Improving speech audibility with wide dynamic range compression in listeners with severe sensorineural loss

OBJECTIVE: In contrast to fitting strategies for linear amplification, which have been refined frequently for listeners with different degrees of hearing loss, we know relatively little about the effects of wide dynamic range compression (WDRC) amplification for listeners with severe auditory thresholds. The primary objective of this study was to determine if increases in audibility with WDRC amplification improved speech recognition to a comparable degree for listeners with different degrees of hearing loss. DESIGN: Listeners with mild to moderate or severe sensorineural loss were tested on recognition of vowel-consonant-vowel (VCV) syllables and sentences digitally processed with linear and WDRC amplification. The speech materials were presented under conditions of controlled audibility, in which WDRC amplification improved speech audibility over linear amplification. Presentation levels were chosen to provide equivalent increases in audibility with WDRC amplification for both listener groups. A control condition in which audibility was equated for the two amplification conditions was also included. RESULTS: Recognition results for VCV stimuli indicated that both listener groups received the same benefit from the improved audibility provided by WDRC amplification. Results for sentence recognition showed a greater benefit of WDRC amplification for listeners with mild to moderate than for listeners with severe loss. CONCLUSIONS: Increasing the amount of audible speech information with WDRC has similar effects on consonant recognition for listeners with different degrees of hearing loss. Differences in sentence recognition for listeners with different degrees of loss may be due to processing effects or to differences in available acoustic information for longer segments of WDRC-amplified speech.     

Quantifying the contribution of audibility to recognition of compression-amplified speech

OBJECTIVE: The speech-recognition advantages of wide-dynamic range compression (WDRC) hearing aids are believed to be a direct result of improved audibility. The main objective of this study was to compare the relationship between increasing audibility and recognition for compression-amplified versus linearly amplified speech, using a quantitative measure of audibility. A second objective was to explore the adequacy of the Aided Articulation Index (Stelmachowicz, Lewis, Kalberer, & Creutz, 1994) in describing performance with WDRC-amplified speech. DESIGN: Sixteen listeners with mild to severe sensorineural hearing loss were tested on recognition of nonsense syllables that had been digitally processed with linear or WDRC amplification. Speech recognition scores were obtained for three input levels that differed in audibility relative to the listeners' hearing thresholds. An Aided Audibility Index (AAI), ranging from 0.0 (inaudible) to 1.0 (fully audible), was calculated for each listener and condition. This importance-weighted index represents the portion of the speech signal that is available to the listener. The relationship between audibility and recognition was then examined for each type of amplification. RESULTS: At low and moderate input levels, AAI values and corresponding recognition scores were higher for the compression-amplified than for the linearly amplified speech. At high input levels, AAIs and recognition scores were essentially the same for both types of amplification. Recognition scores increased monotonically as a function of the AAI for both linear and WDRC amplifiers. There was no significant difference between the functions for linearly amplified versus compression-amplified speech. In other words, a given increase in audibility resulted in the same increase in recognition for both types of amplification. CONCLUSIONS: Increasing the amount of audible speech information plays the same role in recognition for compression-amplified as for linearly amplified speech. This suggests that, at least for the conditions tested here, compression does not introduce detrimental changes to the speech signal that offset the benefits of improved audibility. Accuracy of the compression-amplified AAI can be improved by substituting measured speech ranges for the estimated speech ranges used in the standard AAI calculation. In the absence of measured speech ranges, the standard AAI provides a close approximation.     

Combining temporal-envelope cues across channels: effects of age and hearing loss.

The goal of this study was to examine the ability to combine temporal-envelope information across frequency channels. Three areas were addressed: (a) the effects of hearing loss, (b) the effects of age and (c) whether such effects increase with the number of frequency channels. Twenty adults aged 23-80 years with hearing loss ranging from mild to severe and a control group of 6 adults with normal hearing participated. Stimuli were vowel-consonant-vowel syllables. Consonant identification was measured for 5 conditions: (a) 1-channel temporal-envelope information (minimal spectral cues), (b) 2-channel, (c) 4-channel, (d) 8-channel, and (e) an unprocessed (maximal spectral cues) speech condition. Performance of listeners with normal hearing and listeners with hearing loss was similar in the 1-channel condition. Performance increased with the number of frequency channels in both groups; however, increasing the number of channels led to smaller improvements in consonant identification in listeners with hearing loss. Older listeners performed more poorly than younger listeners but did not have more difficulty combining temporal cues across channels than in a simple, 1-channel temporal task. Age was a significant predictor of nonsense syllable identification, whereas amount of hearing loss was not. The results support an age-related deficit in use of temporal-envelope information with age, regardless of the number of channels.     

Effect of a single-channel wide dynamic range compression circuit on perception of stop consonant place of articulation.

Previous studies have shown that altering the amplitude of a consonant in a specific frequency region relative to an adjacent vowel's amplitude in the same frequency region will affect listeners' perception of the consonant place of articulation. Hearing aids with single-channel, fast-acting wide dynamic range compression (WDRC) alter the overall consonant-vowel (CV) intensity ratio by increasing consonant energy. Perhaps one reason WDRC has had limited success in improving speech recognition performance is that the natural amplitude balances between consonant and vowel are altered in crucial frequency regions, thus disturbing the aforementioned amplitude cue for determining place of articulation. The current study investigated the effect of a WDRC circuit on listeners' perception of place of articulation when the relative amplitude of consonant and vowel was manipulated. The stimuli were a continuum of synthetic CV syllables stripped of all place cues except relative consonant amplitudes. Acoustic analysis of the CVs before and after hearing aid processing showed a predictable increase in high-frequency energy, particularly for the burst of the consonant. Alveolar bursts had more high-frequency energy than labial bursts. Twenty-five listeners with normal hearing and 5 listeners with sensorineural hearing loss labeled the consonant sound of the CV syllables in unaided form and after the syllables were recorded through a hearing aid with single-channel WDRC. There were significantly more listeners who were unable to produce a category boundary when labeling the aided stimuli. Of those listeners who did yield a category boundary for both aided and unaided stimuli, there were significantly more alveolar responses for the aided condition. These results can be explained by the acoustic analyses of the aided stimuli.     

Using multichannel wide-dynamic range compression in severely hearing-impaired listeners: effects on speech recognition and quality

OBJECTIVE: The objective of this study was to compare speech recognition across a sampling of amplification choices available for listeners with severe loss. This includes conventional options (linear with peak clipping and linear with compression limiting) and newer strategies (multichannel wide-dynamic range compression [WDRC]) theorized to better accommodate reduced dynamic range. A second objective was to compare speech quality across the same conditions using a paired-comparison test. DESIGN: Participants were 13 adults with severe sensorineural hearing loss and a control group of seven adults with normal hearing. Test materials included consonant-vowel syllables (speech recognition) and sentences (speech quality). Four amplification conditions were included: peak clipping; compression limiting; two-channel WDRC; and three-channel WDRC, with overall audibility similar across conditions. In the WDRC conditions, the compression ratio was fixed at 3:1 in each channel. Consonant recognition was measured using a closed-set task, and speech quality was measured using a paired-comparison test. RESULTS: For the listeners with severe loss, recognition and preference were lower for a three-channel WDRC system than for a compression limiting system. Specific errors were consistent with poorer transmission of amplitude envelope information by the multichannel WDRC systems. CONCLUSIONS: Under some conditions, the benefit of fast-acting, multichannel WDRC systems relative to more linear amplification strategies may be reduced in listeners with severe loss. Performance decrements with these systems are consistent with consequences of broader auditory filters.     

Prediction of speech recognition from audibility in older listeners with hearing loss: effects of age, amplification, and background noise

The extent to which audibility determines speech recognition depends on a number of signal and listener factors. This study focused on three factors: age, background noise modulation, and linear versus wide-dynamic compression amplification. Three audiometrically matched groups of older listeners with hearing loss were tested to determine at what age performance declined relative to that expected on the basis of audibility. Recognition fell below predicted scores by greater amounts as age increased. Scores were higher for steady versus amplitude-modulated noise. Scores for WDRC-amplified speech were slightly lower than for linearly amplified speech across all groups and noise conditions. We found no interaction between age and type of noise. The small reduction in scores for amplitude-modulated compared to steady noise and lack of age interaction suggests that the substantial deficit seen with age in multitalker babble for previous studies was due to some effect not elicited here, such as informational masking.     

Temporal envelope changes of compression and speech rate: combined effects on recognition for older adults.

PURPOSE: When understanding speech in complex listening situations, older adults with hearing loss face the double challenge of cochlear hearing loss and deficits of the aging auditory system. Wide-dynamic range compression (WDRC) is used in hearing aids as remediation for the loss of audibility associated with hearing loss. WDRC processing has the additional effect of altering the acoustics of the speech signal, particularly the temporal envelope. Older listeners are negatively affected by other types of temporal distortions, but this has not been found for the distortion of WDRC processing for simple signals. The purpose of this research was to determine the circumstances under which older adults might be negatively affected by WDRC processing and what compensatory mechanisms those listeners might be using for the listening conditions when speech recognition performance is not affected. METHOD: Two groups of adults with mild to moderate hearing loss were tested: (a) young-old (62-74 years, n=11) and (b) old-old (75-88 years, n=14). The groups did not differ in hearing loss, cognition, working memory, or self-reported health status. Participants heard low-predictability sentences compressed at each of 4 compression settings. The effect of compression on the temporal envelope was quantified by the envelope difference index (EDI; T. W. Fortune, B. D. Woodruff, & D. A. Preves, 1994). The sentences were presented at three rates: (a) normal rate, (b) 50% time compressed, and (c) time restored. RESULTS: There was no difference in performance between age groups, or any interactions involving age. There was a significant interaction between speech rate and EDI value; as the EDI value increased, representing higher amounts of temporal envelope distortion, speech recognition was significantly reduced. At the highest EDI value, this reduction was greater for the time-compressed than the normal rate condition. When time was restored to the time-compressed signals, speech recognition did not improve. CONCLUSION: Temporal envelope changes were detrimental to recognition of low-context speech for older listeners once a certain threshold of distortion was reached, particularly for rapid rate speech. For this sample tested, the effect was not age related within the age range tested here. The results of the time-restored condition suggested that listeners were using acoustic redundancy to compensate for the negative effects of WDRC distortion in the normal rate condition.     

The contribution of amplitude envelope cues to sentence identification in young and aged listeners

OBJECTIVE: The purpose of this study was to examine the importance of amplitude envelope cues to sentence identification for aged listeners. We also examined the effect of increasing alterations (i.e., compression ratio) and amount of available frequency content (i.e., number of channels) for this population. DESIGN: Thirty-six listeners were classified according to their age (35 or younger versus 65 and older) and hearing status (normal hearing versus hearing impaired). Within each hearing status, mean hearing threshold thresholds for the young and aged listeners were matched as closely as possible through 4 kHz to control for sensitivity differences across age, and all listeners passed a cognitive screening battery. Accuracy of synthetic sentence identification was measured using stimuli processed to restrict spectral information. Performance was measured as a function of age, hearing status, amount of spectral information, and degradation of the amplitude envelope (using fast-acting compression with compression ratios ranging from 1:1 to 5:1). RESULTS: Mean identification scores decreased significantly with increasing age, the presence of hear- c ing loss, the removal of spectral information, and with increasing distortion of the amplitude envelope (i.e., higher compression ratios). There was a consistent performance gap between young and aged listeners, regardless of the magnitude of change to the amplitude envelope. This suggests that some cue other than amplitude envelope variations is inaccessible to the aged listeners. CONCLUSIONS: Although aged listeners performed more poorly overall, they did not show greater susceptibility to alterations in amplitude-envelope cues, such as those produced by fast-acting amplitude compression systems. It is therefore unlikely that compression parameters such as attack and release time or compression ratio would need to be a differentially programmed for aged listeners. Instead, the data suggest two possibilities: aged listeners have difficulty accessing the fine-structure temporal cues present in speech, and/or performance is degraded by age-related loss of function at a central processing level.     

Sources of age-related recognition difficulty for time-compressed speech.

Older people frequently show poorer recognition of rapid speech or time-compressed speech than younger listeners. The present investigation sought to determine if the age-related problem in recognition of time-compressed speech could be attributed primarily to a decline in the speed of information processing or to a decline in processing brief acoustic cues. The role of the availability of linguistic cues on recognition performance was examined also. Younger and older listeners with normal hearing and with hearing loss participated in the experiments. Stimuli were sentences, linguistic phrases, and strings of random words that were unmodified in duration or were time compressed with uniform time compression or with selective time compression of consonants, vowels, or pauses. Age effects were observed for recognition of unmodified random words, but not for sentences and linguistic phrases. Analysis of difference scores (unmodified speech versus time-compressed speech) showed age effects for time-compressed sentences and phrases. The forms of time compression that were notably difficult for older listeners were uniform time compression and selective time compression of consonants. Indeed, poor performance in recognizing uniformly time-compressed speech was attributed primarily to difficulty in recognizing speech that incorporated selective time compression of consonants. Hearing loss effects were observed also for most of the listening conditions, although these effects were independent of the aging effects. In general, the findings support the notion that the problems of older listeners in recognizing time-compressed speech are associated with difficulty in processing the brief, limited acoustic cues for consonants that are inherent in rapid speech.     

Effect of preferred volume setting on speech audibility in different hearing aid circuits

This study compared preferred volume setting for linear peak clipping, compression limiting, and wide dynamic range compression (WDRC) amplification and quantified speech audibility at the preferred volume setting for each amplification type. Ten listeners with mild-to-moderate hearing loss were fitted monaurally with a behind-the-ear hearing aid programmed sequentially with WDRC, compression limiting, and linear peak clipping amplification. Speech was presented in quiet and in noise at a range of input levels. In each condition, the listener adjusted the volume for maximum clarity. Signal levels were measured using a probe microphone system. There was no significant difference in speech audibility between amplification strategies for any speech level regardless of the presence or absence of background noise. These results imply that the improved audibility advantage of WDRC over linear amplification demonstrated in controlled environments may not be maintained in situations where the hearing aid wearer controls the volume.     

Effect of slow-acting wide dynamic range compression on measures of intelligibility and ratings of speech quality in simulated-loss listeners.

The purpose of this study was twofold: (a) to determine the extent to which 4-channel, slow-acting wide dynamic range amplitude compression (WDRC) can counteract the perceptual effects of reduced auditory dynamic range and (b) to examine the relation between objective measures of speech intelligibility and categorical ratings of speech quality for sentences processed with slow-acting WDRC. Multiband expansion was used to simulate the effects of elevated thresholds and loudness recruitment in normal hearing listeners. While some previous studies have shown that WDRC can improve both speech intelligibility and quality, others have found no benefit. The current experiment shows that moderate amounts of compression can provide a small but significant improvement in speech intelligibility, relative to linear amplification, for simulated-loss listeners with small dynamic ranges (i.e., flat, moderate hearing loss). This benefit was found for speech at conversational levels, both in quiet and in a background of babble. Simulated-loss listeners with large dynamic ranges (i.e., sloping, mild-to-moderate hearing loss) did not show any improvement. Comparison of speech intelligibility scores and subjective ratings of intelligibility showed that listeners with simulated hearing loss could accurately judge the overall intelligibility of speech. However, in all listeners, ratings of pleasantness decreased as the compression ratio increased. These findings suggest that subjective measures of speech quality should be used in conjunction with either objective or subjective measures of speech intelligibility to ensure that participant-selected hearing aid parameters optimize both comfort and intelligibility.     

Perceptual integration between target speech and target-speech reflection reduces masking for target-speech recognition in younger adults and older adults

This study evaluated unmasking functions of perceptual integration of target speech and simulated target-speech reflection, which were presented by two spatially separated loudspeakers. In both younger-adult listeners with normal hearing and older-adult listeners in the early stages of presbycusis, reducing the time interval between target speech and target-reflection simulation (inter-target interval, ITI) from 64 to 0ms not only progressively enhanced perceptual integration of target-speech signals, but also progressively released target speech from either speech masking or noise masking. When the signal-to-noise ratio was low, the release from speech masking was significantly larger than the release from noise masking in both younger listeners and older listeners, but the longest ITI at which a significant release from speech masking occurred was significantly shorter in older listeners than in younger listeners. These results suggest that in reverberant environments with multi-talker speech, perceptual integration between the direct sound wave and correlated reflections, which facilitates perceptual segregation of various sources, is critical for unmasking attended speech. The age-related reduction of the ITI range for releasing speech from speech masking may be one of the causes for the speech-recognition difficulties experienced by older listeners in such adverse environments.     

relative contributions of spectral and temporal cues for speech recognition in patients with sensorineural hearing loss

The present study was designed to examine speech recognition in patients with sensorineural hearing loss when the temporal and spectral information in the speech signals were co-varied. Four subjects with mild to moderate sensorineural hearing loss were recruited to participate in consonant and vowel recognition tests that used speech stimuli processed through a noise-excited voeoder. The number of channels was varied between 2 and 32, which defined spectral information. The lowpass cutoff frequency of the temporal envelope extractor was varied from 1 to 512 Hz, which defined temporal information. Results indicate that performance of subjects with sensorineural heating loss varied tremendously among the subjects. For consonant recognition, patterns of relative contributions of spectral and temporal information were similar to those in normal-hearing subjects. The utility of temporal envelope information appeared to be normal in the hearing-impaired listeners. For vowel recognition, which depended predominately on spectral information, the performance plateau was achieved with numbers of channels as high as 16-24, much higher than expected, given that the frequency selectivity in patients with sensorineural hearing loss might be compromised. In order to understand the mechanisms on how hearing-impaired listeners utilize spectral and temporal cues for speech recognition, future studies that involve a large sample of patients with sensorineural hearing loss will be necessary to elucidate the relationship between frequency selectivity as well as central processing capability and speech recognition performance using vocoded signals.     

Recognition of time-compressed and natural speech with selective temporal enhancements by young and elderly listeners.

PURPOSE: The goal of this experiment was to determine whether selective slowing of speech segments improves recognition performance by young and elderly listeners. The hypotheses were (a) the benefits of time expansion occur for rapid speech but not for natural-rate speech, (b) selective time expansion of consonants produces greater score increments than other forms of selective time expansion, and (c) older listeners benefit from time expansion of speech METHOD: Participants (n=10-16 per group) were younger and older adults with normal hearing or with hearing loss. A repeated-measures design was used to assess recognition of sentence-length stimuli presented in 2 baseline speech rates: natural and 50% time compression. Selective time expansion of consonants, vowels, or pauses was applied to the natural-rate and time-compressed sentence-length stimuli. RESULTS: Listeners showed excellent performance for natural-rate speech, regardless of time-expansion method. Recognition was significantly poorer for the time-compressed sentences, but performance by elderly listeners and listeners with hearing loss improved with selective time expansion, particularly when applied to consonant segments. CONCLUSION: The findings support the hypothesis that older listeners and listeners with hearing impairment benefit from selective time expansion of consonants applied to rapid speech, without a corresponding decrement when applied to normal-rate speech.     

Improving audibility with nonlinear amplification for listeners with high-frequency loss

In contrast to fitting strategies for linear amplification that have been frequently refined for listeners with different degrees of hearing loss, we know relatively little about the effects of nonlinear amplification for differing audiometric configurations. The purpose of the current study was to determine whether increases in audibility with nonlinear amplification improved speech recognition to a comparable degree for listeners with sloping sensorineural loss as for a comparison group of listeners with flat sensorineural loss. Consonant recognition was examined as a function of audibility with wide dynamic range compression amplification and with linear amplification. For linearly amplified speech, listeners with flat and sloping loss showed similar improvements in recognition given the same increases in audibility. Results for nonlinearly amplified speech indicated that the listeners with flat loss showed a greater rate of improvement as audibility increases than the listeners with sloping loss. This difference is largely due to superior performance by the listeners with sloping loss for low-audibility speech in comparison to equivalent group performance for high-audibility speech.     

Effects of aging on auditory processing of speech

The focus of this paper is on the effects of age on speech perception, with reference to pertinent psychoacoustic findings. The difficulties of older listeners are related to the well-known effects of high-frequency hearing loss on speech perception in quiet, and to temporal processing declines not predictable from the audiogram that account for reduced ability to listen in complex, noisy conditions. We also discuss issues of research interpretation; e.g. the need for researchers and clinicians to be alert to the frequent confound between degree of hearing loss and age. The implications of age-related changes in auditory speech processing for future practice and research are discussed relative to interactions between older individuals and their acoustic environments.     

Effect of consonant-vowel ratio modification on amplitude envelope cues for consonant recognition

This investigation examined the degree to which modification of the consonant-vowel (C-V) intensity ratio affected consonant recognition under conditions in which listeners were forced to rely more heavily on waveform envelope cues than on spectral cues. The stimuli were 22 vowel-consonant-vowel utterances, which had been mixed at six different signal-to-noise ratios with white noise that had been modulated by the speech waveform envelope. The resulting waveforms preserved the gross speech envelope shape, but spectral cues were limited by the white-noise masking. In a second stimulus set, the consonant portion of each utterance was amplified by 10 dB. Sixteen subjects with normal hearing listened to the unmodified stimuli, and 16 listened to the amplified-consonant stimuli. Recognition performance was reduced in the amplified-consonant condition for some consonants, presumably because waveform envelope cues had been distorted. However, for other consonants, especially the voiced stops, consonant amplification improved recognition. Patterns of errors were altered for several consonant groups, including some that showed only small changes in recognition scores. The results indicate that when spectral cues are compromised, nonlinear amplification can alter waveform envelope cues for consonant recognition.     

Binaural advantage for younger and older adults with normal hearing.

PURPOSE: Three experiments measured benefit of spatial separation, benefit of binaural listening, and masking-level differences (MLDs) to assess age-related differences in binaural advantage. METHOD: Participants were younger and older adults with normal hearing through 4.0 kHz. Experiment 1 compared spatial benefit with and without head shadow. Sentences were at 0 degrees, and speech-shaped noise was at 0 degrees, 90 degrees, or +/-90 degrees. Experiment 2 measured binaural benefit with the near ear unplugged compared with plugged for sentences at 0 degrees and masker at 90 degrees. Experiment 3 measured MLDs under earphones for 0.5-kHz pure tones in Gaussian and low-noise noise, and spondees in speech-shaped noise. RESULTS: Spatial-separation benefit for speech did not differ significantly for younger and older adults but was smaller than predicted by an audibility-based model for older adults and larger than predicted for younger adults. Binaural listening benefit was observed for younger participants only. Tonal MLDs suggested that listeners benefit from interaural difference cues during noise dips for signals out of phase. Neither tonal nor speech MLDs differed significantly between younger and older participants. CONCLUSION: Binaural processing of sentences revealed some age-related deficits in the use of interaural difference cues, whereas no deficits were observed for more simple detection or recognition tasks.     

Temporal-Envelope Reconstruction for Hearing-Impaired Listeners

Recent studies suggest that normal-hearing listeners maintain robust speech intelligibility despite severe degradations of amplitude-modulation (AM) cues, by using temporal-envelope information recovered from broadband frequency-modulation (FM) speech cues at the output of cochlear filters. This study aimed to assess whether cochlear damage affects this capacity to reconstruct temporal-envelope information from FM. This was achieved by measuring the ability of 40 normal-hearing listeners and 41 listeners with mild-to-moderate hearing loss to identify syllables processed to degrade AM cues while leaving FM cues intact within three broad frequency bands spanning the range 65–3,645 Hz. Stimuli were presented at 65 dB SPL for both normal-hearing listeners and hearing-impaired listeners. They were presented as such or amplified using a modified half-gain rule for hearing-impaired listeners. Hearing-impaired listeners showed significantly poorer identification scores than normal-hearing listeners at both presentation levels. However, the deficit shown by hearing-impaired listeners for amplified stimuli was relatively modest. Overall, hearing-impaired data and the results of a simulation study were consistent with a poorer-than-normal ability to reconstruct temporal-envelope information resulting from a broadening of cochlear filters by a factor ranging from 2 to 4. These results suggest that mild-to-moderate cochlear hearing loss has only a modest detrimental effect on peripheral, temporal-envelope reconstruction mechanisms.