Non-native speech perception in adverse conditions: A review <s> The next steps <s> A physical method for measuring the quality of speech-transmission channels has been developed. Essentially, the method represents an extension of the Articulation Index (AI) concept, which was developed mainly to account for distortions in the frequency domain (noise, bandpass-limiting). The underlying concept of the present approach, based on the Modulation Transfer Function (MTF) of a transmission channel, has been adapted to account for nonlinear distortions (peak clipping) as well as for distortions in the time domain (reverberation, echoes, AGC). The resulting index, the Speech-Transmission Index (STI), has been correlated with subjective intelligibility scores obtained on 167 different transmission channels with a wide variety of disturbances. The relative predictive power of the STI, expressed in PB-word score, appeared to be 5%. This accuracy is comparable with results obtained from subjective measurements when about four talkers and four listeners are used. Expressed in terms of signal-to-noise ratio, the accuracy is about 1 dB. Pilot studies have been carried out to evaluate the use of the STI for testing digital-speech transmission channels. <s> BIB001 </s> Non-native speech perception in adverse conditions: A review <s> The next steps <s> Because the field of bilingualism is still relatively new, studies in the linguistics, psycholinguistics, language development and neurolinguistics of bilingualism have often produced conflicting results. It will be argued in this paper that some of the difficulties encountered by researchers could have been lessened, if not avoided, had close attention been paid to methodological and conceptual issues. Among the issues covered are bilingual participants, language mode, stimuli, tasks as well as models of bilingual representation and processing. Each issue is dealt with in the following way: first it is explained, then the problems it causes are discussed, and, finally, tentative solutions are proposed. Examples are taken from descriptive and experimental studies of normal bilingual adults and children as well as bilinguals suffering from aphasia and dementia. <s> BIB002 </s> Non-native speech perception in adverse conditions: A review <s> The next steps <s> This article provides an empirically grounded account of what happens when more persons than one talk at once in conversation. It undertakes to specify when such occurrences are problematic for the participants, and for the organization of interaction; what the features of such overlapping talk are; and what constraints an account of overlapping talk should meet. It describes the practices employed by participants to deal with such simultaneous talk, and how they form an organization of practices which is related to the turn-taking organization previously described by Sacks et al. 1974. This “overlap resolution device” constitutes a previously unexplicated component of that turn-taking organization, and one that provides solutions to underspecified features of the previous account. <s> BIB003 </s> Non-native speech perception in adverse conditions: A review <s> The next steps <s> When listening to languages learned at a later age, speech intelligibility is generally lower than when listening to one's native language. The main purpose of this study is to quantify speech intelligibility in noise for specific populations of non-native listeners, only broadly addressing the underlying perceptual and linguistic processing. An easy method is sought to extend these quantitative findings to other listener populations. Dutch subjects listening to Germans and English speech, ranging from reasonable to excellent proficiency in these languages, were found to require a 1-7 dB better speech-to-noise ratio to obtain 50% sentence intelligibility than native listeners. Also, the psychometric function for sentence recognition in noise was found to be shallower for non-native than for native listeners (worst-case slope around the 50% point of 7.5%/dB, compared to 12.6%/dB for native listeners). Differences between native and non-native speech intelligibility are largely predicted by linguistic entropy estimates as derived from a letter guessing task. Less effective use of context effects (especially semantic redundancy) explains the reduced speech intelligibility for non-native listeners. While measuring speech intelligibility for many different populations of listeners (languages, linguistic experience) may be prohibitively time consuming, obtaining predictions of non-native intelligibility from linguistic entropy may help to extend the results of this study to other listener populations. <s> BIB004 </s> Non-native speech perception in adverse conditions: A review <s> The next steps <s> Previous work has established that naturally produced clear speech is more intelligible than conversational speech for adult hearing-impaired listeners and normal-hearing listeners under degraded listening conditions. The major goal of the present study was to investigate the extent to which naturally produced clear speech is an effective intelligibility enhancement strategy for non-native listeners. Thirty-two non-native and 32 native listeners were presented with naturally produced English sentences. Factors that varied were speaking style (conversational versus clear), signal-to-noise ratio (-4 versus -8 dB) and talker (one male versus one female). Results showed that while native listeners derived a substantial benefit from naturally produced clear speech (an improvement of about 16 rau units on a keyword-correct count), non-native listeners exhibited only a small clear speech effect (an improvement of only 5 rau units). This relatively small clear speech effect for non-native listeners is interpreted... <s> BIB005 </s> Non-native speech perception in adverse conditions: A review <s> The next steps <s> Recent evidence on the perceptual performance of bilingual listeners suggests that a nonaudibility-based cost exists in processing a second language. That is, when compared to monolingual English speakers and early bilinguals, listeners who acquired English as a second language after puberty show reduced performance when listening to the second language in background noise, despite normal auditory thresholds. However, past studies have not controlled for the homogeneity of the bilingual participants used in auditory research; therefore, it is unknown whether the deficit observed in bilingual function is due to a lack of control for language-related variables. The ability of a homogeneous group of Spanish-English bilinguals and English monolingual listeners to perceive sentences in quiet and in noise at threshold levels was evaluated using the Hearing in Noise Test (HINT). Both groups performed similarly in quiet conditions. In the noise conditions, the bilingual group's performance was significantly poorer than the monolingual group's performance. However, both groups showed a binaural advantage of 7-8 dB when the signal and noise were separated by 90 degrees. <s> BIB006 </s> Non-native speech perception in adverse conditions: A review <s> The next steps <s> The similarities and contrasts between native and non-native identification of multi-speaker Mandarin tones in quiet and in noise were explored in a perception experiment. Mandarin tone materials produced by three male and three female speakers were presented with five levels of signal-to-noise ratios (quiet, 0, -5, -10, and -15dB) in two presentation formats (blocked by speaker and mixed across speakers) to listeners with various Mandarin experience (native, first-year, second-year, third-year, and fourth-year students). Stimuli blocked by speaker yielded higher accuracy and shorter reaction time. The additional demand of processing mixed-speaker stimuli, however, did not compromise non-native performance more than native performance. Noise expectedly compromised identification performance, although it did not compromise non-native identification more than native identification. Native listeners expectedly outperformed non-native listeners, although identification performance did not vary systematically as a function of duration of Mandarin experience. It is speculated that sources of variability in speech would affect non-native more than native tone identification only if syllable-internal, canonical F0 information is removed or altered. <s> BIB007 </s> Non-native speech perception in adverse conditions: A review <s> The next steps <s> This study investigated the relative contribution of auditory and visual information to speech perception by looking at the effect of visual and auditory degradation on the weighting given to visual cues for native and non-native speakers. Multiple iterations of /ba/, /da/ and /ga/ by five Australian English and five Mandarin Chinese speakers were presented to Australian English, British English and Mandarin Chinese participants. Tokens were presented in auditory, visual and congruent/incongruent audiovisual (AV) modes, either in clear or with visual degradation (blurring), auditory degradation (noise) or combined degradations. In the AV clear condition, English-speaking participants showed greater visual weighting for non-native speakers, but this was not found for Chinese participants. In 'single-channel degradation' conditions, the weighting of the intact channel increased significantly, with little influence of speaker language. There was no strong evidence of native-language effects on the weighting of visual cues. The degree of visual weighting varied widely across individual participants, and was also affected by individual speaker characteristics. The weighting of auditory and visual cues is therefore highly flexible and dependent on the information load of each channel; non-native speaker and language-background effects may influence visual weighting but individual perceiver and speaker strategies also have a strong impact. <s> BIB008 </s> Non-native speech perception in adverse conditions: A review <s> The next steps <s> The goal of the present study was to devise a means of representing languages in a perceptual similarity space based on their overall phonetic similarity. In Experiment 1, native English listeners performed a free classification task in which they grouped 17 diverse languages based on their perceived phonetic similarity. A similarity matrix of the grouping patterns was then submitted to clustering and multidimensional scaling analyses. In Experiment 2, an independent group of native English listeners sorted the group of 17 languages in terms of their distance from English. Experiment 3 repeated Experiment 2 with four groups of non-native English listeners: Dutch, Mandarin, Turkish and Korean listeners. Taken together, the results of these three experiments represent a step towards establishing an approach to assess the overall phonetic similarity of languages. This approach could potentially provide the basis for developing predictions regarding foreign-accented speech intelligibility for various listener groups, and regarding speech perception accuracy in the context of background noise in various languages. <s> BIB009
This review examined the impact of listener, task and noise-related factors on speech perception by non-native listeners in adverse conditions. We conclude by noting some of the limitations of existing studies and suggest research themes which may become important in the near future. One key issue is realism. As noted at the outset, much of our experience as non-native communicators comes from everyday situations rather than formal teaching settings, yet most of what we know about the effect of adverse conditions has been gleaned from controlled laboratory conditions and narrow tasks. For instance, only one study to date has combined additive noise and reverberation even though that is by far the most common adverse condition. Noise also affects speech production  , so it will be important to explore how non-native listeners cope with speech modifications induced by noise. Little work has been carried out using conversational tasks, so we do not know how noise affects the ability of non-native listeners to process and generate high-level interactional cues (e.g. BIB003 and the extent to which non-native listeners are capable of aligning in conversations (e.g. Pickering and Garrod, 2004). One step towards more realistic tasks has been taken by BIB007 , who consider the case of processing multiple talkers rather than a fixed talker, demonstrating that changes of talker do affect native and non-native listeners equally. Another factor from which non-native listeners appear to derive equivalent benefit is the release from masking due to spatial separation of target and masker, irrespective of whether the masker was energetic or informational in nature  . The potential differential use of multimodal information is a further realworld aspect which is being tackled in non-native speech perception studies (e.g. BIB008 . The lack of unified criteria when selecting and describing NN populations makes some of the results less robust or reliable than they appear to be, as pointed out by BIB004 , and renders comparisons between studies tentative (see BIB002 von Hapsburg and Peña, 2002; BIB006   . Some of the variables and results that apply to L2 populations cannot be extrapolated to FL listeners because of their smaller degree of contact with the TL BIB005 , which makes it crucial to distinguish the two types of population. A related issue concerns the distance between languages, which is predicted to affect how different NN populations perform in speech tasks. BIB009 make a start at deriving a phonetic similarity space for languages. While models of non-native sound perception exist, few cater for adverse conditions. An exception is that of van Wijngaarden et al., (2004) , which adapts the speech transmission index BIB001 to nonnative listeners in additive noise, reverberation and bandlimiting using a single parameter modification to the L1 psychometric function. While the non-native index provides a global indication of intelligibility, we currently lack explanatory models which produce more detailed predictions of, for example, specific sound confusions suffered by non-native listeners in noise. A feature of all studies involving non-native listeners is the large degree of between-listener variation in identification scores, with the most competent listeners often achieving native levels of performance. Further work is needed to study the relationship between individual performance in L1 and L2 tasks. A related issue concerns non-native listener baselines. Few studies have measured the performance of monolingual non-native listeners in low-level tasks involving L2 sound perception in noise. Also, the role of formal training in noise is currently unknown, but it is certainly worth asking whether exposure in adverse conditions can be beneficial, (for example, by highlighting robust cues to L2 distinctions). Finally, although research focussing on the recognition of spoken words has addressed non-native listening in ever growing detail in recent years (see Section 3.4), and has also used noise as an important tool in critically distinguishing between theories (Section 4.4), the intersection of these topics (that is to say, lexical processing in adverse conditions by non-native listeners) is as yet untrodden research terrain.