). In both cases, simple measures based on energetic masking provide a good quantitative explanation for the gains. In addition to a durational account, other non-EM factors also have a potential role for the observed residual. For instance, changes in vowel formant frequencies of Lombard speech that lead to a change in vowel space dispersion is likely to contribute since the perceptual confusion between different vowels could be reduced in an expanded vowel formant space. The improved Lombard speech intelligibility could also result from the enhancement of speech regions which contain acoustic cues to phonemic contrasts.

Various studies have attempted to improve speech intelligibility by enhancing perceptually-relevant acoustic cues, typically by identifying information-bearing regions of the signal, including those which contain important acoustic cues to phonetic contrasts, and increasing their relative intensity. Using a consonant identification task in a set of nonsense CV/VCV syllables, consonant intelligibility has been found to increase in a background of noise when their intensity relative to that of vowels was enhanced [START_REF] Gordon-Salant | Recognition of natural and time/intensity altered CVs by young and elderly subjects with normal hearing[END_REF][START_REF] Hazan | The effect of cue-enhancement on the intelligibility of nonsense word and sentence materials presented in noise[END_REF][START_REF] Skowronski | Applied principles of clear and Lombard speech for automated intelligibility enhancement in noisy environments[END_REF]. [START_REF] Hazan | The effect of cue-enhancement on the intelligibility of nonsense word and sentence materials presented in noise[END_REF] reported significant improvement by applying amplitude enhancement to the formant transition regions at vowel onset and offset as well as the perceptually-important spectral regions of consonants. [START_REF] Tallal | Language comprehension in language-learning impaired children improved with acoustically modified speech[END_REF] also observed a benefit of amplifying regions of rapid spectral change to auditory training. However, these speech enhancement approaches are difficult to apply in a robust manner in real-time. The finding from the current study that speech enhancement can be realised by spectrum flattening is encouraging since it is certainly feasible to implement spectrum modifications online. Indeed, the successful application of real-time processing approach to speech enhancement in noise has been shown by [START_REF] Lee | Real-time speech intelligibility enhancement based on the background noise analysis[END_REF], for instance. By increasing the speech energy relative to noise in the frequency bands where the SNR is low, they were able to enhance speech intelligibility in noise in communication situations requiring realtime processing, such as in mobile phone applications.

Conclusions

The current study investigated the effects of an upward shift in F0 and a flattening of spectral tilt on speech intelligibility in noise with a speech-shaped spectrum. The results showed a significant contribution to Lombard speech intelligibility of spectrum flattening and failed to find a perceptual influence of an increase in F0. The possibility that a lengthened duration helps to improve the intelligibility of Lombard speech in noise was also suggested. Echoing [START_REF] Lu | Speech production modifications produced by competing talkers, babble and stationary noise[END_REF], the current study also found a high correlation between speech intelligibility and the amount of the time-frequency plane glimpsed. These findings suggest that speech modifications which reassign speech energy in time and frequency to introduce more glimpses in the presence of noise can be used in an attempt to improve speech intelligibility in everyday conditions.

Acknowledgment

The second author acknowledges support from the EU Marie Curie Network "Sound to Sense". The authors would like to thank Hideki Kawahara for providing the Matlab implementation of STRAIGHT v40.

i STRAIGHT uses pitch-adaptive spectral analysis combined with a surface reconstruction method in the time-frequency region, and an excitation source design based on phase manipulation. It preserves the bilinear surface in the time-frequency region and allows for over 600% manipulation of such speech parameters as pitch, vocal tract length, and speaking rate, without introducing the artificial timbre specific to synthetic speech signals while maintaining a high reproductive quality [START_REF] Kawahara | Speech representation and transformation using adaptive interpolation of weighted spectrum: Vocoder revisited[END_REF][START_REF] Kawahara | Restructuring speech representations using a pitch-adaptive time-frequency smoothing and an instantaneous-frequency-based F0 extraction: Possible role of a repetitive structure in sounds[END_REF]. The baseline identification score for utterances produced in quiet was 56%. relative intelligibility gain (%) relative glimpse increase (%)

Glimpse area (r=.988 p<.001)

Figure 1 .

 1 Figure 1. Spectro-temporal excitation patterns (left column) and glimpses (right

Figure 2 .

 2 Figure2. The spectrum of a Grid sentence in the conditions of "Quiet", "Lomb_89"

Figure 3 .

 3 Figure 3. Long-term average spectrum of the Grid corpus.

Figure 4 .

 4 Figure 4. Relative improvements in keyword identification scores for speech with

Figure 5 .

 5 Figure 5. Glimpse area and proportion for the stimuli used in the intelligibility

Figure 6 .

 6 Figure 6. Relation between increase in glimpse area and intelligibility, together with

Figure 7 .

 7 Figure7. Long-term average spectrum over sentences in "Quiet" and "F0_89"


R Herrero
email: ruth.herrero@upct.es

CHARACTERIZATION OF A SEMI-INDIRECT EVAPORATIVE COOLER

Keywords: energy recovery, porous evaporative cooling, DOE, ANOVA

This paper presents the experimental study of a semi-indirect evaporative cooler (SIEC), which acts as an energy recovery device in air conditioning systems. Experimental measurements were achieved for the characterization of the thermal performance of the device, employing the design of experiments (DOE) methodology and an analysis of variance (ANOVA).

INTRODUCTION

The energy cost, the environmental impact, the necessity of fulfilling established protocols, etc. force to reduce energy demand. This can be achieved using more energy efficient equipment. Evaporative cooling provides a lowcost, energy efficient and environmentally benign method of cooling. [START_REF] Watt | Evaporative Air Conditioning Handbook[END_REF] Most conventional direct evaporative cooling methods involve collection and recirculation of water to keep the wetting media or misting region saturated. There exists an environment of almost stagnant water in direct contact with the outside air, which aids, if not properly maintained at significant costs, the spread of many liquid phase-born bacterial diseases, most notably Legionnaire's disease. [START_REF] Johnson | Analysis of heat and mass transfer phenomena in hollow fiber membranes used for evaporative cooling[END_REF].

Due to this fact, indirect evaporative cooling application has been commonly used, in spite of the fact that indirect systems have lower efficiency in comparison with direct systems [START_REF] Rey | Estudio de un Recuperador Evaporativo Indirecto[END_REF].

Semi-indirect evaporative systems with porous media are a successful attempt to improve the efficiency of indirect systems. Several prototypes for building cooling using porous ceramic materials as wetting media, as this paper proposes, have been built such as those by Ibrahim et al. [START_REF] Ibrahim | Performance of porous ceramic evaporators for building cooling application[END_REF] or Riffat and Zhu [START_REF] Riffat | Mathematical model of indirect evaporative cooler using porous ceramic and heat pipe[END_REF] who also combined an indirect evaporative cooler using porous ceramic as the cooling source and a heat pipe as the heat transfer device.

This paper presents the experimental study of a semi-indirect evaporative cooler (SIEC), which acts as an energy recovery device in air conditioning systems. For experimental characterization purposes, a complete factorial design was developed and an analysis of variance was performed to account for the contributory percentage of the analyzed factors and interactions which allowed to better understand the behaviour of the system.

EXPERIMENTAL APPARATUS

The semi-indirect evaporative cooler (SIEC) has two independent air flow supplies, one used for cooling, together with a second, the return air flow, in direct contact with water to favour heat and mass transfer. Water is forced against the return air flow and it is constantly circulating.

The cooling effect of the impulsed air would thus be the addition of two processes: the heat exchange between the two air flows (supply and return) plus the heat exchange process, through evaporation, between the air supply and the external wall.

The semi-indirect evaporative cooler works with the following mechanisms:

-Heat and mass transfer in the return air flow.

-Spread of mass due to porosity and heat transport through the solid wall.

-Evaporation or condensation as well as heat and mass exchange in the air flow supply.

All of these features are presented together, thus combining heat and mass transfer, increasing the cooling effect of the air to be conditioned and achieving optimisation of the thermal process. [START_REF] Kaviany | Principles of Heat Transfer in Porous Media[END_REF] Depending on the permeability of the wall of the solid porous cooler which separates the two air flows, there is greater or lower liquid diffusion (water) towards the air flow supply from the external pores, in all cases. The partial pressure of the water vapour in the supply air is the controlling factor in this mass transport process. In this respect we have called it semi-indirect.

In Tables 1, 2 the geometric dimensions and the staggered configuration of the ceramic exchanger are presented.

EXPERIMENTAL STUDY

As mentioned, the system is located in a recovery configuration to condition a room. The equipments used to carry out the experiments are the following: (Fig. 1) Fig. 1 -Supply system: it consists on a fan with a potentiometer to keep the air flows under control.

-Air Handling Unit (A.H.U.): this equipment allows us to simulate the conditions of the air supplied (temperature and humidity).

-Air distribution System: all the measuring instruments are inserted there.

-Water distribution System: a water pump convey water from the tank to the pressure spray system with downward directed nozzles.

-A semi-indirect evaporative cooler.

-The conditioned room: the dimensions are 2x2x2.5 m, which contains a heat pump inside to guarantee when needed that the space is properly conditioned.

-Monitoring and Data-Acquisition system: a computer controls and stores all the results from the measuring instruments.

From the A.H.U. the air goes inside the SIEC. This main airstream is called the primary airflow and when this airstream is conditioned in the SIEC enters the room. Thus, the air expelled from the room goes through the SIEC in a cross-flow (recovery configuration), going up the SIEC until it leaves in the upper part (secondary airflow).

The measurement sensors used are:

-T: temperature measurement sensor. Type: Technoterm 60, Accuracy: 0.1ºC.

-HR: relative humidity measurement sensor. Type: Honeywell HIH-3610, Accuracy: : ±2 % RH (0 -100 % RH) -DP: differential pressure transducer. Type: DWYER 603-2. Accuracy: (70 ºF) ±2% scale range.

The characterization of the recovery system was carried out by following the experimental design method (DOE). A complete factorial design was applied to analyse latent and sensible heat recovered in order to obtain the total heat recovered by the evaporative cooling system.

The three factors analyzed were:

-Air Flow (V)

The three airflow levels are: 140 m 3 /h (V1), 260 m 3 /h (V2) and 380 m 3 /h (V3).

-Humidity level (HL)

The humidity levels corresponds to: Relative Humidity less than 30% (HL1), Relative Humidity between 30 and 60% (HL2), Relative Humidity more than 60% (HL3).

-Temperature (T)

The temperature levels are: 40ºC (T5), 36.5ºC (T4), 33ºC (T3), 29.5ºC (T2) and 26ºC (T1).

Once the controlled factors and the corresponding levels are selected, an orthogonal design matrix was generated.

The tests were randomized to avoid the possibility of creating an "order effect" as influenced by the sequence of the tests. All tests were replicated five times. Interactions between factors as well as the effects of individual factors were investigated. All the observations were corroborated by performing an analysis of variance (ANOVA). [START_REF] Ross | Taguchi techniques for quality engineering. Loss function, orthogonal experiments, parameter and tolerance design[END_REF] The uncertainty values for the characteristics analyzed are shown in Table 3, where U Vi represent the uncertainty values for each volumetric flow described [START_REF]ISO: Guide to the expression of uncertainty in measurements[END_REF].

EXPERIMENTAL DESIGN RESULTS

Sensible Heat Recovered:

The most remarkable results are shown in Figure 2 andTable 4. In Figure 2 The most significant conclusions which can be drawn from these results are presented below:

Airflow Analysis:

This factor has a medium contributory percentage close to 12% (Table 4). The energy recovered is proportional to the air flow due to the rise in the convection coefficient in the external flow.

Temperature Analysis:

As in common heat exchangers, the heat transfer rate is related to the temperature difference. When the temperature difference between the outdoor and the return airstreams rises, the sensible heat recovered is higher. Its contribution in terms of variance analysis is close to 12% (Table 4).

VxT

This is the highest contributory percentage with a value of 45% (Table 4). There is no interaction between the factors, as can be clearly seen in Figure 2 which corroborates the individual factor analysis made. When the temperature and the airstream rise the sensible heat recovered also does. The continuity and the linearity of the results are shown. A trend to negative values is clearly observed, thereby increasing the heat recovered. Thus, the best values are obtained for V3 and T5.

HLxT

It has a medium contributory percentage, close to 13% (Table 4). The maximum sensible heat recovered is linked to the lowest relative humidity value and the highest temperature level. Nevertheless, the near values of the different humidity levels for all the temperatures found explain the low contribution of the humidity single-factor.

Error

It should be underlined the high contributory percentage of the error (9%), higher than the humidity factor (4.5%).

The explanation to this fact might be found after analysing the triple interaction VxHLxT, included in this percentage.

Latent Heat Recovered:

The most remarkable results are shown in Figure 3 and Table 5. In Figure 3 The most significant conclusions which can be drawn from these results are presented below:

Humidity Analysis:

This factor has the highest contributory percentage 38% (see Table 5). When the humidity level is low, evaporation takes place from the surface of the pipe. This humidity increment can leads to the appearance of condensation in the primary airflow, decreasing the humidity content (dehumidification process, which is linked to negative values of latent heat). (See Figure 2)

VxT

The contributory percentage is close to 15% (see Table 5), which expresses the importance of considering the synergy between both factors. Furthermore, it should be emphasized the predominant effect of dehumidification, (represented by negative mean values) and the proximity of the values, which explains the no contribution of the single factors.

HLxT

The contributory percentage is close to 29% (see Table 5). The behaviour of HLxT corroborates the single factor analysis. Air with low relative humidity and high temperature has a large evaporative capacity, while for the same temperatures, high humidity levels result in high specific humidity values and thus there are good conditions for the appearance of condensation on the pipes (negative values of latent heat). The same effect can also be explained for low temperature values but the effect in the humidity gradient is lower. This fact yields to a decrease of the latent heat exchanged.

Total Heat Recovered:

The most remarkable results are shown in Figure 4 andTable &. In Figure 4 The most significant conclusions which can be drawn from these results are presented below: Temperature This is an important factor especially in terms of sensible heat recovered. The total mean values become linearly more negative at higher temperature levels, when sensible and latent heat effects are added.

Humidity Level

This is the most contributory factor (40%). This is the determining factor in terms of latent heat. The mass transfer phenomena has priority over sensible heat recovered, fact in which lies the superiority of the SIEC over conventional indirect evaporative cooling systems where only sensible heat recovered is involved.

HLxT

At the lowest humidity level (HL1) the mean total heat recovered by adding sensible (negative values) and latent heat (positive values) is almost null. Nevertheless, this trend changes for the other humidity levels, especially for the highest temperature levels (T3, T4 and T5) where sensible heat and latent heat are both negative, due to the appearance of condensation, which corroborates the tendency previously observed in the single factors.

CONCLUSIONS

-The characterization of the SIEC system in terms of heat recovered was carried out by following the experimental design method. A complete factorial design was developed and an ANOVA was also performed to account the contributory percentage of the analyzed factors and interactions which allowed to better understand the behaviour of the system.

-For low relative humidity contents and high temperatures of the air supply, the main effect is evaporative from the surface of the ceramic pipes. For high temperatures and relative humidities of the air supply, dehumidification takes place and thus condensation appears in the exterior surface of the pipes and the latent and sensible heat recovered are added (negative values). However, the mass transfer phenomena takes priority over sensible heat recovered.

-A possible use of this recovery system can be in climates with high temperatures and humidity, such as tropical environments where the system could reduce the humidity of the primary air supply, by using the cooling power of the secondary air.

  the most contributory interaction VxT, in terms of analysis of variance, is plotted against the mean sensible heat recovered. The results of the ANOVA carried out are shown in Table 4. Column 1 shows the factors analyzed, which are: volumetric flow rate, relative humidity level, temperature and the interactions between the aforementioned factors (VxHL, VxT and HLxT). Column 2 represents the degree of freedom (Dof) of the factors analyzed. Column 3 shows the sum of squares (SS) of the factors and column 4 gives the associated variance values (V) and in column 5 the contributory percentage is given (%).

  single factors are plotted against the mean latent heat recovered. The results of the ANOVA carried out are shown in Table 5. Column 1 shows the factors analyzed, which are: volumetric flow rate, relative humidity level, temperature and the interactions between the aforementioned factors (VxHL, VxT and HLxT). Column 2 represents the degree of freedom (Dof) of the factors analyzed. Column 3 shows the sum of squares (SS) of the factors and column 4 gives the associated variance values (V) and in column 5 the contributory percentage is given (%).

  single factors are plotted against the mean total heat recovered. The most contributory interaction in terms of analysis of variance is HLxT. The results of the ANOVA carried out are shown in Table 6. Column 1 shows the factors analyzed, which are: volumetric flow rate, relative humidity level, temperature and the interactions between the aforementioned factors (VxHL, VxT and HLxT). Column 2 represents the degree of freedom (Dof) of the factors analyzed. Column 3 shows the sum of squares (SS) of the factors and column 4 gives the associated variance values (V) and in column 5 the contributory percentage is given (%).

  Figure 3.-Average Latent Heat Recovered: Factors Contribution


Youssef Habibi

Waleed K El-Zawawy

Maha M Ibrahim

Alain Dufresne
email: alain.dufresne@efpg.inpg.fr

Processing and characterization of reinforced polyethylene composites made with lignocellulosic fibers from Egyptian agro-industrial residues

Keywords: Cellulose Fibres (A), polymer-matrix composites (A), fibre/matrix bond (B), interface (B)

Lignocellulosic fibers were extracted from Egyptian industrial crops, viz. cotton stalk, rice straw, bagasse, and banana plant waste. The chemical composition of these fibers was determined.

Composite materials were processed from these natural lignocellulosic fibers using low density polyethylene and acid stearic as compatipilizer, or maleated low density polyethylene. The thermal and mechanical properties were studied by differential scanning calorimetry (DSC) and tensile tests, respectively. The morphology of processed composites was studied by scanning electronic microscopy (SEM). Better compatibility and enhanced mechanical properties were obtained when using maleated LDPE as compatibilizer. The chemical composition of fibers, in terms of lignin, cellulose and hemicelluloses contents, was found to have a strong influence on the mechanical properties of the composites.

Introduction

Natural fibers are gaining progressive account as renewable, environmentally acceptable, and biodegradable starting material for industrial applications, technical textiles, composites, pulp and paper, as well as for civil engineering and building activities. Natural fibers reinforced composites combine acceptable mechanical properties with a low density. Such composites offer a number of well-known advantages which include low cost, availability of renewable natural resources, biodegradability, etc [START_REF] Eichhorn | Review : Current international research into cellulosic fibres and composites[END_REF][START_REF] Mohanty | Natural fibres, biopolymers, and biocomposites[END_REF].

The utilization of biomass for the processing of novel composites has attracted growing interest because of its eco-friendly and renewable nature. Indeed enormous interest in the development of new composite materials filled with natural fibers has been shown by important industries such as the automotive, construction or packaging industry. Composites have encompassed almost all material domains. All synthetic polymers (thermoplastics, thermosets, and elastomers) can be used as matrices. As fillers, besides inorganic powders, extensive use has been made of inorganic manmade fibers such as glass, silicium carbide, or organic fibers like carbon and aramid, in the form of individual fibers (chopped or continuous), mats, or 2D-3D fabrics [START_REF] Nando | Short Fiber-Thermoplastic Elastomer Composites[END_REF].

Compared to inorganic fibers, natural fibers present some well-known advantages such as lower density and lower price. They are less abrasive to the processing equipment, harmless, biodegradable, renewable, and their mechanical properties can be comparable to those of inorganic fibers [START_REF] Bledzki | Composites reinforced with cellulose based fibres[END_REF][START_REF] Bledzki | Thermoplastics reinforced with wood fillers: a literature review[END_REF][START_REF] Plackett | Biodegradable composites based on Lpolylactide and jute fibres[END_REF][START_REF] Ratajska | Biodegradation of some natural polymers in blends with polyolefines[END_REF].

Many studies have been developed based on composites containing natural lignocellulosic fibers from agro-industrial or agricultural residues. Depending on their origin, natural fibers can be grouped into bast (jute, flax, hemp, kenaf, mesta), leaf (pineapple, sisal, henequen, screw pine), and seed or fruit fibers (coir, cotton, oil palm) [START_REF] Sanadi | Agro-Fiber/Thermoplastic Composites[END_REF][START_REF] Young | Utilization of Natural Fibers: Characterization, Modification and Applications[END_REF]. Cellulose is the main component of natural fibers, and the elementary unit of a cellulose macromolecule is anhydro-D-glucose, which contains three hydroxyl (OH) groups. These hydroxyl groups form hydrogen bonds inside the macromolecule itself (interamolecular) and between other cellulose macromolecule (intermolecular). Therefore, all natural fibers are hydrophilic in nature.

However, the main disadvantage of natural fibers/plastic composites is the poor compatibility between the mainly hydrophobic polymer matrix and the hydrophilic fibers. This leads to the formation of a weak interface, which results in poor mechanical properties; where the stress transfer at the interface between two different phases is determined by the degree of adhesion. A strong adhesion at the interfaces is needed for an effective transfer of stress and load distribution throughout the interface [START_REF] Fung | Interface modification on the properties of sisal fiber-reinforced polypropylene composites[END_REF][11][START_REF] Caulfield | Interphase effects on the mechanical and physical aspects of natural fiber composites[END_REF]. Thus, in order to improve the mechanical properties of composites, an improvement of the compatibility between the fibers and the matrix has to be made, i.e. a third component, the so-called compatibilizer, has to be used or the fibers have to be surface modified prior to the preparation of the composites [START_REF] Caulfield | Interphase effects on the mechanical and physical aspects of natural fiber composites[END_REF][START_REF] Beshay | Improved interface bonding between cellulosic fibers and thermoplastics[END_REF]. Formation of an interphase between the lignocellulosic phase and the polymer matrix phase, with polymer chains bonded onto fibers particles, is probably the best solution to the problem. For such a purpose, polyethylene (PE) was treated with maleic anhydride to form maleated polyethylene (MPE).

Egypt is a country with a large agricultural production and agricultural harvesting residues abundantly available in the River Delta of Northern Egypt constitute a bulky and wet resource of significant quantities. It is estimated that 23 million ton/year of agricultural residues result from cotton and rice.

Beside that, there are wastes from sugar-cane and banana plant, which pose major disposal problems.

The Egyptian Environmental Ministry most recent legislation regarding particulate matter discharge from burning, will force to dispose wastes by composting or land filling. Consequently, it is important to find uses for these waste materials. The potential to use lignocellulosic fibers extracted from these agricultural residues has been evaluated. Both PE and MPE; that can be bonded on cellulosic fibers under extrusion, were used as polymeric matrices for different lignocellulosic fibers extracted from Egyptian agricultural wastes, i.e. cotton stalk, rice straw, bagasse, and banana plant waste. The influence of the type of matrix and the type and amount of the fibers on the mechanical properties of the prepared composites was analyzed.

Experimental

Materials

Low density polyethylene (LDPE) (Lacqtene 1008 FE 24, density 0.924 g.cm -3 ) and maleated low density polyethylene (MLDPE, Polybond 3109 with 0.8% maleic anhydride content, density 0.926 g.cm -3 ) were supplied by Atofina S.A. and Crompton Corporation, respectively. Stearic acid (98%) was purchased from Aldrich.

Cotton stalk, rice straw, sugarcane and banana fibers were collected from Egypt. These raw materials were obtain from the farmers after harvesting, i.e. we got the rice straw after harvesting the rice and the same for the cotton stalk we got it after harvesting the cotton and so on. The fibers used in this study were chemically characterized. The chemical characterization consisted in the determination of the carbohydrates, the Klason lignin and the ash content.

Fibers preparation

All the fibers were ground for a few minutes in a mechanical grinder (Fristch Pulverisette, Germany) with speed 3000 rpm and sieved on sieves with various mesh sizes (18, 35 and 60 mesh) to produce homogeneous fractions.

Fiber length measurements

The diameter and length of fibers were measured automatically with the MorFi LB-01 fiber analyzer (Techpap, France) by a computer analysis of images of the suspension flowing through a flat cell observed by a digital CCD video-camera.

Composites preparation

The samples of fiber powders were extruded with the polymeric matrix at 75 rpm with a Micro 15cc Twin Screw Compounder system (DSM Research, Geleen, The Netherlands) at 130°C for 10 min. The LDPE or MLDPE was first added to the mixer and after the thermoplastic matrix was melted, the fiber powders were added and the mixing maintained for additional 10 min. For LDPE based composites, stearic acid was added as compatibilizer (2% of weight of fibers content) with fibers. The resulting composites were chopped and then molded into films 0.5 mm thick, using compression molding at 130°C and 200 bar for few minutes.

Differential Scanning Calorimetry (DSC)

DSC experiments were performed using a DSC Q100 differential scanning calorimeter (TA Instruments, USA). Around 10 mg of samples were placed in a DSC cell in glove box. Each sample was heated from -100 to 200°C at a heating rate of 10°C min -1 . The melting temperature T m was taken as the peak temperature of the melting endotherm.

Tensile tests

The mechanical behavior of the composites prepared with different fibers was analyzed with an RSA3 (TA Instruments, USA) with a load cell of 100 N. Experiments were performed with a cross head speed of 10 mm.min -1 at room temperature, 25°C. The sample dimensions were 10 × 5 × 0.25 mm 3 , and the results were averaged over five measurements.

Scanning Electron Microscopy observations

Scanning electron microscopy (SEM) was used to investigate the morphology of the different types of materials and the filler/matrix interface by using an ABT-55 microscope. The specimens were frozen under liquid nitrogen, fractured, mounted, coated with gold/palladium and observed using an applied tension of 10 kV.

Results and discussion

Fibers characterization

The chemical characterization revealed the proportion of each component of the fibers from agricultural residues. Results are collected in Table 1. Important differences were found depending on the different origin of the fibers, especially in regard to lignin, cellulose and minerals contents.

Cotton stalk and banana plant waste fibers exhibited the highest lignin content, in contrast with the cellulose being the lowest. Comparing these further, these fibers were taken from the plant trunk, which presents high rigidity and hardness, indicating its high content of lignin. An important amount of cellulose and lower lignin content were observed for bagasse and rice straw fibers. One can observe a higher amount of ashes in banana plant waste and rice straw fibers showing their high content in minerals. The wax content was identical for the different fibers.

All the fibers were grounded and sieved with different sieves to produce homogenous fractions.

The 18 mesh sizes retained fraction is the main fraction for all fibers. It was used to process the composites. Both the average diameter and length of all fibers were calculated using MorFi analysis. Results are reported in Table 2 and it was found that they were short fibers. The fiber length used in the preparation of composites is critical. It should not be too long, otherwise the fibers may get entangled with each other [START_REF] Derringer | Short Fiber-Elastomer Composites[END_REF]. This will cause problems with fiber dispersion. If fibers are too short, the stress transfer area will be too small for the fibers to offer effective reinforcement. According to Derringer [START_REF] Derringer | Short Fiber-Elastomer Composites[END_REF], commercially available fibers such as nylon, rayon, and polyester need to be cut into lengths, of approximately 0.4 mm for best dispersion. However, it is worth noting that values reported in Table 2 refer to the initial fiber length before extrusion and this initial fiber length will not be retained in the final composite. The fibers undergoes severe attrition in the extrusion barrel due to extreme shear and the fiber distribution is probably much shorter coming out of the extrusion machine.

LDPE or MLDPE-based composites

First, composite materials were prepared using either LDPE or MLDPE as matrix. For LDPEbased composites stearic acid was used as compatibilizer. It is known to improve the compatibilization between the hydrophobic LDPE matrix and hydrophilic fibers without any covalent chemical bonds between them. Raj and Kokta [START_REF] Raj | Compounding of cellulose fibers with polypropylene: effect of fiber treatment on dispersion in the polymer matrix[END_REF] investigated the influence of using various dispersing aids (stearic acid and mineral oil) and a coupling agent (maleated ethylene) in cellulose fiber reinforced polypropylene composites. Both the tensile strength and modulus of the composites studied were found to increase with the fiber content when either strearic acid or mineral oil (1 wt% by weight of fiber) were added as processing aids during the compounding.

Stearic acid was found to perform better in improving the fiber dispersion compared to mineral oil.

In contrast, anhydride maleic groups in the MLDPE act as coupling agents and covalent bonds can be created between fibers and matrix (Figure 1). Many studies were concerned with the effectiveness of maleic-anhydride-polypropylene copolymers as a coupling agent. Mieck et al.

determined increased shear and tensile strengths of about 100% and 25% respectively for flaxpolypropylene composites, when the coupling agent was applied to the flax fibres before the composite was processed [16][17]. The chemical bonding between the anhydride-and the hydroxyl groups caused a better stress transfer from the matrix into the fibres, leading to a higher tensile strength.

The thermal properties of both LDPE and MLDPE-based composites reinforced with each kind of fiber were investigated by DSC. The thermograms (not shown) obtained for the neat matrix, as well as for all fibers composites were conventional and very similar among them. The temperature position of the endothermal peak corresponding to the melting and degree of crystallinity of all composites obtained from DSC curves are given in Table 3. For neat matrices, it was found that both the melting point and degree of crystallinity were higher for MLDPE compared to LDPE. No significant change of the melting temperature was reported when adding lignocellulosic fibers regardless the nature of the matrix. It remains roughly constant around 110-112°C and 121-124°C for LDPE-and MLDPE-based composites, respectively. The degree of crystallinity of MLDPEbased composites was roughly constant compared to the unfilled matrix. On the contrary, it seems to globally slightly increase when adding lignocellulosic fibers to the LDPE matrix. χc values were found to be higher for cotton stalk-based materials compared with other fibers. This may be due to the high amount of lignin and hemicelluloses in this fiber, which are amorphous and display strong molecular interactions with the compatibilizer.

The evolution of the mechanical properties (Young's modulus, strength and elongation at break) as a function of fiber content for both sets of composites is shown in Figures 2, 3 and4. The Young's modulus (Figure 2) of the unfilled matrix was found to be higher for MLDPE compared to LDPE as a consequence of its higher degree of crystallinity. It more or less continuously increased when adding lignocellulosic fibers, being independent on the nature of the matrix. However, it is found to remain higher for MLDPE-based composites compare to its LDPE-based counterparts because of the higher degree of crystallinity of the former. However, there is a slight increase in crystallinity and much of the increase is probably due to crosslinking and covalent adhesion of fiber to anhydridemodified matrix. The strength (Figure 3) clearly increased as the fiber content was increased in the case of MLDPE, and in contrast it tends to decrease in the case of LDPE, the value for the unfilled matrix being similar. These results show the beneficial effect of the copolymer and that using maleated PE certainly improved the tensile strength owing to better adhesion between the filler and matrix and better stress transfer from the matrix to the filler. The elongation at break (Figure 4) of the unfilled matrix is higher for LDPE than for MLDPE. This is most probably due to the lower degree of crystallinity of the former. Amorphous domains being in the rubbery state at room temperature are more able to undergo the strain. The elongation at break decreased upon fiber addition for both sets of composites regardless the nature of the fiber. The lignocellulosic fibers are responsible for the decrease of the deformation capability. The PE matrix provides ductility whereas the lignocellulosic fibers exhibit brittle behavior with a subsequent loss of toughness of the composite material. The elongation at break mainly depends on the fiber content, even though MLDPE based composites show slightly higher values at high filler content.

The fractured surfaces of the composite materials filled with 30 wt% of lignocellulosic fibers were examined using SEM. Some other compositions were examined as well (i.e., 20 and 40 wt% fibers), but the results were found to be most apparent for the 30 wt% fiber content level. For LDPE-based composites (Figures 5A i ) fiber pull-out as well as holes resulting from fiber delamination were observed regardless the nature of the fiber. The fractured surfaces of each composite revealed extensive interfacial delamination between the fiber and the matrix. It indicates that lignocellulosic fibers were hardly wetted by the LDPE matrix with a gap developing around the fibers. The fibers also stay intact after fracture. Pulled-out fibers and gaps are generally observed when the adhesion between fibers and matrix is not sufficient, and this suggests poor fiber-matrix compatibility. These observations are in agreement with the low strength values reported for these materials.

In contrast to LDPE-based composites, fibers in MLDPE-based composites were found to be uniformly coated by the matrix (Figures 5B i ). This indicates that the fibers are more compatible with the MLDPE matrix. The fibers were also severely fractured after the fracture suggesting that the fibers did get strained up to their maximum stress values. This suggests improved interfacial adhesion between the fibers and the MLDPE matrix. This must be attributed to the covalent bonds created between the fiber and the matrix and which were ensured by the maleate groups.

LDPE and MLDPE blends-based composites

In order to improve the mechanical properties of LDPE-based composites, blends of LDPE and MLDPE, at different proportions, were used as matrix for composite materials reinforced with 30 wt% of lignocellulosic fibers. The thermal properties (melting point and degree of crystallinity) of resulting composites are reported in table 4. For each type of fiber, the melting point increased when increasing the MLDPE content. Compared to pure LDPE-or MLDPE-based composites, the degree of crystallinity of the composites obtained from the blend of these two polymers was found to be significantly lower. This is probably due to the disturbance of the crystalline structure.

For each blend-based composite, the mechanical properties were estimated and the results are reported in Figure 6. The strength (Figure 6B) continuously increased as the MLDPE weight ratio in the matrix increased. This suggests improved interfacial adhesion between the fibers and the matrix which must be attributed to the covalent bonds created between the fiber and the MLDPE

Conclusion

The potential to use lignocellulosic fibers extracted from Egyptian agricultural wastes, viz. cotton stalk, rice straw, bagasse, and banana plant waste has been evaluated. It was found that cotton stalk and banana plant waste fibers exhibited the highest lignin content, in contrast with the cellulose being the lowest. A high amount of minerals was reported for banana plant waste and rice straw fibers. After grounding, fibers around 300-500 µm long and 40 µm in diameter were used to process composite materials using low density polyethylene (LDPE) or maleated LDPE (MLDPE) as matrix. The degree of crystallinity of LDPE-based composites was found to slightly increase when adding lignocellulosic fibers, mainly in the case of lignin-and hemicellulose-rich cotton stalk fibers. A significant increase of the strength was reported upon fibers addition when using MLDPE as matrix showing the beneficial effect of this copolymer as compatibilizer, as confirmed by microscopic observations;

  moieties. Except for the LDPE:MLDPE 0.75:0.25 blend-based composites, the Young's modulus also displays a continuous increase upon adding of MLDPE in the blend, whereas the elongation at break remains roughly constant. The LDPE:MLDPE 0.75:0.25 blend-based composites, low values of the Young's modulus and high values of the elongation at break were observed. These phenomena are very clear for the composites containing lignin-rich fibers, i.e. cotton stalk and banana plant waste (open diamond-shape and filled circles in Figures 6A and 6C). Lignin moieties, which are amorphous and behave like a co-compatibilizer or plasticization agent, display strong molecular interactions with MLDPE and probably chemical bonds can be easily created and firstly between them.

Figure 1 :BFigure 2 .Figure 3 .

 123 Figure 1: Adhesion mechanism for (A) LDPE-stearic acid and (B) MLDPE based composites reinforced

Figure 4 .

 4 Figure 4. Elongation at break versus fibers content for LDPE-(™) and MLDPE-(£) based composites:

Figure 5 .

 5 Figure 5. SEM micrographs of the fractured LDPE-(A i ) and MLDPE-(B i ) based composites reinforced

Figure 6 :

 6 Figure 6: Young's Modulus (A), strength (B) and elongation at break (C) vs MLDPE weight ratio for

Table 1 .

 1 Chemical composition of the agricultural residues *

	Raw material	a-cellulose	Hemicellulose	Lignin	Ash	Wax
	Bagasse	69.4	21.1	4.4	0.6	5.5
	Cotton stalk	50.6	28.4	23.1	0.5	5.1
	Banana plant waste	43.5	31.7	16.9	9.9	6.1
	Rice straw	59.1	18.4	5.3	13.7	6.3
	* As percentage of dry raw material

Table 2 :

 2 Diameter and length of the fibers

	Fiber	Length (µm)	Diameter (µm)
	Bagasse
	Cotton stalk
	Banana plant waste	380	38.5
	Rice straw

Table 3 :

 3 Melting characteristics of LDPE-and MLDPE-based composites reinforced with lignocellulosic fibers obtained from DSC curves: melting temperature (T m ), associated heat of fusion (∆H m ) and degree of crystallinity (χ c ). χ c =∆H m /w∆H m °where ∆H m °=288,83 J/g is the heat of fusion for 100% crystalline Polyethylene [18] and w is the weight fraction of polymeric matrix material in the composite.

	LDPE	MLDPE

a

Table 4 :

 4 Melting characteristics of LDPE and MLDPE blends-based composites reinforced with lignocellulosic fibers obtained from DSC curves: melting temperature (T m ), associated heat of fusion (∆H m ) and degree of crystallinity (χ c ). χ c =∆H m /w∆H m °where ∆H m °=288,83 J/g is the heat of fusion for 100% crystalline Polyethylene and w is the weight fraction of polymeric matrix material in the composite.

	Fiber	Weight Ratio	T m (°C)	∆H m (J.g -1 )	χ c	a
		LDPE : MPELD
	Bagasse			88.9	0.44
			112.5	47.6	0.24
			116.7	46.4	0.23
			123.0	60.4	0.30
			124.8	83.5	0.41
	Banana		110.8	86.3	0.42
			113.0	59.5	0.30
			115.5	47.6	0.23
			120.0	49.8	0.24
			122.4	99.6	0.49
	Cotton Stalk		111.6	90.4	0.44
			113.9	39.8	0.20
			120.1	63.5	0.31
			121.9	57.4	0.28
			123.7	90.7	0.45
	Rice Straw		112.1	76.0	0.37
			113.6	50.3	0.25
			117.7	51.3	0.25
			119.2	54.5	0.27
			123.8	88.3	0.47

a

Handbook of Polyethylene: Structures, Properties, and Applications. Peacock AJ, New York: Marcel Dekker, Inc. 2000.

Acknowledgment We acknowledge Prof. A. Maazouz (INSA-Lyon) for supplying polyethylene samples, Mrs. Bertine Khelifi (EFPG-INPG) for her help in SEM observations and the French Ministry of Foreign Affairs for the financial support (Imhotep program 12437TE).