The selection and validation of output sound pressure level in multichannel hearing aids.

OBJECTIVE: To validate the Australian National Acoustic Laboratories' (NAL) procedure for prescribing output sound pressure level (OSPL) for multichannel hearing aids (Dillon & Storey, 1998) DESIGN: The NAL OSPL prescriptive procedure for multichannel hearing aids was used to calculate Predicted OSPL, Predicted Maximum Acceptable OSPL and Predicted Minimum Acceptable OSPL for 20 subjects with sensorineural hearing loss fitted with a 2-channel linear hearing aid. Subjects rated the speech clarity and quality of average (65 dBA) and loud (80 dBA) speech, in quiet and in noise, with the hearing aid set to a number of OSPL settings. These data were used to evaluate the validity of the Predicted OSPL. Frequency-specific loudness discomfort levels (LDLs) were measured to determine whether use of measured LDLs would improve the accuracy of the prediction. RESULTS: The Predicted Minimum Acceptable OSPL was in good agreement with the measured minimum acceptable OSPL for both the low- and high-frequency channels. The Predicted Maximum Acceptable OSPL was in good agreement with the measured maximum acceptable OSPL for the low-frequency channel, but was only a fair predictor for the high-frequency channel. The use of measured LDLs rather than predicted LDLs did little to improve the accuracy of the fitting. A direct comparison between the NAL single-channel and multichannel prescribed OSPL settings showed that most listeners rated speech clarity higher for the multichannel settings. CONCLUSIONS: In two channel hearing aids, the NAL Predicted Minimum Acceptable OSPL and Predicted Maximum Acceptable OSPL are reasonable predictors of minimum and maximum OSPL levels measured using sound clarity and quality ratings. The results of this study support the use of the NAL prescriptive formula for setting OSPL in multichannel hearing aids. Such settings should be verified by having the listener rate the loudness of an intense speech signal. If tolerance problems are evident, the OSPL in the high-frequency channel(s) should be reduced first.

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.

Selective visual masking in speechreading.

Using digital video technology, selective aspects of a face can be masked by identifying the pixels that represent it and then, by adjusting the gray levels, effectively eliminate that facial aspect. In groups of young adults with normal vision and hearing, consonant-viseme recognition was measured for closed sets of vowel-consonant-vowel disyllables. In the first experiment viseme recognition was measured while the tongue and teeth were masked and while the entire mouth was masked. The results showed that masking of the tongue and teeth had little effect on viseme recognition, and when the entire mouth was masked, participants continued to identify consonant visemes with 70% or greater accuracy in the /a/ and /(ball)/ vowel contexts. In the second experiment, viseme recognition was measured when the upper part of the face and the mouth were masked and when the lower part of the face and the mouth were masked. The results showed that when the mouth and the upper part of the face were masked, performance was poor, but information was available to identify the consonant-viseme /f/. When the mouth and the lower part of the face were masked, viseme recognition was quite poor, but information was available to discriminate the consonant-viseme /p/ from other consonant visemes.

Quantifying the relation between speech quality and speech intelligibility.

The purpose of the present research was to examine the relation between speech quality and speech intelligibility. Speech quality measurements were made using continuous discourse and a category rating procedure for the following dimensions: intelligibility, pleasantness, loudness, effort, and total impression. Measurements were made using a group of listeners with normal hearing for a set of stimulus conditions in which intelligibility varied, and for a set of stimulus conditions in which intelligibility was held constant near 100%. When ratings were made for a set of stimulus conditions in which intelligibility was allowed to vary (a) intersubject reliability was high (i.e., different listeners interpreted the dimensions in a similar manner); and (b) the speech quality dimensions of intelligibility, effort, and loudness were indistinguishable. When ratings were made for a set of stimulus conditions in which intelligibility was held constant (a) intersubject reliability was reduced, indicating that different listeners interpreted the dimensions in different ways; (b) most listeners rated each dimension differently, indicating that the dimensions were unique; and (c) across listeners, no single dimension was highly correlated with total impression. These results can be used in order to examine the relation between speech quality and speech intelligibility.

Measurement of speech quality as a tool to optimize the fitting of a hearing aid.

The purpose of the present research was to develop a theoretical basis for the adjustment of hearing aid frequency response based on speech quality measurements. Speech quality measurements were made using continuous discourse and a category rating procedure for the following dimensions: intelligibility, pleasantness, loudness, effort, noisiness, and total impression. Speech quality ratings were obtained from a group of listeners with hearing loss who wore hearing aids. The stimulus conditions simulated hearing aid frequency response alterations within a frequency response range where intelligibility was held constant at or near 100%. The subject ratings revealed that (a) different listeners interpreted the individual dimensions in different ways; (b) within listeners, most of the dimensions were unique; that is, they were rated differently; and (c) across listeners, pleasantness was the dimension most highly correlated with total impression.

Is useful speech information carried by fibers with high characteristic frequencies?

It has been proposed that auditory-nerve fibers with characteristic frequencies (CFs) above the speech-frequency range are important in speech perception when the signal-to-noise ratio in the speech frequency range is low [S. Greenberg, J. Phon. 16, 139-149 (1988)]. If this is true, then it might be expected that recognition of speech at low signal-to-noise ratios would worsen with the addition of high-pass noise sufficiently intense to mask information in high-CF fibers. This hypothesis was tested for closed-set recognition of vowels and spondees. Recognition was measured as a function of signal-to-noise ratio in speech-shaped noise, with and without an intense high-pass noise. The addition of high-pass noise did not degrade vowel recognition. Spondee recognition decreased with the addition of the high-pass noise at the highest levels of the speech-shaped noise. However, this same decrease was seen when the spondees were low-pass filtered to simulate downward spread of masking by the high-pass noise. This indicates that the decrease in spondee recognition with high-pass noise was due to masking of information in fibers with CFs in the speech range. Overall, these results suggest that fibers whose CFs are above the speech range are not necessary for speech perception in noise or in quiet.

Frequency selectivity and consonant intelligibility in sensorineural hearing loss.

The relations between frequency selectivity and consonant intelligibility were investigated in subjects with sensorineural hearing loss in an attempt to derive predictive indices. Three matched pairs of subjects with similar audiometric configurations (high-frequency, flat or low-frequency hearing loss) but significantly different word-intelligibility scores were tested. Characteristics of psychophysical tuning curves (PTCs) for high- and low-frequency probes were compared with speech-intelligibility performance for high- and low-frequency consonant-vowel syllables. Frequency-specific relations between PTC characteristics and consonant-intelligibility performance were observed in the subject pairs with high-frequency and flat sensorineural hearing loss. Corresponding results for the subject pair with low-frequency sensorineural hearing loss were equivocal.