Influence of audibility and distortion on the recognition of reverberant speech for children and adults with hearing aid amplification

Background: Adults and children with sensorineural hearing loss (SNHL) have trouble understanding speech in rooms with reverberation when using hearing aid amplification. While the use of amplitude compression signal processing in hearing aids may contribute to this difficulty, there is conflicting evidence on the effects of amplitude compression settings on speech recognition. Less clear is the effect of a fast release time for adults and children with SNHL when using compression ratios derived from a prescriptive procedure. Purpose: To determine whether release time impacts speech recognition in reverberation for children and adults with SNHL and to determine if these effects of release time and reverberation can be predicted using indices of audibility or temporal and spectral distortion. Research Design: A quasi-experimental cohort study. Participants used a hearing aid simulator set to the Desired Sensation Level algorithm m[i/o] for three different amplitude compression release times. Reverberation was simulated using three different reverberation times. Participants: Participants were 20 children and 16 adults with SNHL. Data Collection and Analyses: Participants were seated in a sound-attenuating booth and then nonsense syllable recognition was measured. Predictions of speech recognition were made using indices of audibility, temporal distortion, and spectral distortion and the effects of release time and reverberation were analyzed using linear mixed models. Results: While nonsense syllable recognition decreased in reverberation; release time did not significantly affect nonsense syllable recognition. Participants with lower audibility were more susceptible to the negative effect of reverberation on nonsense syllable recognition. Conclusions: We have extended previous work on the effects of reverberation on aided speech recognition to children with SNHL. Variations in release time did not impact the understanding of speech. An index of audibility best predicted nonsense syllable recognition in reverberation and, clinically, these results suggest that patients with less audibility are more susceptible to nonsense syllable recognition in reverberation.

Use of Emotional and Neutral Speech in Evaluating Compression Speeds

Background Emotional speech differs from neutral speech in its envelope characteristics. Use of emotional speech materials may be more sensitive for evaluating signal processing algorithms that affect the temporal envelope. Purpose Subjective listener preference was compared between variable speed compression (VSC) and fast acting compression (FAC) amplitude compression algorithms using neutral and emotional speech. Research Design The study used a single-blinded, repeated measures design. Study Sample Twenty hearing-impaired (HI) listeners with a bilaterally symmetrical, mild- to-moderately severe sensorineural hearing loss and 21 listeners with normal hearing (NH) participated. Intervention Speech was processed using FAC and VSC algorithms. Data Collection and Analysis A paired-comparison paradigm assessed subjective preference for FAC versus VSC using emotional and neutral speech materials. The significance of subjective preference for compression algorithm (FAC or VSC) was evaluated using a linear mixed effects model at each combination of stimulus type (emotional or neutral speech) and hearing group (NH or HI). Results HI listeners showed a preference for VSC over FAC when listening to emotional speech. The same listeners showed a nonsignificant, preference for VSC over FAC when listening to neutral speech. NH listeners showed preference for VSC over FAC for both neutral and emotional speech materials. Conclusion These results suggest that the subjective sound quality of emotional speech is more susceptible than neutral speech to changes in the signal introduced by FAC. Clinicians should consider including emotional speech materials when evaluating listener preference for different compression speeds in the clinic.

Effects of Fast, Slow, and Adaptive Amplitude Compression on Children’s and Adults’ Perception of Meaningful Acoustic Information

Background: Fast- and slow-acting amplitude compression parameters have complementary strengths and weaknesses that limit the full benefit of this feature to hearing aid users. Adaptive time constants have been suggested in the literature as a means of optimizing the benefits of amplitude compression. Purpose: The purpose of this study was to compare the effects of three amplitude compression release times (slow, fast, and adaptive) on children’s and adults’ accuracy for categorizing speech and environmental sounds. Research Design: Participants were asked to categorize speech or environmental sounds embedded in short trials containing low-level playground noise. Stimulus trials included either a high-level environmental sound followed by a lower-level speech stimulus (word) or a high-level speech stimulus followed by a lower-level environmental sound. The listeners responded to the second (low-level) stimulus in each trial. The two stimuli overlapped temporally in half of the trials but not in the other half. The stimulus trials were processed to simulate amplitude compression having fast (40 msec), slow (800 msec), or adaptive release times. The adaptive-compression parameters operated in a slow fashion until a sudden increase/decrease in level required a rapid change in gain. Study Sample: Participants were 15 children and 26 adults with hearing loss (HL) as well as 20 children and 21 adults with normal hearing (NH). Data Collection and Analyses: Performance (in % correct) was arcsine transformed and subjected to repeated-measures analysis of variance with pairwise comparisons of significant main effects using Bonferroni adjustments for multiple comparisons. Results: Overall, the performance of listeners with HL was poorer than that of the listeners with NH and performance for the environmental sounds was poorer than for the speech stimuli, particularly for the adults and children with HL. Significant effects of age group, stimulus overlap, and compression speed were observed for the listeners with NH, whereas effects of stimulus overlap and compression speed were found for the listeners with HL. Whereas listeners with NH achieved optimal performance with slow-acting compression, the listeners with HL achieved optimal performance with adaptive compression. Conclusions: Although slow, fast, and adaptive compression affects the acoustic signal in a subtle fashion, amplitude compression significantly affects perception of speech and environmental sounds.

Active Artificial Hair Cells Using Nonlinear Feedback Control

There is interest in developing devices that mimic the sound transduction of the cochlear hair cells. Current artificial hair cell (AHC) designs have focused on passive transduction of sound into electrical signals. However, measurements inside living cochleae have revealed that a nonlinear amplification is at work in mammalian hearing. This amplification lowers the threshold for sound detection allowing mammals to hear faint sounds. The nonlinearity results in an amplitude compression whereby a large range of sound pressure levels produces a smaller range of displacements. This compressive nonlinearity gives the ear a large dynamic range. This work seeks to develop and analyze active artificial hair cells which employ a bio-inspired amplification to improve performance. This paper examines two artificial hair cell designs. The first is an 18.5 in long aluminum cantilever beam which is excited and controlled using piezoelectric actuators along the length of the beam. The second design is a one inch piezoelectric bimorph beam subject to a base excitation. In both cases a nonlinear feedback control law is implemented which reduces the beam’s linear viscous damping and introduces a cubic damping term. Model and experimental results show the control law amplified the response of the artificial hair cell to low excitation levels near the resonance frequency. Increasing input levels produced a compressive nonlinearity at resonance similar to that observed in measurements from mammalian cochleae. This work could lead to the development of new bio-inspired sensors with a lower threshold of detection, improved frequency sensitivity, and larger dynamic range.