Non-native speech perception in adverse conditions: A review <s> Additive noise: energetic masking <s> It is obvious that the intelligibility of a given speech message depends strongly on how the message is spoken. Not only does intelligibility vary with the identity of the speaker, but also, for a given speaker, with the effort to speak clearly. Assuming that the variations in intelligibility associated with such factors are substantial, detailed knowledge of these factors could prove useful in the development of improved signal‐processing schemes for hearing aids. Our initial goal is to estimate the magnitude of the intelligibility changes associated with a given speaker's attempt to speak clearly. Among the variables to be considered are presentation level of material, frequency‐gain characteristic of amplification system, severity of hearing loss, identity of speaker, and degree of familiarity with speaker's voice. Preliminary results obtained using sentence material for a number of hearing‐impaired listeners will be presented. [Work supported by NIH.] <s> BIB001 </s> Non-native speech perception in adverse conditions: A review <s> Additive noise: energetic masking <s> The speech-reception threshold (SRT) for sentences presented in a fluctuating interfering background sound of 80 dBA SPL is measured for 20 normal-hearing listeners and 20 listeners with sensorineural hearing impairment. The interfering sounds range from steady-state noise, via modulated noise, to a single competing voice. Two voices are used, one male and one female, and the spectrum of the masker is shaped according to these voices. For both voices, the SRT is measured as well in noise spectrally shaped according to the target voice as shaped according to the other voice. The results show that, for normal-hearing listeners, the SRT for sentences in modulated noise is 4-6 dB lower than for steady-state noise; for sentences masked by a competing voice, this difference is 6-8 dB. For listeners with moderate sensorineural hearing loss, elevated thresholds are obtained without an appreciable effect of masker fluctuations. The implications of these results for estimating a hearing handicap in everyday conditions are discussed. By using the articulation index (AI), it is shown that hearing-impaired individuals perform poorer than suggested by the loss of audibility for some parts of the speech signal. Finally, three mechanisms are discussed that contribute to the absence of unmasking by masker fluctuations in hearing-impaired listeners. The low sensation level at which the impaired listeners receive the masker seems a major determinant. The second and third factors are: reduced temporal resolution and a reduction in comodulation masking release, respectively. <s> BIB002 </s> Non-native speech perception in adverse conditions: A review <s> Additive noise: energetic masking <s> Do listeners process noisy speech by taking advantage of "glimpses"-spectrotemporal regions in which the target signal is least affected by the background? This study used an automatic speech recognition system, adapted for use with partially specified inputs, to identify consonants in noise. Twelve masking conditions were chosen to create a range of glimpse sizes. Several different glimpsing models were employed, differing in the local signal-to-noise ratio (SNR) used for detection, the minimum glimpse size, and the use of information in the masked regions. Recognition results were compared with behavioral data. A quantitative analysis demonstrated that the proportion of the time-frequency plane glimpsed is a good predictor of intelligibility. Recognition scores in each noise condition confirmed that sufficient information exists in glimpses to support consonant identification. Close fits to listeners' performance were obtained at two local SNR thresholds: one at around 8 dB and another in the range -5 to -2 dB. A transmitted information analysis revealed that cues to voicing are degraded more in the model than in human auditory processing. <s> BIB003 </s> Non-native speech perception in adverse conditions: A review <s> Additive noise: energetic masking <s> Consonant identification rates were measured for vowel-consonant-vowel tokens gated with N-talker babble noise and babble-modulated noise for an extensive range of N, at a fixed signal-to-noise ratio. In the natural babble condition, intelligibility was a nonmonotonic function of N, with a broad performance minimum from N=6 to N=128. Identification rates in babble-modulated noise fell gradually with N. The contributions of factors such as energetic masking, linguistic confusion, attentional load, peripheral adaptation, and stationarity to the perception of consonants in N-talker babble are discussed. <s> BIB004
Most environments contain energy intruding from other sound sources at a range of signal-to-noise ratios (SNRs) and varying in attentional capture capacity from sources with a slowly-varying spectrum to competing speech. Sound source distance is often a critical factor too: even relatively quiet noise sources close to the listener can make a more distant target source more difficult to comprehend. In the following we adopt a wide definition of noise to mean any signal other than the target. Fig. 1 illustrates predicted masking effects of the nonsense sentence His travels show in our fear from the Picheny corpus BIB001 by speech-shaped noise, 6-talker babble and competing speech. In each case, the noise signal was added to the target speech to produce a global SNR of Fig. 1 . Upper panel: Conventional spectrogram of the utterance "his travels show in our fear" spoken by a male American English speaker. The remaining panels depict auditory spectrograms of this utterance mixed with speech-shaped noise, 6-talker babble and competing female speech, in each case at an SNR of 0 dB. The red patches highlight those time-frequency regions where the male speech contains more energy than the masker. The frequency range for the auditory spectrograms is linear in ERB-rate: the middle of the range corresponds to about 1000 Hz. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 0 dB, which is typically near the middle of the range of SNRs employed in non-native studies. The regions picked out in red indicate those portions of the time-frequency plane where the target speech has more energy than the masker. These regions are derived from a "glimpsing" account of speech perception in noise based on the idea that the auditory system makes use of only those time-frequency regions where the target source is locally dominant BIB003 . Since two sound sources are unlikely to possess similar amounts of energy at any given time-frequency location, the speech signal is either clearly 'visible' or masked in nearly all spectro-temporal locations: the proportion of ambiguous locations is very small. Regions which survive masking are shown superimposed upon auditory spectrograms since this is the domain in which the masking model operates. For reference, a conventional spectrogram of the clean utterance is shown in the top panel of Fig. 1 . Fig. 1 depicts energetic masking (EM), a form of masking caused by the interaction of speech and masker at the level of the auditory periphery. Energetic masking affects speech perception by rendering unavailable potential cues to the identity of segments and their boundaries as well as interfering with access to prosodic cues (although complete energetic masking of prosody is uncommon due to the longer-term nature of suprasegmental information). Sounds such as the dental fricatives which are inherently low in energy are most susceptible to EM. In addition, the spectral profile of the masker also dictates the likelihood of cues surviving in different frequency regions. White, pink and speech-shaped noise have a different masking effects which influence segmental confusions in speech perception. Fig. 1 illustrates several features of masking by additive noise. The overall quantity of information available about the target speech signal is not determined solely by SNR but depends on the properties of the masker. Speechshaped noise obscures significantly more information than a competing talker, with babble noise producing an intermediate degree of masking. While long runs of all three maskers would have the same long-term spectrum equal to the average spectrum of speech, they differ both in degree of temporal modulation and preservation of spectral detail. The spectro-temporal variation in masker energy is greatest for the competing speech source and least for speech-shaped noise. In essence, for stationary maskers like speech-shaped noise, glimpses are determined by the modulations of the target speech, while for nonstationary maskers it is the interaction of target and masker modulations which define those parts of the target which lies above the noise. Thus, for speech-shaped noise, only those parts of the speech signal which are intrinsically energetic (typically resolved harmonics, formants and strong frication) are likely to survive at SNRs below 0 dB, while for a modulated masker, weaker target signal components may well be audible if they coincide with a spectro-temporal gap in the masker. In the example utterance, cues such as the /t/ burst in travels and the weak fricative in fear are virtually obliterated by the stationary masker yet retain some presence for the other maskers. Temporal modulations in the competing speech masker produce epochs when information from across much of the spectrum is visible. Indeed, at 0 dB SNR, most words from each of a pair of talkers are intelligible, demonstrating that less than half of the time-frequency plane is perfectly adequate for speech perception for listeners with normal hearing. Masker modulation depth declines as increasing numbers of talkers are added to create babble until, in the limit, a stationary signal is reached. For six talkers, significant modulations remain, giving rise to a greater number of glimpses than in the stationary speech-shaped noise case. Studies of the effect of masker modulation on speech perception  BIB002 BIB004 confirm that, when the global SNR is held constant, highly-modulated maskers such as speech or speech-modulated noise are significantly less effective than maskers with smaller modulation depth such as multi-talker babble or speech-shaped noise: typically, 6-8 dB of additional modulated noise is required to reduce sentence intelligibility to the level seen in the stationary condition. Spectral variations are also larger in the competing speech case than for speech-shaped noise since the latter represents an average spectrum while, for most speech sounds, at any instant a speech masker is likely to contain energy concentrated in certain spectral regions joined by spectral valleys of lower energy. These valleys provide an opportunity for glimpses of the target signal.