An examination of the practicality of the simplex procedure.

OBJECTIVE: The practical importance of the simplex procedure, a subjective technique used to refine the frequency gain characteristic (FGC) of a hearing aid according to listener preference, was determined for individual listeners by measuring hearing aid benefit using both laboratory studies and field studies. DESIGN: A digital research hearing aid with two memories was used as the test hearing aid. The modified simplex procedure was used to select the FGC judged to yield the best speech clarity in the presence of low-level vent noise and again in higher-level cafeteria noise by 10 experienced hearing aid users. The FGCs assessed by the listeners varied systematically from The National Acoustic Laboratories Revised (NAL-R) response in the amount of low-frequency or high-frequency amplification. The benefit obtained with these two simplex-selected settings was compared with that obtained using the NAL-R FGC. Measures of benefit included speech recognition testing in the laboratory and ratings of speech intelligibility in the field. In the first field study, the two simplex settings were compared. In the second field study, the simplex-selected setting for higher level noise and the NAL-R setting were compared. RESULTS: In the laboratory, the majority of listeners selected an increase in the low-frequency channel gain compared with the NAL-R. Desired high-frequency channel gain was correlated with degree of hearing loss and type of background noise. The benefit as measured using nonsense syllables did not differ significantly among the three fittings, but differences in benefit were measurable with the rating procedure. Five of eight participants noticed a significant difference in their speech understanding in the real world for the FGCs selected in different background noises. Two of seven participants reported significantly better speech intelligibility with a simplex-selected FGC compared with the NAL-R FGC in the real world. The remaining subjects reported similar speech understanding capabilities with both hearing aid settings. CONCLUSIONS: The majority of subjects included in this study selected an FGC with real ear insertion gain different than the NAL-R prescription to improve subjective speech understanding in the laboratory. A small number of these listeners rated the selected FGC as providing improved speech intelligibility over the NAL-R FGC in the real world. This finding indicates that the simplex procedure should be used selectively to modify the NAL-R prescription. A screening technique would be useful in selecting those who might benefit from a modified fitting. The simplex procedure may also prove to be useful in selecting listeners who would benefit from multiple memory hearing aids.

The effect of compression ratio and release time on the categorical rating of sound quality.

Two experiments were carried out to determine how manipulating the compression ratio and release time of a single-band wide dynamic range hearing aid affects sound quality. In experiment I, compression ratio was varied over the range from linear to 10:1 (low compression threshold, attack time = 5 ms, release time = 200 ms). In experiment II, compression ratios of 1.5, 2, and 3:1 were combined with release times of 60, 200, and 1000 ms (attack time = 5 ms). Twenty listeners with sensorineural hearing loss rated the clarity, pleasantness, background noise, loudness, and the overall impression of speech-in-noise (Ventilation, Apartment, Cafeteria) processed through a compression hearing aid. Results revealed that increasing compression ratio caused decreases in ratings on all scales. Increasing release time caused ratings of pleasantness to increase, and ratings of background noise and loudness to decrease. At the 3:1 compression ratio, increasing the release time caused increases in ratings of clarity, pleasantness, and overall impression, and a decrease in background noise. Significant correlations were found between scales. Regression analysis revealed that the contributions of the scales of clarity, pleasantness, background noise, and loudness to the prediction of overall impression differed as a function of the competing noise condition.

Effect of release time in compression hearing aids: paired-comparison judgments of quality.

Paired-comparison judgments of quality were obtained from 20 hearing-impaired listeners for speech processed through simulated compression hearing aids varying in release time (60, 200, 1000 ms) at three different compression ratios (1.5, 2, 3:1) and for three different background noises (ventilation, apartment, cafeteria). Analysis revealed that the main effect of release time did not have a significant effect on perceived quality. The interaction between release time and noise type was found to be significant. While no significant difference in preference for release times was evident for the ventilation noise, the longer release times (200 and 1000 ms) were preferred for the higher level noises (apartment noise, cafeteria noise). Post hoc testing revealed that the mean preference scores for the 200- and 1000-ms release time were significantly greater than that of the 60-ms release time with the competing cafeteria noise (p < 0.05). Analysis of individual subject data revealed statistically significant preferences that differed from the group mean, suggesting that individualized fitting of this parameter of a compression hearing aid might be warranted.

Preferred listening levels for linear and slow-acting compression hearing aids.

OBJECTIVE: The purpose of the present experiment was to determine the relationship between most comfortable listening level and preferred listening levels for linear and slow-acting compression hearing aids as a function of variations in speech and noise level. DESIGN: A digital hearing aid test system was used to simulate six hearing aids having compression ratios of 1, 1.5, 2, 3, 5, and 10:1. Speech was presented in three different noises (vent, apartment, and cafeteria), with speech input level being varied (55, 70, 85 dB SPL). Subjects were 20 listeners with sensorineural hearing loss (half with a dynamic range < or = 30 dB and half with a dynamic range >30 dB). The boundaries of the most comfortable listening range were measured to estimate most comfortable listening level. Preferred listening level was measured by having subjects adjust the output of the hearing aid for satisfactory listening. RESULTS: On average, the deviation of preferred listening level from most comfortable loudness (MCL) was less than 5 dB. Dynamic range, noise type, and input level were all found to have small, but significant, effects on the deviation of preferred listening level from MCL. On average, subjects with a small dynamic range listened slightly below MCL, and subjects with a larger dynamic range listened slightly above MCL. For favorable signal-to-noise ratios, preferred listening levels were highest for high input levels and for conditions that resulted in high output levels before level adjustment. Although the pattern of average performance differed slightly at poorer signal-to-noise ratios, all preferred listening levels were close to MCL. CONCLUSIONS: The gain of a slow-acting compression hearing aid should place the output within 5 dB of MCL. The output for low and medium inputs should approximate MCL and the output for high input levels should be slightly above MCL. This pattern of gain may be obtained with mild compression ratios and a gain rule that places a speech input of 70 dB at MCL.

Effect of compression ratio in a slow-acting compression hearing aid: paired-comparison judgments of quality.

Paired-comparison judgments of quality were obtained from 20 hearing-impaired listeners (half with a small dynamic range and half with a large dynamic range) for speech-in-noise (vent, apartment, and cafeteria) processed through a slow-acting compression hearing aid. Compression ratio was varied (1, 1.5, 2, 3, 5, and 10:1). Compression threshold, attack time, and release time were fixed. Sound quality judgments were significantly affected by compression ratio, noise, and dynamic range. Preference decreased with increasing compression ratio. The selection of compression ratio. The selection of compression ratios < or = 2:1 was significantly higher than of compression ratios > 3:1. Less compression (no compression or 1.5:1) was preferred with the highest level noise (cafeteria noise) than with the lower level noises (vent or apartment). In particular, the small dynamic range group preferred compression with the vent and apartment noises (noise below the compression threshold), but preferred a linear hearing aid with the cafeteria noise (above the compression threshold). The large dynamic range group showed a slightly greater preference for the linear hearing aid for all three noises.