Perception of clear fricatives by normal-hearing and simulated hearing-impaired listeners

Speakers may adapt the phonetic details of their productions when they anticipate perceptual difficulty or comprehension failure on the part of a listener. Previous research suggests that a speaking style known as clear speech is more intelligible overall than casual, conversational speech for a variety of listener populations. However, it is unknown whether clear speech improves the intelligibility of fricative consonants specifically, or how its effects on fricative perception might differ depending on listener population. The primary goal of this study was to determine whether clear speech enhances fricative intelligibility for normal-hearing listeners and listeners with simulated impairment. Two experiments measured babble signal-to-noise ratio thresholds for fricative minimal pair distinctions for 14 normal-hearing listeners and 14 listeners with simulated sloping, recruiting impairment. Results indicated that clear speech helped both groups overall. However, for impaired listeners, reliable clear speech intelligibility advantages were not found for non-sibilant pairs. Correlation analyses comparing acoustic and perceptual data indicated that a shift of energy concentration toward higher frequency regions and greater source strength contributed to the clear speech effect for normal-hearing listeners. Correlations between acoustic and perceptual data were less consistent for listeners with simulated impairment, and suggested that lower-frequency information may play a role.     

The effects of the addition of low-level, low-noise noise on the intelligibility of sentences processed to remove temporal envelope information

The intelligibility of sentences processed to remove temporal envelope information, as far as possible, was assessed. Sentences were filtered into N analysis channels, and each channel signal was divided by its Hilbert envelope to remove envelope information but leave temporal fine structure (TFS) intact. Channel signals were combined to give TFS speech. The effect of adding low-level low-noise noise (LNN) to each channel signal before processing was assessed. The addition of LNN reduced the amplification of low-level signal portions that contained large excursions in instantaneous frequency, and improved the intelligibility of simple TFS speech sentences, but not more complex sentences. It also reduced the time needed to reach a stable level of performance. The recovery of envelope cues by peripheral auditory filtering was investigated by measuring the intelligibility of 'recovered-envelope speech', formed by filtering TFS speech with an array of simulated auditory filters, and using the envelopes at the output of these filters to modulate sinusoids with frequencies equal to the filter center frequencies (i.e., tone vocoding). The intelligibility of TFS speech and recovered-envelope speech fell as N increased, although TFS speech was still highly intelligible for values of N for which the intelligibility of recovered-envelope speech was low.     

The clear speech effect for non-native listeners

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 as a consequence of the fact that clear speech is essentially native-listener oriented, and therefore is only beneficial to listeners with extensive experience with the sound structure of the target language.     

Relations between intelligibility of narrow-band speech and auditory functions, both in the 1-kHz frequency region

Relations between perception of suprathreshold speech and auditory functions were examined in 24 hearing-impaired listeners and 12 normal-hearing listeners. The speech intelligibility index (SII) was used to account for audibility. The auditory functions included detection efficiency, temporal and spectral resolution, temporal and spectral integration, and discrimination of intensity, frequency, rhythm, and spectro-temporal shape. All auditory functions were measured at 1 kHz. Speech intelligibility was assessed with the speech-reception threshold (SRT) in quiet and in noise, and with the speech-reception bandwidth threshold (SRBT), previously developed for investigating speech perception in a limited frequency region around 1 kHz. The results showed that the elevated SRT in quiet could be explained on the basis of audibility. Audibility could only partly account for the elevated SRT values in noise and the deviant SRBT values, suggesting that suprathreshold deficits affected intelligibility in these conditions. SII predictions for the SRBT improved significantly by including the individually measured upward spread of masking in the SII model. Reduced spectral resolution, reduced temporal resolution, and reduced frequency discrimination appeared to be related to speech perception deficits. Loss of peripheral compression appeared to have the smallest effect on the intelligibility of suprathreshold speech.     

Bidirectional clear speech perception benefit for native and high-proficiency non-native talkers and listeners: intelligibility and accentedness

This study investigated how native language background interacts with speaking style adaptations in determining levels of speech intelligibility. The aim was to explore whether native and high proficiency non-native listeners benefit similarly from native and non-native clear speech adjustments. The sentence-in-noise perception results revealed that fluent non-native listeners gained a large clear speech benefit from native clear speech modifications. Furthermore, proficient non-native talkers in this study implemented conversational-to-clear speaking style modifications in their second language (L2) that resulted in significant intelligibility gain for both native and non-native listeners. The results of the accentedness ratings obtained for native and non-native conversational and clear speech sentences showed that while intelligibility was improved, the presence of foreign accent remained constant in both speaking styles. This suggests that objective intelligibility and subjective accentedness are two independent dimensions of non-native speech. Overall, these results provide strong evidence that greater experience in L2 processing leads to improved intelligibility in both production and perception domains. These results also demonstrated that speaking style adaptations along with less signal distortion can contribute significantly towards successful native and non-native interactions.     

Acoustic properties of naturally produced clear speech at normal speaking rates

Sentences spoken "clearly" are significantly more intelligible than those spoken "conversationally" for hearing-impaired listeners in a variety of backgrounds [Picheny et al., J. Speech Hear. Res. 28, 96-103 (1985); Uchanski et al., ibid. 39, 494-509 (1996); Payton et al., J. Acoust. Soc. Am. 95, 1581-1592 (1994)]. While producing clear speech, however, talkers often reduce their speaking rate significantly [Picheny et al., J. Speech Hear. Res. 29, 434-446 (1986); Uchanski et al., ibid. 39, 494-509 (1996)]. Yet speaking slowly is not solely responsible for the intelligibility benefit of clear speech (over conversational speech), since a recent study [Krause and Braida, J. Acoust. Soc. Am. 112, 2165-2172 (2002)] showed that talkers can produce clear speech at normal rates with training. This finding suggests that clear speech has inherent acoustic properties, independent of rate, that contribute to improved intelligibility. Identifying these acoustic properties could lead to improved signal processing schemes for hearing aids. To gain insight into these acoustical properties, conversational and clear speech produced at normal speaking rates were analyzed at three levels of detail (global, phonological, and phonetic). Although results suggest that talkers may have employed different strategies to achieve clear speech at normal rates, two global-level properties were identified that appear likely to be linked to the improvements in intelligibility provided by clear/normal speech: increased energy in the 1000-3000-Hz range of long-term spectra and increased modulation depth of low frequency modulations of the intensity envelope. Other phonological and phonetic differences associated with clear/normal speech include changes in (1) frequency of stop burst releases, (2) VOT of word-initial voiceless stop consonants, and (3) short-term vowel spectra.     

A method to determine the speech transmission index from speech waveforms

A method for computing the speech transmission index (STI) using real speech stimuli is presented and evaluated. The method reduces the effects of some of the artifacts that can be encountered when speech waveforms are used as probe stimuli. Speech-based STIs are computed for conversational and clearly articulated speech in several noisy, reverberant, and noisy-reverberant environments and compared with speech intelligibility scores. The results indicate that, for each speaking style, the speech-based STI values are monotonically related to intelligibility scores for the degraded speech conditions tested. Therefore, the STI can be computed using speech probe waveforms and the values of the resulting indices are as good predictors of intelligibility scores as those derived from MTFs by theoretical methods.     

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.     

Speech perception from monaural and binaural information

Two experiments explored the concept of the binaural spectrogram [Culling and Colburn, J. Acoust. Soc. Am. 107, 517-527 (2000)] and its relationship to monaurally derived information. In each experiment, speech was added to noise at an adverse signal-to-noise ratio in the NoS pi binaural configuration. The resulting monaural and binaural cues were analyzed within an array of spectro-temporal bins and then these cues were resynthesized by modulating the intensity and/or interaural correlation of freshly generated noise. Experiment 1 measured the intelligibility of the resynthesized stimuli and compared them with the original NoSo and NoS pi stimuli at a fixed signal-to-noise ratio. While NoS pi stimuli were approximately equal to 50% intelligible, each cue in isolation produced similar (very low) intelligibility to the NoSo condition. The resynthesized combination produced approximately equal to 25% intelligibility. Modulation of interaural correlation below 1.2 kHz and of amplitude above 1.2 kHz was not as effective as their combination across all frequencies. Experiment 2 measured three-point psychometric functions in which the signal-to-noise ratio of the original NoS pi stimulus was increased in 3-dB steps from the level used in experiment 1. Modulation of interaural correlation alone proved to have a flat psychometric function. The functions for NoS pi and for combined monaural and binaural cues appeared similar in slope, but shifted horizontally. The results indicate that for sentence materials, neither fluctuations in interaural correlation nor in monaural intensity are sufficient to support speech recognition at signal-to-noise ratios where 50% intelligibility is achieved in the NoS pi configuration; listeners appear to synergistically combine monaural and binaural information in this task, to some extent within the same frequency region.     

Differential contribution of envelope fluctuations across frequency to consonant identification in quiet

Two experiments investigated the effects of critical bandwidth and frequency region on the use of temporal envelope cues for speech. In both experiments, spectral details were reduced using vocoder processing. In experiment 1, consonant identification scores were measured in a condition for which the cutoff frequency of the envelope extractor was half the critical bandwidth (HCB) of the auditory filters centered on each analysis band. Results showed that performance is similar to those obtained in conditions for which the envelope cutoff was set to 160 Hz or above. Experiment 2 evaluated the impact of setting the cutoff frequency of the envelope extractor to values of 4, 8, and 16 Hz or to HCB in one or two contiguous bands for an eight-band vocoder. The cutoff was set to 16 Hz for all the other bands. Overall, consonant identification was not affected by removing envelope fluctuations above 4 Hz in the low- and high-frequency bands. In contrast, speech intelligibility decreased as the cutoff frequency was decreased in the midfrequency region from 16 to 4 Hz. The behavioral results were fairly consistent with a physical analysis of the stimuli, suggesting that clearly measurable envelope fluctuations cannot be attenuated without affecting speech intelligibility.     

Role of spectral and temporal cues in restoring missing speech information

This study measured the role of spectral details and temporal envelope (E) and fine structure (TFS) cues in reconstructing sentences from speech fragments. Four sets of sentences were processed using a 32-band vocoder. Twenty one bands were either processed or removed, leading to sentences differing in their amount of spectral details, E and TFS information. These sentences remained perfectly intelligible, but intelligibility significantly fell after the introduction of periodic silent gaps of 120-ms. While the role of E was unclear, the results unambiguously showed that TFS cues and spectral details influence the ability to reconstruct interrupted sentences.     

Vowel intelligibility in clear and conversational speech for normal-hearing and hearing-impaired listeners

Several studies have demonstrated that when talkers are instructed to speak clearly, the resulting speech is significantly more intelligible than speech produced in ordinary conversation. These speech intelligibility improvements are accompanied by a wide variety of acoustic changes. The current study explored the relationship between acoustic properties of vowels and their identification in clear and conversational speech, for young normal-hearing (YNH) and elderly hearing-impaired (EHI) listeners. Monosyllabic words excised from sentences spoken either clearly or conversationally by a male talker were presented in 12-talker babble for vowel identification. While vowel intelligibility was significantly higher in clear speech than in conversational speech for the YNH listeners, no clear speech advantage was found for the EHI group. Regression analyses were used to assess the relative importance of spectral target, dynamic formant movement, and duration information for perception of individual vowels. For both listener groups, all three types of information emerged as primary cues to vowel identity. However, the relative importance of the three cues for individual vowels differed greatly for the YNH and EHI listeners. This suggests that hearing loss alters the way acoustic cues are used for identifying vowels.     

Effects of cochlear implant processing and fundamental frequency on the intelligibility of competing sentences

Speech perception in the presence of another competing voice is one of the most challenging tasks for cochlear implant users. Several studies have shown that (1) the fundamental frequency (F0) is a useful cue for segregating competing speech sounds and (2) the F0 is better represented by the temporal fine structure than by the temporal envelope. However, current cochlear implant speech processing algorithms emphasize temporal envelope information and discard the temporal fine structure. In this study, speech recognition was measured as a function of the F0 separation of the target and competing sentence in normal-hearing and cochlear implant listeners. For the normal-hearing listeners, the combined sentences were processed through either a standard implant simulation or a new algorithm which additionally extracts a slowed-down version of the temporal fine structure (called Frequency-Amplitude-Modulation-Encoding). The results showed no benefit of increasing F0 separation for the cochlear implant or simulation groups. In contrast, the new algorithm resulted in gradual improvements with increasing F0 separation, similar to that found with unprocessed sentences. These results emphasize the importance of temporal fine structure for speech perception and demonstrate a potential remedy for difficulty in the perceptual segregation of competing speech sounds.     

Predicting speech intelligibility based on the signal-to-noise envelope power ratio after modulation-frequency selective processing

A model for predicting the intelligibility of processed noisy speech is proposed. The speech-based envelope power spectrum model has a similar structure as the model of Ewert and Dau [(2000). J. Acoust. Soc. Am. 108, 1181-1196], developed to account for modulation detection and masking data. The model estimates the speech-to-noise envelope power ratio, SNR(env), at the output of a modulation filterbank and relates this metric to speech intelligibility using the concept of an ideal observer. Predictions were compared to data on the intelligibility of speech presented in stationary speech-shaped noise. The model was further tested in conditions with noisy speech subjected to reverberation and spectral subtraction. Good agreement between predictions and data was found in all cases. For spectral subtraction, an analysis of the model's internal representation of the stimuli revealed that the predicted decrease of intelligibility was caused by the estimated noise envelope power exceeding that of the speech. The classical concept of the speech transmission index fails in this condition. The results strongly suggest that the signal-to-noise ratio at the output of a modulation frequency selective process provides a key measure of speech intelligibility.     

The influence of spectral characteristics of early reflections on speech intelligibility

The auditory system takes advantage of early reflections (ERs) in a room by integrating them with the direct sound (DS) and thereby increasing the effective speech level. In the present paper the benefit from realistic ERs on speech intelligibility in diffuse speech-shaped noise was investigated for normal-hearing and hearing-impaired listeners. Monaural and binaural speech intelligibility tests were performed in a virtual auditory environment where the spectral characteristics of ERs from a simulated room could be preserved. The useful ER energy was derived from the speech intelligibility results and the efficiency of the ERs was determined as the ratio of the useful ER energy to the total ER energy. Even though ER energy contributed to speech intelligibility, DS energy was always more efficient, leading to better speech intelligibility for both groups of listeners. The efficiency loss for the ERs was mainly ascribed to their altered spectrum compared to the DS and to the filtering by the torso, head, and pinna. No binaural processing other than a binaural summation effect could be observed.     

The ability of listeners to use recovered envelope cues from speech fine structure

Recent work has demonstrated that auditory filters recover temporal-envelope cues from speech fine structure when the former were removed by filtering or distortion. This study extended this work by assessing the contribution of recovered envelope cues to consonant perception as a function of the analysis bandwidth, when vowel-consonant-vowel (VCV) stimuli were processed in order to keep their fine structure only. The envelopes of these stimuli were extracted at the output of a bank of auditory filters and applied to pure tones whose frequency corresponded to the original filters' center frequencies. The resulting stimuli were found to be intelligible when the envelope was extracted from a single, wide analysis band. However, intelligibility decreases from one to eight bands with no further decrease beyond this value, indicating that the recovered envelope cues did not play a major role in consonant perception when the analysis bandwidth was narrower than four times the bandwidth of a normal auditory filter (i.e., number of analysis bands > or =8 for frequencies spanning 80 to 8020 Hz).     

Effects of degradation of intensity, time, or frequency content on speech intelligibility for normal-hearing and hearing-impaired listeners

Many hearing-impaired listeners suffer from distorted auditory processing capabilities. This study examines which aspects of auditory coding (i.e., intensity, time, or frequency) are distorted and how this affects speech perception. The distortion-sensitivity model is used: The effect of distorted auditory coding of a speech signal is simulated by an artificial distortion, and the sensitivity of speech intelligibility to this artificial distortion is compared for normal-hearing and hearing-impaired listeners. Stimuli (speech plus noise) are wavelet coded using a complex sinusoidal carrier with a Gaussian envelope (1/4 octave bandwidth). Intensity information is distorted by multiplying the modulus of each wavelet coefficient by a random factor. Temporal and spectral information are distorted by randomly shifting the wavelet positions along the temporal or spectral axis, respectively. Measured were (1) detection thresholds for each type of distortion, and (2) speech-reception thresholds for various degrees of distortion. For spectral distortion, hearing-impaired listeners showed increased detection thresholds and were also less sensitive to the distortion with respect to speech perception. For intensity and temporal distortion, this was not observed. Results indicate that a distorted coding of spectral information may be an important factor underlying reduced speech intelligibility for the hearing impaired.     

Improvement of intelligibility of ideal binary-masked noisy speech by adding background noise

When a target-speech/masker mixture is processed with the signal-separation technique, ideal binary mask (IBM), intelligibility of target speech is remarkably improved in both normal-hearing listeners and hearing-impaired listeners. Intelligibility of speech can also be improved by filling in speech gaps with un-modulated broadband noise. This study investigated whether intelligibility of target speech in the IBM-treated target-speech/masker mixture can be further improved by adding a broadband-noise background. The results of this study show that following the IBM manipulation, which remarkably released target speech from speech-spectrum noise, foreign-speech, or native-speech masking (experiment 1), adding a broadband-noise background with the signal-to-noise ratio no less than 4 dB significantly improved intelligibility of target speech when the masker was either noise (experiment 2) or speech (experiment 3). The results suggest that since adding the noise background shallows the areas of silence in the time-frequency domain of the IBM-treated target-speech/masker mixture, the abruption of transient changes in the mixture is smoothed and the perceived continuity of target-speech components becomes enhanced, leading to improved target-speech intelligibility. The findings are useful for advancing computational auditory scene analysis, hearing-aid/cochlear-implant designs, and understanding of speech perception under "cocktail-party" conditions.     

Effects of periodic interruptions on the intelligibility of speech based on temporal fine-structure or envelope cues

The intelligibility of speech signals processed to retain either temporal envelope (E) or fine structure (TFS) cues within 16 0.4-oct-wide frequency bands was evaluated when processed stimuli were periodically interrupted at different rates. The interrupted E- and TFS-coded stimuli were highly intelligible in all conditions. However, the different patterns of results obtained for E- and TFS-coded speech suggest that the two types of stimuli do not convey identical speech cues. When an effect of interruption rate was observed, the effect occurred at low interruption rates (<8 Hz) and was stronger for E- than TFS-coded speech, suggesting larger involvement of modulation masking with E-coded speech.     

Effect of spectral resolution on the intelligibility of ideal binary masked speech

Most binary-mask studies assume a fine time-frequency representation of the signal that may not be available in some applications (e.g., cochlear implants). This study assesses the effect of spectral resolution on intelligibility of ideal-binary masked speech. In Experiment 1, speech corrupted in noise at -5 to 5 dB signal-to-noise ratio (SNR) was filtered into 6-32 channels and synthesized using the ideal binary mask. Results with normal-hearing listeners indicated substantial improvements in intelligibility with 24-32 channels, particularly in -5 dB SNR. Results from Experiment 2 indicated that having access to the ideal binary mask in the F1/F2 region is sufficient for good performance.