Wide-dynamic Range Compression Amplification Research Articles

Measuring the Long-Term SNRs of Static and Adaptive Compression Amplification Techniques for Speech in Noise

Multichannel wide-dynamic-range compression (WDRC) is a widely adopted amplification scheme in modern digital hearing aids. It attempts to provide individuals with loudness recruitment with superior speech intelligibility and greater listening comfort over a wider range of input levels. However, recent surveys have shown that compression processing (operating in the nonlinear regime) usually reduces the long-term signal-to-noise ratio (SNR). The purpose of this study was to determine the long-term SNR in an adaptive compression-ratio (CR) amplification scheme called adaptive wide-dynamic-range compression (AWDRC), and to determine whether this concept is better than static WDRC amplification at improving the long-term SNR for speech in noise. AWDRC uses the input short-term dynamic range to adjust the CR to maximize audibility and comfort. Various methods for evaluating the long-term SNR were used to observe the relationship between the CR and output SNR performance in AWDRC for seven typical audiograms, and to compare the results with those for static WDRC amplification. The results showed that the variation of the CR in AWDRC amplification can maintain the comfort and audibility of the output sound. In addition, the average long-term SNR improved by 0.1-5.5 dB for a flat hearing loss, by 0.2-3.4 dB for a reverse sloping hearing loss, by 1.4-4.8 dB for a high-frequency hearing loss, and by 0.3-5.7 dB for a mild-to-moderate-sloping high-frequency hearing loss relative to static WDRC amplification. The output long-term SNR differed significantly (p < .001) between static WDRC and AWDRC amplification. The results of this study show that AWDRC, which uses the characteristics of the input signal to adaptively adjust the CR, provides better long-term SNR performance than static WDRC amplification.

Amplification and Consonant Modulation Spectra

In previous work, a simplified version of the modulation spectrum, the Spectral Correlation Index, was shown to be related to consonant error patterns. It is unknown what effect clinical amplification strategies will have on the modulation spectrum. Accordingly, the goals of this study were to examine the effect of clinical amplification strategies on the consonant modulation spectrum and to determine whether there was a relationship between the modulation spectrum and consonant errors for spectrally robust, amplified speech presented to listeners with hearing loss. Participants were 13 adults (mean age, 67 yrs) with mild to moderate sensorineural hearing loss. Each listener was fit monaurally in the test ear with a 16-band, four-channel behind the ear hearing aid. One memory of the hearing aid was programmed with compression limiting amplification and one with fast-acting wide-dynamic range compression (WDRC) amplification. Twenty-two consonant-vowel syllables were presented to the listener and recorded at the output of the hearing aid using a probe microphone system. A modulation spectrum was obtained for each amplified and unamplified consonant-vowel. Consonant recognition was also measured for each listener. Results show that (1) WDRC increased heterogeneity of the modulation spectrum across consonants and (2) for spectrally robust speech processed with either compression limiting or WDRC amplification, two consonants with similar modulation spectra are more likely to be confused with one another than are the two consonants with dissimilar modulation spectra. These data expand and confirm earlier results linking the modulation spectrum to specific consonant errors.

Effects of Audibility and Multichannel Wide Dynamic Range Compression on Consonant Recognition for Listeners with Severe Hearing Loss

This study examined the effects of multichannel wide-dynamic range compression (WDRC) amplification and stimulus audibility on consonant recognition and error patterns. Listeners had either severe or mild to moderate sensorineural hearing loss. Each listener was monaurally fit with a wearable hearing aid using typical clinical procedures, frequency-gain parameters, and a hybrid of clinically prescribed compression ratios for desired sensation level () and NAL-NL (). Consonant-vowel nonsense syllables were presented in soundfield at multiple input levels (50, 65, 80 dB SPL). Test conditions were four-channel fast-acting WDRC amplification and a control compression limiting (CL) amplification condition. Listeners identified the stimulus heard from choices presented on an on-screen display. A between-subject repeated measures design was used to evaluate consonant recognition and consonant confusion patterns. Fast-acting WDRC provided a considerable audibility advantage at 50 dB SPL, especially for listeners with severe hearing loss. Listeners with mild to moderate hearing loss received less audibility improvement from the fast-acting WDRC amplification, for conversational and high level speech, when compared with listeners with severe hearing loss. Analysis of WDRC benefit scores revealed that listeners had slightly lower scores with fast-acting WDRC amplification (relative to CL) when WDRC provided minimal improvement in audibility. The negative effect was greater for listeners with mild to moderate hearing loss compared with their counterparts with severe hearing loss. All listeners, but particularly the severe loss group, benefited from fast-acting WDRC amplification for low-level speech. For conversational and higher speech levels (i.e., when WDRC does not confer a significant audibility advantage), fast-acting WDRC amplification seems to slightly degrade performance. Listeners' consonant confusion patterns suggest that this negative effect may be partly due to fast-acting WDRC-induced distortions, which alter specific consonant features. In support of this view, audibility accounted for a greater percentage of the variance in listeners' performance with CL amplification compared with fast-acting WDRC amplification.

Effects of Three Amplification Strategies on Speech Perception by Children With Severe and Profound Hearing Loss

Traditionally in the United Kingdom, children with severe and profound hearing loss have been fitted with linear, analog hearing aids. Fast-acting, wide-dynamic-range compression (WDRC) has been shown to give better discrimination of speech than linear amplification for moderately hearing-impaired young adults. For severe and profound hearing losses, higher compression ratios are needed. The resultant distortion of the temporal envelope and reduced modulation depth may offset improvements in audibility offered by WDRC. In this study, speech recognition and discrimination were assessed for severely and profoundly hearing-impaired children, using three different amplification strategies, including WDRC. Fifteen children (ages 7 to 15 yr) with severe and profound hearing loss were fitted bilaterally with high-power, multichannel compression hearing aids, incorporating one of three different amplification strategies: linear with peak clipping, linear with compression limiting, or WDRC. Output responses were matched to Desired Sensation Level (DSL i/o) targets. The children wore hearing aids programmed with each of the amplification strategies in turn, for at least 1 wk, in a counterbalanced order across children. After using a particular amplification strategy for at least 1 wk, speech perception tests were carried out. Speech scores on closed-set testing for the profound group showed significant benefit for WDRC over the other two algorithms. None of the other results showed a statistically significant effect of algorithm on speech performance. WDRC amplification sometimes led to better performance than linear amplification with peak clipping or output limiting, and it never led to poorer performance. Therefore, it appears to be safe to use well-designed WDRC for hearing-impaired children with severe or profound hearing loss.

Effect of compression ratio on speech recognition and speech-quality ratings with wide dynamic range compression amplification.

This project examined the effect of varying compression ratio on speech recognition and quality. Both listeners with mild-to-moderate sensorineural hearing loss and a control group of listeners with normal hearing participated. Test materials were sentences from the Connected Speech Test (R. M. Cox, G. C. Alexander, & C. Gilmore, 1987) which were digitally processed with linear amplification and wide dynamic range compression amplification with 3 compression ratios. Speech-recognition scores were obtained with sentences in quiet and in noise at a 10-dB signal-to-noise ratio for each amplification condition. Additionally, the participants rated each amplification condition in terms of clarity, pleasantness, ease of understanding, and overall impression. Results indicated that, for speech in quiet, compression ratio had no effect on speech-recognition scores; however, speech-quality ratings decreased as compression ratio increased. For speech in noise, both speech-recognition scores and ratings decreased with increasing compression ratio for the listeners with hearing loss. These results suggest that selection of compression ratio on the basis of speech-quality judgments does not compromise speech recognition.

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.

Quantifying the Contribution of Audibility to Recognition of Compression-Amplified Speech

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. 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. 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. 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.