Preferred hearing aid gain in everyday environments

Thirty-three hearing-impaired individuals were each fitted with three hearing aids. The instruments conformed to three frequency-gain prescriptions, differing by a total of 8 dB/octave, with the middle prescription derived using the MSU version 3.0 procedure. The subjects were divided into three matched groups of eleven. Each group used the fitted hearing aids in one of three everyday listening environments representing quiet, reverberant, and noisy situations, respectively. In each listening environment, preferred hearing aid gain for conversationally produced speech was measured in each hearing aid condition for each subject. Preferred gain in daily listening situations was compared to prescribed gain. Results indicated that: (1) preferred gain averaged across all three environments was about equal to prescribed gain, (2) mean preferred gain in each separate environment was substantially different from the prescribed level, (3) volume control adjustments of about +/- 8 dB relative to the prescribed level would be necessary to accommodate the preferred gain settings of the typical hearing aid wearer in daily life. Guidelines are presented for establishing recommended volume control settings for hearing aid users who may be unable to set the volume control independently.

Hearing aid benefit in everyday environments

Hearing aid benefit was measured for three matched groups of eleven hearing-impaired subjects, each serving in one typical listening environment. Benefit was quantified in terms of improvement in intelligibility score for the Connected Speech Test. Each subject was individually fitted with three hearing aids, differing in nominal frequency response slope by a total of 8 dB/octave. Research questions centered on the amount of benefit typically realized in everyday environments and the interactions of this benefit with frequency response and/or visual cues. Results revealed: (1) mean benefit in a living-room type setting was about 24% and significantly greater than in a reverberant setting (7%) and a noisy setting (-1%); (2) despite the relatively large mean difference in benefit between the reverberant and noisy environments, the difference was not statistically significant (p greater than 0.05); (3) the addition of visual cues did not change hearing aid benefit in any tested environment; (4) there was no significant overall trend for any of the three different frequency-response slopes to give superior benefit in any environment; (5) 76% of the subjects achieved significantly different benefit (p less than 0.05) in at least one hearing aid condition when data were considered on an individual basis; and (6) articulation indices in the aided conditions did not successfully predict the observed within-subject benefit differences. Benefit was significantly related to speech reception threshold in the living-room environment. However, in the less favorable environments, benefit and hearing loss were not related despite the fact that benefit varied considerably across subjects.

Evaluation of an in-situ output probe-microphone method for hearing aid fitting verification

Several hearing aid prescription procedures specify frequency gain functions in terms of the desired levels of amplified speech. The most direct method for verifying hearing aid fittings based on these procedures requires a measurement of amplified speech in the ear canal. This paper describes the evaluation of a probe microphone measurement procedure designed to measure amplified speech to verify hearing aid fittings using the MSUv3 prescription procedure. With minor modifications, the same protocol could be used with other prescriptive procedures. Hearing aids were fitted to a group of subjects with sensorineural hearing impairment. Data derived from the fittings were analyzed to assess several issues relating to the procedure itself and to prescriptive fittings in general. The main results were: (1) with over the ear hearing aids, most prescriptions can be matched with an RMS error of 5 dB or less through the frequency range from 500 to 2500 Hz; (2) even though actual fittings usually do not perfectly correspond with their prescriptions, differences among frequency gain prescriptions are preserved in hearing aid fittings if the RMS error of the fitting is 5 dB or less; (3) if sound field stimuli presented to the hearing aid are precisely controlled, an in situ output verification method produces valid results; (4) when hearing aids are fitted with gain similar to that prescribed by the MSUv3 procedure, the maximum comfortable speech input level is typically about 72 dB; (5) the SSPL90 prescription generated by the MSUv3 procedure overestimates desired SSPL90 by 7 dB on average.

The Connected Speech Test version 3: audiovisual administration

The Connected Speech Test (CST) is used to measure the intelligibility of everyday speech; it is intended primarily for quantifying hearing aid benefit. The test consists of 48 passages of conversationally produced connected speech, each passage concerning a familiar topic and comprising 10 sentences. Listeners are apprised of the passage topic in advance and are required to repeat the sentences one at a time. Each passage contains 25 scoring words. The test is recorded audiovisually. In previous papers, we have reported the development of the test materials and investigations of the use of the audio portion with normally hearing and hearing-impaired listeners. Audio versions of the test have been developed for use with normal hearers (CST version 1), and for hearing-impaired listeners (CST version 2). In the present paper, we report a study of the test administered audiovisually. Twenty-six normally hearing subjects responded to audiovisual presentation of all 48 test passages. On the basis of the results, a new version of the test (CST version 3) was generated. In this new version, the test passages are presented in designated sets of four or six passages. The critical difference between two scores is estimated to be the same for both audio and audiovisual administration.

Use of the Connected Speech Test (CST) with hearing-impaired listeners

Two studies were performed in which hearing-impaired subjects responded to the Connected Speech Test (CST). In experiment 1, 40 subjects, divided into four groups according to extent and configuration of hearing loss, responded to the CST version 1 (CSTv1). This version of the test consisted of 57 passages of connected speech: 48 test passages and 9 practice passages. It was developed on the basis of data for normal-hearing listeners. Performance of hearing-impaired listeners for the CSTv1 revealed that, although the passages were equal in average intelligibility for normal hearers, they were not equally intelligible for hearing-impaired persons. Based on results of data analyses, the 57 passages were reconstituted into 28 pairs of passages: 24 test pairs and 4 practice pairs. The pairs were equal in average intelligibility for both normal and hearing-impaired listeners. This form of the test was named the CST version 2 (CSTv2). In experiment 2, an additional 23 hearing-impaired subjects responded to the CSTv2. Critical differences and the slope of the signal to babble ratio (SBR) function were determined for the CSTv2 for hearing-impaired listeners. When two CSTv2 pairs were used per score, the 95% critical difference for hearing-impaired subjects was about 15.5 rationalized arcsine units (rau). The mean SBR function slope for hearing-impaired listeners was 8.5 rau/dB. Comparing the critical difference with the SBR function slope, it may be seen that, for hearing-impaired listeners, differences in intelligibility equivalent to a 2 dB change in SBR can be detected with CST scores based on mean performance across two passage pairs.

Development of the Connected Speech Test (CST)

This paper describes the first phase in the development of the Connected Speech Test (CST). This test of intelligibility of everyday speech has been developed primarily for use as a criterion measure in investigations of hearing aid benefit. The test consists of 48 passages of conversationally produced connected speech. Each passage contains 25 key words for scoring. All passages are of equal intelligibility for the average normal hearer. Key words vary in intelligibility within a passage but span the same intelligibility range in all passages. Several passages are administered, and the results averaged, to yield a single intelligibility score. For pairs of scores, each based on mean performance across 4 randomly-chosen passages, the 95% critical difference is estimated to be about 14 rationalized arcsine units (rau). The performance-intensity function for the CST has a slope of 12 rau/dB signal-to-babble ratio. Investigations of the test are continuing with hearing-impaired listeners.

Acoustic versus electronic modifications of hearing aid low-frequency output

The rated quality and intelligibility of speech processed by hearing aids in which the low-frequency output had been reduced by either electronic modification (low-cut tone controls) or acoustic modification (vented or open earmolds) was investigated. Fifteen subjects with high-frequency hearing loss provided data for nine commercial hearing aids and both high and low background noise levels. Results for both background noise levels indicated that for hearing aids with a low-frequency cut off at or above 750 Hz (as measured in this investigation), the use of a vented or open earmold significantly improved both quality and intelligibility even when it had essentially no effect on the hearing aid's low-frequency output. The implication of the outcome is that for an individual with essentially normal low-frequency sensitivity and a high-frequency hearing loss, an earmold incorporating an opening should be used whenever possible, even though it may not be used for the purpose of controlling low-frequency amplification.