Contributions of cochlea-scaled entropy and consonant-vowel boundaries to prediction of speech intelligibility in noise

Recent evidence suggests that spectral change, as measured by cochlea-scaled entropy (CSE), predicts speech intelligibility better than the information carried by vowels or consonants in sentences. Motivated by this finding, the present study investigates whether intelligibility indices implemented to include segments marked with significant spectral change better predict speech intelligibility in noise than measures that include all phonetic segments paying no attention to vowels/consonants or spectral change. The prediction of two intelligibility measures [normalized covariance measure (NCM), coherence-based speech intelligibility index (CSII)] is investigated using three sentence-segmentation methods: relative root-mean-square (RMS) levels, CSE, and traditional phonetic segmentation of obstruents and sonorants. While the CSE method makes no distinction between spectral changes occurring within vowels/consonants, the RMS-level segmentation method places more emphasis on the vowel-consonant boundaries wherein the spectral change is often most prominent, and perhaps most robust, in the presence of noise. Higher correlation with intelligibility scores was obtained when including sentence segments containing a large number of consonant-vowel boundaries than when including segments with highest entropy or segments based on obstruent/sonorant classification. These data suggest that in the context of intelligibility measures the type of spectral change captured by the measure is important.     

Application of spectral subtraction method on enhancement of electrolarynx speech

Although electrolarynx (EL) serves as an important method of phonation for the laryngectomees, the resulting speech is of poor intelligibility due to the presence of a steady background noise caused by the instrument, even worse in the case of additive noise. This paper investigates the problem of EL speech enhancement by taking into account the frequency-domain masking properties of the human auditory system. One approach is incorporating an auditory masking threshold (AMT) for parametric adaptation in a subtractive-type enhancement process. The other is the supplementary AMT (SAMT) algorithm, which applies a cross-correlation spectral subtraction (CCSS) approach as a post-processing scheme to enhancing EL speech dealt with the AMT method. The performance of these two algorithms was evaluated as compared to the power spectral subtraction (PSS) algorithm. The best performance of EL speech enhancement was associated with the SAMT algorithm, followed by the AMT algorithm and the PSS algorithm. Acoustic and perceptual analyses indicated that the AMT and SAMT algorithms achieved the better performances of noise reduction and the enhanced EL speech was more pleasant to human listeners as compared to the PSS algorithm.     

Perception of sinewave vowels

There is a significant body of research examining the intelligibility of sinusoidal replicas of natural speech. Discussion has followed about what the sinewave speech phenomenon might imply about the mechanisms underlying phonetic recognition. However, most of this work has been conducted using sentence material, making it unclear what the contributions are of listeners' use of linguistic constraints versus lower level phonetic mechanisms. This study was designed to measure vowel intelligibility using sinusoidal replicas of naturally spoken vowels. The sinusoidal signals were modeled after 300 /hVd/ syllables spoken by men, women, and children. Students enrolled in an introductory phonetics course served as listeners. Recognition rates for the sinusoidal vowels averaged 55%, which is much lower than the ～95% intelligibility of the original signals. Attempts to improve performance using three different training methods met with modest success, with post-training recognition rates rising by ～5-11 percentage points. Follow-up work showed that more extensive training produced further improvements, with performance leveling off at ～73%-74%. Finally, modeling work showed that a fairly simple pattern-matching algorithm trained on naturally spoken vowels classified sinewave vowels with 78.3% accuracy, showing that the sinewave speech phenomenon does not necessarily rule out template matching as a mechanism underlying phonetic recognition.     

A study on the optimal English speech level for Chinese listeners in classrooms

Speech intelligibility in classrooms affects the learning efficiency of students directly, especially for the students who are using a second language. The speech intelligibility value is determined by many factors such as speech level, signal to noise ratio, and reverberation time in the rooms. This paper investigates the contributions of these factors with subjective tests, especially speech level, which is required for designing the optimal gain for sound amplification systems in classrooms. The test material was generated by mixing the convolution output of the English Coordinate Response Measure corpus and the room impulse responses with the background noise. The subjects are all Chinese students who use English as a second language. It is found that the speech intelligibility increases first and then decreases with the increase of speech level, and the optimal English speech level is about 71 dBA in classrooms for Chinese listeners when the signal to noise ratio and the reverberation time keep constant. Finally, a regression equation is proposed to predict the speech intelligibility based on speech level, signal to noise ratio, and reverberation time.     

Effects of frequency response characteristics on speech discrimination and perceived intelligibility and pleasantness of speech for hearing-impaired listeners

Frequency response characteristics were selected for 14 hearing-impaired ears, according to six procedures. Three procedures were based on MCL measurements with speech bands of three bandwidths (1/3 octave, 1 octave, and 1 2/3 octaves). The other procedures were based on hearing thresholds, pure-tone MCLs, and pure-tone LDLs. The procedures were evaluated by speech discrimination testing, using nonsense syllables in noise, and by paired comparison judgments of the intelligibility and pleasantness of running speech. Speech discrimination testing showed significant differences between pairs of responses for only seven test ears. Nasals and glides were most affected by frequency response variations. Both intelligibility and pleasantness judgments showed significant differences for all test ears. Intelligibility in noise was less affected by frequency response differences than was intelligibility in quiet or pleasantness in quiet or in noise. For some ears, the ranking of responses depended on whether intelligibility or pleasantness was being judged and on whether the speech was in quiet or in noise. Overall, the three speech band MCL procedures were far superior to the others. Thus the studies strongly support the frequency response selection rationale of amplifying all frequency bands of speech to MCL. They also highlight some of the complications involved in achieving this aim.     

Speech intelligibility of normal listeners and persons with impaired hearing in traffic noise

Speech intelligibility (PB words) in traffic-like noise was investigated in a laboratory situation simulating three common listening situations, indoors at 1 and 4 m and outdoors at 1 m. The maximum noise levels still permitting 75% intelligibility of PB words in these three listening situations were also defined. A total of 269 persons were examined. Forty-six had normal hearing, 90 a presbycusis-type hearing loss, 95 a noise-induced hearing loss and 38 a conductive hearing loss. In the indoor situation the majority of the groups with impaired hearing retained good speech intelligibility in 40 dB(A) masking noise. Lowering the noise level to less than 40 dB(A) resulted in a minor, usually insignificant, improvement in speech intelligibility. Listeners with normal hearing maintained good speech intelligibility in the outdoor listening situation at noise levels up to 60 dB(A), without lip-reading (i.e., using non-auditory information). For groups with impaired hearing due to age and/or noise, representing 8% of the population in Sweden, the noise level outdoors had to be lowered to less than 50 dB(A), in order to achieve good speech intelligibility at 1 m without lip-reading.     

Speech signal modification to increase intelligibility in noisy environments

The role of transient speech components on speech intelligibility was investigated. Speech was decomposed into two components--quasi-steady-state (QSS) and transient--using a set of time-varying filters whose center frequencies and bandwidths were controlled to identify the strongest formant components in speech. The relative energy and intelligibility of the QSS and transient components were compared to original speech. Most of the speech energy was in the QSS component, but this component had low intelligibility. The transient component had much lower energy but was almost as intelligible as the original speech, suggesting that the transient component included speech elements important to speech perception. A modified version of speech was produced by amplifying the transient component and recombining it with the original speech. The intelligibility of the modified speech in background noise was compared to that of the original speech, using a psychoacoustic procedure based on the modified rhyme protocol. Word recognition rates for the modified speech were significantly higher at low signal-to-noise ratios (SNRs), with minimal effect on intelligibility at higher SNRs. These results suggest that amplification of transient information may improve the intelligibility of speech in noise and that this improvement is more effective in severe noise conditions.     

Recognition of accented English in quiet and noise by younger and older listeners

This study investigated the effects of age and hearing loss on perception of accented speech presented in quiet and noise. The relative importance of alterations in phonetic segments vs. temporal patterns in a carrier phrase with accented speech also was examined. English sentences recorded by a native English speaker and a native Spanish speaker, together with hybrid sentences that varied the native language of the speaker of the carrier phrase and the final target word of the sentence were presented to younger and older listeners with normal hearing and older listeners with hearing loss in quiet and noise. Effects of age and hearing loss were observed in both listening environments, but varied with speaker accent. All groups exhibited lower recognition performance for the final target word spoken by the accented speaker compared to that spoken by the native speaker, indicating that alterations in segmental cues due to accent play a prominent role in intelligibility. Effects of the carrier phrase were minimal. The findings indicate that recognition of accented speech, especially in noise, is a particularly challenging communication task for older people.     

Quantifying the intelligibility of speech in noise for non-native listeners

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.     

Reverberation time and maximum background-noise level for classrooms from a comparative study of speech intelligibility metrics

Speech intelligibility metrics that take into account sound reflections in the room and the background noise have been compared, assuming diffuse sound field. Under this assumption, sound decays exponentially with a decay constant inversely proportional to reverberation time. Analytical formulas were obtained for each speech intelligibility metric providing a common basis for comparison. These formulas were applied to three sizes of rectangular classrooms. The sound source was the human voice without amplification, and background noise was taken into account by a noise-to-signal ratio. Correlations between the metrics and speech intelligibility are presented and applied to the classrooms under study. Relationships between some speech intelligibility metrics were also established. For each noise-to-signal ratio, the value of each speech intelligibility metric is maximized for a specific reverberation time. For quiet classrooms, the reverberation time that maximizes these speech intelligibility metrics is between 0.1 and 0.3 s. Speech intelligibility of 100% is possible with reverberation times up to 0.4-0.5 s and this is the recommended range. The study suggests "ideal" and "acceptable" maximum background-noise level for classrooms of 25 and 20 dB, respectively, below the voice level at 1 m in front of the talker.     

一种抑制方向性噪声的双耳语音增强算法

为了给双耳听力设备佩戴者带来更好的语音可懂度,提出了一种利用双耳时间差与声级差的近场语音增强算法,该方法首先利用这两种差异来估计语音的功率谱和语音的相干函数,然后计算干扰噪声在左右耳间的头相关传输函数的比值,最后构造两个维纳滤波器。客观评价的参数显示该算法去噪效果优于对比算法而目标语音的时间差误差和声级差误差低于对比算法。主观的言语接受阈测试表明该方法能有效提高语音可懂度。结果表明,该算法在能够有效去除干扰噪声的同时,保留了目标语音的空间信息。      

Evaluating airborne sound insulation in terms of speech intelligibility

This paper reports on an evaluation of ratings of the sound insulation of simulated walls in terms of the intelligibility of speech transmitted through the walls. Subjects listened to speech modified to simulate transmission through 20 different walls with a wide range of sound insulation ratings, with constant ambient noise. The subjects' mean speech intelligibility scores were compared with various physical measures to test the success of the measures as sound insulation ratings. The standard Sound Transmission Class (STC) and Weighted Sound Reduction Index ratings were only moderately successful predictors of intelligibility scores, and eliminating the 8 dB rule from STC led to very modest improvements. Various previously established speech intelligibility measures (e.g., Articulation Index or Speech Intelligibility Index) and measures derived from them, such as the Articulation Class, were all relatively strongly related to speech intelligibility scores. In general, measures that involved arithmetic averages or summations of decibel values over frequency bands important for speech were most strongly related to intelligibility scores. The two most accurate predictors of the intelligibility of transmitted speech were an arithmetic average transmission loss over the frequencies from 200 to 2.5 kHz and the addition of a new spectrum weighting term to R(w) that included frequencies from 400 to 2.5 kHz.     

Traffic noise annoyance and speech intelligibility in persons with normal and persons with impaired hearing

Annoyance ratings in speech intelligibility tests at 45 dB(A) and 55 dB(A) traffic noise were investigated in a laboratory study. Subjects were chosen according to their hearing acuity to be representative of 70-year-old men and women, and of noise-induced hearing losses typical for a great number of industrial workers. These groups were compared with normal hearing subjects of the same sex and, when possible, the same age. The subjects rated their annoyance on an open 100 mm scale. Significant correlations were found between annoyance expressed in millimetres and speech intelligibility in percent when all subjects were taken as one sample. Speech intelligibility was also calculated from physical measurements of speech and noise by using the articulation index method. Observed and calculated speech intelligibility scores are compared and discussed. Also treated is the estimation of annoyance by traffic noise at moderate noise levels via speech intelligibility scores.     

Enhancing intelligibility of narrowband speech with out-of-band noise: evidence for lateral suppression at high-normal intensity

Previous studies have shown that the intelligibility of filtered speech can be enhanced by filling stopbands with noise. The present study found that this enhancement occurred only when speech intensity was sufficiently high to degrade performance. Intelligibility decreased by about 15% when narrowband speech was increased from 45 to 65 dBA (corresponding to broadband speech levels of about 60 and 80 dBA), and decreased by 20% at a level of 75 dBA. However, when flanking bands of low-pass and high-pass filtered white noise were added at spectrum levels of -40 to -20 dB relative to the speech, intelligibility of the 75-dBA speech band increased by about 13%. Additional findings confirm that this enhancement of intelligibility depends upon out-of-band stimulation, in agreement with theories proposing that lateral suppressive interactions extend the dynamic range of intensity coding by counteracting effects of auditory-nerve firing-rate saturation at high signal levels.     

A comparative intelligibility study of single-microphone noise reduction algorithms

The evaluation of intelligibility of noise reduction algorithms is reported. IEEE sentences and consonants were corrupted by four types of noise including babble, car, street and train at two signal-to-noise ratio levels (0 and 5 dB), and then processed by eight speech enhancement methods encompassing four classes of algorithms: spectral subtractive, sub-space, statistical model based and Wiener-type algorithms. The enhanced speech was presented to normal-hearing listeners for identification. With the exception of a single noise condition, no algorithm produced significant improvements in speech intelligibility. Information transmission analysis of the consonant confusion matrices indicated that no algorithm improved significantly the place feature score, significantly, which is critically important for speech recognition. The algorithms which were found in previous studies to perform the best in terms of overall quality, were not the same algorithms that performed the best in terms of speech intelligibility. The subspace algorithm, for instance, was previously found to perform the worst in terms of overall quality, but performed well in the present study in terms of preserving speech intelligibility. Overall, the analysis of consonant confusion matrices suggests that in order for noise reduction algorithms to improve speech intelligibility, they need to improve the place and manner feature scores.     

声学头模对语言清晰度测量的影响

借助声学头模考察了水平面不同语声源和噪声源位置对语言清晰度测量的影响,比较了有声学头模的双耳STIPA与无声学头模常规STIPA测量结果的差异,分别采用录听和现场测听方式进行了同等条件下的汉语听感清晰度主观评价实验,并分析了清晰度主客观结果的相关性。结果表明:声源位置对有声学头模的STIPA以及头模录制信号和真人现场实测的听感清晰度影响显著。无声学头模的STIPA更接近有声学头模时左右耳中较差的劣势耳的STIPA结果。单侧耳与语声源同侧或与噪声源异侧对应的单侧耳听感清晰度更高,语声源和噪声源重叠对应的双耳听感清晰度最低,声源分离可以显著提高双耳听感清晰度。头模录制信号和真人现场实测的听感清晰度与无声学头模STIPA不相关,与有声学头模的STIPA高度相关,其中单侧耳听感清晰度与该单侧耳STIPA高度相关,双耳听感清晰度与左右耳STIPA的较高值相关性最高。      

The effect of timing errors on the intelligibility of deaf children's speech.

The purpose of this study was to quantify the effect of timing errors on the intelligibility of deaf children's speech. Deviant timing patterns were corrected in the recorded speech samples of six deaf children using digital speech processing techniques. The speech waveform was modified to correct timing errors only, leaving all other aspects of the speech unchanged. The following six-stage approximation procedure was used to correct the deviant timing patterns: (1) original, unaltered utterances, (2) correction of pauses only, (3) correction of relative timing, (4) correction of absolute syllable duration, (5) correction of relative timing and pauses, and (6) correction of absolute syllable duration and pauses. Measures of speech intelligibility were obtained for the original and the computer-modified utterances. On the average, the highest intelligibility score was obtained when relative timing errors only were corrected. The correction of this type of error improved the intelligibility of both stressed and unstressed words within a phrase. Improvements in word intelligibility, which occurred when relative timing was corrected, appeared to be closely related to the number of phonemic errors present within a word. The second highest intelligibility score was obtained for the original, unaltered sentences. On the average, the intelligibility scores obtained for the other four forms of timing modification were poorer than those obtained for the original sentences. Thus, the data show that intelligibility improved, on the average, when only one type of error, relative timing, was corrected.     

The influence of dummy head on measuring speech intelligibility

In order to investigate the influence of dummy head on measuring speech intelligibility,the objective and subjective speech intelligibility evaluation experiments were respectively carried out for different spatial configurations of a target source and a noise source in the horizontal plane.The differences between standard STIPA measured without a dummy head and binaural STIPA measured with a dummy head were compared and the correlation of subjective speech intelligibility and objective STIPA was analyzed.It is showed that the position of sound source affects significantly on binaural STIPA and subjective intelligibility measured by a dummy head or measured in a real-life scenario.The standard STIPA is closer to the lower value of the two binaural STIPA values.The speech intelligibility is higher for a single ear which is on the same side with the target source or on the other side of the noise source.Binaural speech intelligibility is always the lowest when both target and noise sources are at the same place but once apart the speech intelligibility will increase sharply.It is also found that the subjective intelligibility measured by a dummy head or measured in a real-life scenario is uncorrelated with standard STIPA,but correlated highly with STIPA measured with a dummy head.The subjective intelligibility of one single ear is correlated highly with STIPA measured at the same ear,and the binaural speech intelligibility is in well agreement with the higher value of the two binaural STIPA values.     

Effects of noise and distortion on speech quality judgments in normal-hearing and hearing-impaired listeners

Noise and distortion reduce speech intelligibility and quality in audio devices such as hearing aids. This study investigates the perception and prediction of sound quality by both normal-hearing and hearing-impaired subjects for conditions of noise and distortion related to those found in hearing aids. Stimuli were sentences subjected to three kinds of distortion (additive noise, peak clipping, and center clipping), with eight levels of degradation for each distortion type. The subjects performed paired comparisons for all possible pairs of 24 conditions. A one-dimensional coherence-based metric was used to analyze the quality judgments. This metric was an extension of a speech intelligibility metric presented in Kates and Arehart (2005) [J. Acoust. Soc. Am. 117, 2224-2237] and is based on dividing the speech signal into three amplitude regions, computing the coherence for each region, and then combining the three coherence values across frequency in a calculation based on the speech intelligibility index. The one-dimensional metric accurately predicted the quality judgments of normal-hearing listeners and listeners with mild-to-moderate hearing loss, although some systematic errors were present. A multidimensional analysis indicates that several dimensions are needed to describe the factors used by subjects to judge the effects of the three distortion types.     

Influences of auditory object formation on phonemic restoration

In phonemic restoration, intelligibility of interrupted speech is enhanced when noise fills the speech gaps. When the broadband envelope of missing speech amplitude modulates the intervening noise, intelligibility is even better. However, this phenomenon represents a perceptual failure: The amplitude modulation, a noise feature, is misattributed to the speech. Experiments explored whether object formation influences how information in the speech gaps is perceptually allocated. Experiment 1 replicates the finding that intelligibility is enhanced when speech-modulated noise rather than unmodulated noise is presented in the gaps. In Experiment 2, interrupted speech was presented diotically, but intervening noises were presented either diotically or with an interaural time difference leading in the right ear, causing the noises to be perceived to the side of the listener. When speech-modulated noise and speech are perceived from different directions, intelligibility is no longer enhanced by the modulation. However, perceived location has no effect for unmodulated noise, which contains no speech-derived information. Results suggest that enhancing object formation reduces misallocation of acoustic features across objects, and demonstrate that our ability to understand noisy speech depends on a cascade of interacting processes, including glimpsing sensory inputs, grouping sensory inputs into objects, and resolving ambiguity through top-down knowledge.