Search

Hearing-impaired listeners are known to suffer from reduced speech intelligibility in noise, even if sounds are above their hearing thresholds. This study examined the possible contribution of reduced acuity of intensity coding to this problem. The "distortion-sensitivity model" was used: the effect of reduced acuity of auditory intensity coding on intelligibility was mimicked by an artificial distortion of the speech intensity coding, and the sensitivity to this distortion for hearing-impaired listeners was compared with that for normal-hearing listeners. Stimuli (speech plus noise) were wavelet coded using a Gaussian wavelet (1/4-octave bandwidth). The intensity coding was distorted by multiplying the modulus of each wavelet coefficient by a random factor. Speech-reception thresholds (SRTs) were measured for various degrees of intensity perturbation. Hearing-impaired listeners were classified as suffering from suprathreshold deficits if intelligibility of undistorted speech was worse than predicted from audibility by the speech intelligibility index model [ANSI, ANSI S3.5-1997 (1997)]. Hearing-impaired listeners without suprathreshold deficits were as sensitive to the intensity distortion as the normal-hearing listeners. Hearing-impaired listeners with suprathreshold deficits appeared to be less sensitive. Results indicate that reduced acuity of auditory intensity coding may be a factor underlying reduced speech intelligibility in noise for the hearing impaired.

In a previous study [Noordhoek et al., J. Acoust. Soc. Am. 105, 2895-2902 (1999)], an adaptive test was developed to determine the speech-reception bandwidth threshold (SRBT), i.e., the width of a speech band around 1 kHz required for a 50% intelligibility score. In this test, the band-filtered speech is presented in complementary bandstop-filtered noise. In the present study, the performance of 34 hearing-impaired listeners was measured on this SRBT test and on more common SRT (speech-reception threshold) tests, namely the SRT in quiet, the standard SRT in noise (standard speech spectrum), and the spectrally adapted SRT in noise (fitted to the individual's dynamic range). The aim was to investigate to what extent the performance on these tests could be explained simply from audibility, as estimated with the SII (speech intelligibility index) model, or require the assumption of suprathreshold deficits. For most listeners, an elevated SRT in quiet or an elevated standard SRT in noise could be explained on the basis of audibility. For the spectrally adapted SRT in noise, and especially for the SRBT, the data of most listeners could not be explained from audibility, suggesting that the effects of suprathreshold deficits may be present. Possibly, such a deficit is an increased downward spread of masking.

The demand on extra effort and concentration during listening are notorious handicapping effects of hearing impairment as is shown by self-assessment studies. In an attempt to explore new ways of assessing hearing handicap, the present study focuses on an objective measure of mental effort during listening. Pupil dilatation is used as the index of mental effort. Results for 14 hearing-impaired and 14 normal hearing listeners show a relation between pupil dilatation and difficulty in speech reception in noise, as manipulated by the speech-to-noise ratio. In addition the study shows that, with regard to effort and concentration, hearing-impaired subjects benefit less than normals from easier listening situations (e.g. at 5 dB above the individual speech-reception threshold). The results show a significant correlation between self-rated handicap and pupil dilatation.

Although required for many practical purposes, adequate measures of hearing disability are not yet available. In an attempt to identify a set of performance tests for predicting hearing disability in daily life the relationship between self-reported disability scores and measures of auditory disability was examined. The Amsterdam Inventory was completed by 51 respondents aged 30 to 70 years who also performed on various tests. Earlier factor analysis of the inventory scores resulted in the distinction of five aspects of auditory disability. Stepwise multiple regression analysis in the present study shows that the tests describe and differentiate quite well between these five aspects. Multiple correlation coefficients range from R = 0.60 to R = 0.74.

In a series of experiments, we introduced peaks of 10, 20, and 30 dB, in various combinations, onto a smooth reference frequency response. For each of the conditions, we evaluated speech intelligibility in noise, using a test as developed by Plomp and Mimpen (1979), and sound quality (for both speech and music), using a rating-scale procedure. We performed the experiments with 26 listeners with sensorineurally impaired hearing and 10 listeners with normal hearing. Signal processing was accomplished digitally; for each listener, the stimuli were filtered and subsequently amplified so that the average speech spectrum was well above the threshold of hearing at all frequencies. The results show that, as a result of the introduction of peaks onto the frequency response, speech intelligibility is affected more for the listeners with impaired hearing than for those with normal hearing. Sound-quality judgments tend to be less different between the listener groups. Conditions with 30-dB peaks especially show serious effects on both speech intelligibility and sound quality.

The long-term average frequency spectrum of speech was modified to 25 target frequency spectra in order to determine the effect of each of these spectra on speech intelligibility in noise and on sound quality. Speech intelligibility was evaluated using the test as developed by Plomp and Mimpen (1979), whereas sound quality was examined through judgments of loudness, sharpness, clearness, and pleasantness of speech fragments. Subjects had different degrees of sensorineural hearing loss and sloping audiograms, but not all of them were hearing aid users. The 25 frequency spectra were defined such that the entire dynamic range of each listener, from 5 dB above threshold to 5 dB below UCL, was covered. Frequency shaping of the speech was carried out on-line by means of Finite Impulse Response (FIR) filters. The tests on speech reception in noise indicated that the Speech-Reception Thresholds (SRTs) did not differ significantly for the majority of spectra. Spectra with high levels, especially at low frequencies (probably causing significant upward spread of masking), and also those with steep negative slopes resulted in significantly higher SRTs. Sound quality judgments led to conclusions virtually identical to those from the SRT data: frequency spectra with an unacceptably low sound quality were in most of the cases significantly worse on the SRT test as well. Because the SRT did not vary significantly among the majority of frequency spectra, it was concluded that a wide range of spectra between the threshold and UCL levels of listeners with hearing losses is suitable for the presentation of speech energy. This is very useful in everyday listening, where the frequency spectrum of speech may vary considerably.

This paper examines the relations among the spectral contrast needed for speech intelligibility, hearing loss for speech in noise, and auditory filter bandwidth. Fifteen hearing-impaired listeners with relatively flat, mild-to-moderate sensorineural losses and eight normal-hearing listeners participated in the study. The spectral contrast needed for speech intelligibility was determined by reducing spectral contrast in the speech signal and measuring the reduction in contrast beyond which the speech-reception threshold (SRT) for sentences in noise increases. Reduction of spectral contrast was accomplished by smearing the envelope of the squared short-time fast Fourier transform by a convolution with a Gaussian-shaped filter, and overlapping additions to reconstruct a continuous signal. Auditory filter bandwidth was determined by estimating auditory filter shapes at center frequencies of 0.8, 1.6, and 3.2 kHz, using a notched-noise masking paradigm. The results show that limited resolution of spectral contrast is only loosely associated with hearing loss for speech in noise. Moreover, the correlations between the SRT for unsmeared speech and the auditory filter bandwidth at various frequencies were weak.

A multichannel signal-processing hearing aid in which the gain is controlled by the level of the minima in the sound envelope [outlined by Festen et al., 1990] was evaluated with hearing-impaired listeners. This evaluation is an extension to the work reported by van Dijkhuizen et al. (1990). A first experiment focused on the speech-reception threshold (SRT), i.e. the S/N ratio for 50% intelligibility. The greatest benefit in terms of the SRT from frequency-dependent control of the amplification is expected in conditions where the spectrum of noise exceeds strongly that of the speech in a limited frequency region. In these conditions frequency-dependent amplification may reduce upward spread of masking. We investigated the upper limit of this benefit in conditions of intense frequency-limited interfering noise. Speech and noise were both spectrally shaped according to the line bisecting the listener's dynamic range; however, the level of the noise in one octave band (0.25-0.5 or 0.5-1 kHz) was increased by 20 dB. The results show that frequency-selective attenuation of the signal in the octave band with the 20-dB increase of noise is more beneficial than wide-band gain control, and gives a decrease in SRT of up to 4 dB relative to a condition without gain control. In a subsequent experiment we investigated, for several very common interfering sounds, the effect of controlling the gain by the minima in the signal envelope on both the SRT and the perceived noisiness. Results show that the condition with gain control does not affect the SRT for sentences in the presence of everyday interfering sounds having spectra that are roughly comparable to that of the speech signal; however, it substantially reduces the perceived noisiness. In line with our expectations, the effect of the gain control on the signal was very small for a single voice, and it was greatest in case of sounds with a more or less continuous character (e.g. stationary noise, music). For these last sounds it was found that the growth in perceived noisiness with the increase of input level is equivalent to the growth produced by only about one-fifth of the increase in input level (in decibels) in a condition without gain control.

The speech-reception threshold (SRT) for sentences presented in a fluctuating interfering background sound of 80 dBA SPL is measured for 20 normal-hearing listeners and 20 listeners with sensorineural hearing impairment. The interfering sounds range from steady-state noise, via modulated noise, to a single competing voice. Two voices are used, one male and one female, and the spectrum of the masker is shaped according to these voices. For both voices, the SRT is measured as well in noise spectrally shaped according to the target voice as shaped according to the other voice. The results show that, for normal-hearing listeners, the SRT for sentences in modulated noise is 4-6 dB lower than for steady-state noise; for sentences masked by a competing voice, this difference is 6-8 dB. For listeners with moderate sensorineural hearing loss, elevated thresholds are obtained without an appreciable effect of masker fluctuations. The implications of these results for estimating a hearing handicap in everyday conditions are discussed. By using the articulation index (AI), it is shown that hearing-impaired individuals perform poorer than suggested by the loss of audibility for some parts of the speech signal. Finally, three mechanisms are discussed that contribute to the absence of unmasking by masker fluctuations in hearing-impaired listeners. The low sensation level at which the impaired listeners receive the masker seems a major determinant. The second and third factors are: reduced temporal resolution and a reduction in comodulation masking release, respectively.

Most sensorineurally hearing-impaired listeners need a better signal-to-noise ratio for speech reception than normal-hearing listeners do. This aspect of hearing loss is probably related to deterioration of the signal-analysing power of the ear. In an impaired ear, noise in one frequency region may have considerable masking effects in other frequency regions. To prevent the hearing-impaired listener from excessive masking, we need an adaptive hearing aid that selectively amplifies only those frequency bands with a signal quality that can contribute to intelligibility. It suffices to present signals in other frequency regions at a just-audible level. A signal-processing scheme is proposed that meets these requirements. The audiological reasoning for the spectral characteristics of such a hearing aid is illustrated with the Articulation Theory. The temporal characteristics required for adaptation to changes in the acoustic environment are discussed in terms of the Modulation Transfer Function. It is shown that amplitude compression with short time constants seriously reduces the quality of speech transmission as expressed in the Speech Transmission Index.

In a study on the effects of a frequency-dependent automatic gain control in hearing-aids, two experiments were carried out with hearing-impaired listeners. In the first experiment, the effect of varying the amplitude-frequency response on the speech-reception threshold (SRT) of sentences presented in noise was studied. The noise had the same spectrum as the long-term average spectrum of the sentences. Results suggest that the amplitude-frequency response may vary within a range from, roughly, -3 to +10 dB/oct relative to the bisector of the dynamic range, without giving an increase in SRT larger than 2 dB. In the second experiment, the effect on masked SRT of adjusting the amplitude-frequency response to situations of seriously interfering low-frequency noise was studied. Again, the noise had a spectrum identical with the long-term average spectrum of the sentences, but this time the noise level in one octave band was increased by 20 dB. Preliminary results indicate that a selective attenuation of the signal in the band containing the extra noise may give a decrease of masked SRT up to 4.5 dB.

The binaural free-field speech-reception threshold (SRT) in 70-dBA noise was measured with conversational sentences for 24 hearing-impaired subjects without hearing aids, with a hearing aid left, right, and left plus right, respectively. The sentences were always presented in front of the listener and the interfering noise, with a spectrum equal to the long-term average spectrum of the sentences, was presented either frontally, from the right, or from the left side. For subjects with only moderate hearing loss, PTA (average air-conduction hearing level at 500, 1000, and 2000 Hz) less than 50 dB, the SRT in 70-dBA noise in both ears is determined by the signal-to-noise ratio even if only one hearing aid is used. For larger hearing losses the SRT appears to be partly determined by the absolute threshold. In conditions with a high noise level relative to the absolute threshold, in which case for both ears the SRT is determined by the signal-to-noise ratio, a second hearing aid, just as a monaural hearing aid, generally does not improve the SRT. However, in the case of a high hearing level, or a low noise level, in which a monaural hearing aid is profitable, the use of two hearing aids is even more profitable. In a separate experiment, acoustic head shadow was measured at the entrance of the ear canal and at the microphone location of a hearing aid. It appeared that, for a lateral noise source and speech frontal, the microphone position of behind-the-ear hearing aids has a negative effect on the signal-to-noise ratio of 2-3 dB.

Relations between auditory functions, as expressed by coefficients of correlation, were studied for a group of 22 sensorineurally hearing-impaired subjects with moderate losses. In addition to the audiogram, we measured frequency resolution, temporal resolution, and speech reception in quiet and in noise. Frequency resolution was derived from masking with comb-filtered noise and from the psychophysical tuning curve, for both paradigms in simultaneous and in nonsimultaneous masking. The critical ratio was also determined. Temporal resolution was determined with intensity-modulated noise and from backward and forward masking. All tests were performed at 1000 Hz. Correlations among tests were gathered in a matrix and subjected to a principal-components analysis. It turned out that tests on frequency resolution form a cluster, and are approximately independent of audiometric loss. Furthermore, hearing loss for speech in noise is closely allied to frequency resolution, whereas hearing loss for speech in quiet is governed by audiometric loss.

In an evaluation of frequency-dependent automatic gain-control systems in hearing aids, the effect of varying the amplitude-frequency response on the speech-reception threshold (SRT) for sentences in noise is studied for 20 hearing-impaired listeners. The noise has a spectrum identical to the long-term average spectrum of the sentences. Speech and noise are shaped by the same amplitude-frequency response; their spectra are varied relative to the bisector of the individual's dynamic range. In four experimental conditions, the effect of a steady-state amplitude-frequency response is studied. Steepening the negative spectral slope of speech and noise appears to cause an increase of masked SRT, possibly due to increased effect of upward spread of masking. The effect of a single transition of the amplitude-frequency response between 10 and -10 dB/oct halfway through the sentence seems to be related to the effect for the fixed -10-dB/oct condition. Two transition times are tested. For a transition time of 0.25 s, the SRT is only a little higher than for 1 s. The results suggest that the amplitude-frequency response may be varied in time without having a detrimental effect on the masked SRT of sentences for hearing-impaired listeners as long as strongly negatively sloping spectra are avoided.