Evaluation of phoneme compression schemes designed to compensate for temporal and spectral masking in background noise.

The effect of phonemic compression has been studied on speech intelligibility in background noise in hearing-impaired listeners with moderate-to-severe high-frequency losses. One configuration, anti-upward-spread-of-masking (anti-USOM) focuses on a release from spectral masking of high-frequency speech cues by selective spectral tilting. Release from temporal masking is the main goal of a second configuration, high-pass filtered compression (HFC), which reduces the speech modulations within a high-pass filtered compression channel. Speech intelligibility was measured with consonant-vowel-consonant (CVC) words in a multi-talker babble and a single-talker background noise. Anti-USOM has a significant negative effect on the phoneme scores in background noise. HFC compression tends to improve vowel intelligibility in a single-talker background noise, especially for the listeners with a relatively poor speech score. In a multi-talker babble noise the effects of HFC compression tend to be negative. It can be concluded that no significant release from spectral or temporal masking is obtained by the applied processing.

Speech intelligibility in noise with fast compression hearing aids.

The effect of fast (syllabic) compression with overshoot reduction was studied in moderately hearing-impaired and in severely hearing-impaired listeners in quiet and in noisy situations. A test battery of daily masking noises was selected using multidimensional scaling techniques. Four relevant noises were selected: a multi-talker babble, the noise in an industrial plant, in a printing office and a city-noise background. The speech measurements show that only selected patients benefit from syllabic compression, i.e. listeners with a poor speech discrimination score. The effect in noisy surroundings was tested at the critical signal-to-noise ratio of each patient, showing whether they benefited from compression in the most critical condition or not. It turns out that the effect depends largely on the speech discrimination score and the modulation of the noise signal. When the speech discrimination score is good, compression tends to impair the results. When the speech discrimination score is poor, compression helps if the noise is modulated.

Compression and its effect on the speech signal.

Compression systems are often used in hearing aids to increase the wearing comfort. A patient has to readjust frequently the gain of a linear hearing aid because of the limited dynamic hearing range and the changing acoustical conditions. A great deal of attention has been given to the static parameters but very little to the dynamic parameters. We present a general method to describe the dynamic behavior of a compression system by comparing modulations at the output with modulations at the input. The use of this method resulted in a single parameter describing the temporal characteristics of a compressor, the cut-off modulation frequency. In this paper its value is compared with known properties of running speech. A limitation of this method is the use of only small modulation depths, and the consequence of this limitation is tested. The use of this method is described for an experimental digital compressor developed by the authors, and the effects of some temporal parameters such as attack and release time are studied. This method shows the rather large effects of some of the parameters on the effectiveness of a compressor on speech. This method is also used to analyze two generally accepted compression systems in hearing aids. The theoretical method is next compared to the effects of compression on the distribution of the amplitude envelope of running speech, and it could be shown that single-channel compression systems do not reduce the distribution width of speech filtered in frequency bands. This finding questions the use of compression systems for fitting the speech banana in the dynamic hearing range of impaired listeners.

Hearing loss in students at a conservatory.

We tend not to think of music as noise but as a pleasant sound. Yet, played loud enough, music can become a threat to the human ear. The question arises whether professional musicians suffer from hearing losses caused by their playing of music. The hearing of students at the Rotterdam conservatory was studied; medical students served as a reference group. High percentages of audiometric noise dips (16%) and high-frequency losses (20%) were found in students of the conservatory, as well as a high percentage (72%) of extended high-frequency losses relative to the reference curves of Dreschler et al. Surprisingly, an equally large (and in the high-frequency region an even higher) percentage of hearing losses was found in the control group of medical students with the same median age. In sum, the exposure of conservatory students to the practice of music has as yet had no effect on their hearing.

Prevalence of hearing impairment and hearing complaints in older adults: a study in general practice.

In one general practice, 660 people aged 60 years or over were screened by means of pure tone audiometry and a specific questionnaire to assess the prevalence of hearing impairment and hearing complaints. Hearing impairment was defined as an average loss of 35 dB or more in the 1, 2 and 4 kHz frequencies in one or both ears. In total, 37.4% (95% CI, 33.3-41.1%) of the participants was hearing impaired. The prevalence was higher in men (55.1%) than in women (44.9%) and clearly increased with age in both sexes. The prevalence of hearing complaints in terms of hearing difficulties and/or tinnitus, was 37.3% (95% CI, 33.6-41.0%), and increased with age, especially in women. Of the subjects with hearing impairment, 64.4% reported hearing complaints. Of the subjects without hearing impairment, 21.1% experienced hearing complaints. This study suggests that screening older adults with relatively simple methods, may identify a large proportion of men and women in general practice with hearing problems. Providing information to both patients and general practitioners about the possibilities of hearing improvement is a crucial step in making people become more aware of hearing problems. This could ultimately lead to improvement of the quality of life of older men and women with hearing problems.

Impaired speech perception in noise in patients with a normal audiogram.

Patients with normal hearing acuity but complaining of impaired speech perception in noisy conditions were submitted to a test battery including the measurement of the speech reception threshold in noise and central auditory tests. In 95% of the cases the complaint was corroborated by an abnormal speech reception threshold (SRT) in noise. Central auditory tests were abnormal in 65% of the patients. Auditory brainstem-evoked potentials were measured in some of these patients and showed normal results in almost all cases. No specific relationship could be found between an elevated SRT in noise or an abnormal central test result.

Directional hearing aid based on array technology.

The hearing impaired often have great difficulty understanding speech in surroundings with background noise or reverberation. A directional hearing aid might be beneficial in reducing background noise in relation to the desired speech signal. To this end microphone systems were developed with strongly directional characteristics, using array techniques. Considerable attention was paid to optimization and stability. Free-field simulations of several robust models showed that a directivity index of 9 dB can be obtained. Simulations were verified with a laboratory model. Based on simulations and measurements, two portable prototypes were developed and tested using a KEMAR-manikin. The KEMAR-measurements showed that the two prototypes gave an improvement of the signal to noise ratio of 7 dB in a fully diffuse sound field. The benefit of these microphone arrays for the hearing impaired was tested in a sound insulated room. One loudspeaker was placed in front of the listener simulating the partner in a discussion, and a diffuse background noise was produced by eight loudspeakers placed on the corners of a cube. The hearing impaired subject was seated in the centre of the cube. The speech-reception threshold in noise for simple Dutch sentences was determined with a normal single omni-directional microphone and with one of the prototypes. The results of the listening tests with 45 hearing impaired subjects showed an average improvement of the S/N-ratio of 7.0 dB for monaural fitting.

Syllabic compression and speech intelligibility in hearing impaired listeners.

Syllabic compression has not been shown unequivocally to improve speech intelligibility in hearing-impaired listeners. This paper attempts to explain the poor results by introducing the concept of minimum overshoots. The concept was tested with a digital signal processor on hearing-impaired subjects. The results show that moderate syllabic compression may raise speech intelligibility, as long as overshoots are minimized and relatively short time constants are used. Frequency equalization also contributes to speech intelligibility.

Effect of hearing aids on speech perception in noisy situations.

Hearing-impaired people often complain about poor speech intelligibility when they are in noisy surroundings. Several authors have published data suggesting that hearing aids add distortion to the signal and thus make speech less intelligible. Given the tendency of people to speak louder in competing noise, it means that some patients might understand better in such circumstances without an aid than with an aid. The present study investigated the effect of modern commercial hearing aids on the ability of patients to understand speech monaurally in noisy circumstances. The results in general showed speech to be equally understandable with and without an aid if the effects of presentation level (amplification) are excluded. The results, however, varied considerably between patients. Statistical analysis showed that the variance could be explained primarily by the degree of high-frequency loss and the slope of the audiogram. Patients with steep sloping audiograms understand better and patients with a conductive hearing loss component understand less in noisy circumstances with a hearing aid. Speech intelligibility in noisy situations did not correlate at all with the hearing loss averaged over the mid-frequencies (500, 1,000 and 2,000 Hz) but it did correlate with the high-frequency hearing loss (2,000 and 4,000 Hz) and the slope of the audiogram, indicating the importance of high-frequency emphasis in hearing aids.

Methods of analysis of large numbers of audiograms.

The analysis of large numbers of audiograms raises the question if and how we can reduce the amount of data without discarding essential information. The present paper compares two ways of data reduction: principal-component analysis and curve fitting. The methods are tested on the audiograms of a large family suffering from a dominant hereditary, progressive hearing loss, beginning in the high frequencies. It is shown that principal-component analysis rejects information on the shape of the audiogram, as do all methods generally referred to as factor analysis. The information concerned is essential for our understanding of the patient's ability to discriminate speech. Curve-fitting procedures are shown to be effective in data reduction.

Follow-up study in a family with dominant progressive hereditary sensorineural hearing impairment. I. Analysis of hearing deterioration.

An autosomal-dominant progressive hereditary sensorineural hearing loss with onset in the high frequencies was studied in a Dutch family with 105 affected individuals. The hearing loss was analyzed by a curve-fitting procedure. A hyperbolic tangent was fitted into the audiogram. Five characteristics were distinguished: the high-frequency plateau, the low-frequency plateau, the slope, the low cut-off frequency and the high cut-off frequency. Six phases are recognizable in the development of the hearing loss, which is characterized by a two-step progression, first in the high and then in the low frequencies. Great differences in the rate of the deterioration were observed. The age of onset is between 5 and 15 years.

Follow-up study in a family with dominant progressive hereditary sensorineural hearing impairment. II. Clinical aspects.

An autosomal-dominant progressive sensorineural hearing loss in six generations of a large family with 105 affected members was studied. The pattern of inheritance is autosomal dominant with an almost complete penetrance. The age of onset is between 5 and 15 years. Individuals with a normal audiogram at the age of 15 and over will not develop the disorder. Different generations show an identical pattern of progression. Because the age of onset is the same, anticipation is excluded. The hearing loss is symmetrical. Over 40 years, low-frequency losses are greater in females than in males. Epistasis possibly plays a role since affected individuals in branch II of this family have a more severe expression than those in the other two affected branches. No abnormal excretion of organic acids in the urine could be established.

A follow-up study in a family with dominant progressive inner ear deafness.

A family with dominant progressive inner ear deafness beginning in the high tones has been examined in 1962, 1968 and 1978. The audiometric data of 86 affected members (376 audiograms) were studied by 1) determination of averages, 2) principal component and factor analysis, and 3) curve fitting procedures. The hearing appeared to deteriorate in six characteristic phases with a separate deterioration for the high and the low frequencies. The hearing loss was equal for the R and L ear; in females low tone loss was greater than in males; in branch II the hearing loss proceeded more rapidly than in other branches. No abnormal excretion of organic acids in the urine could be established.

Just audible pitch and loudness modulations with cochlear impairment.

Just noticeable combined modulations in amplitude and frequency were determined for an observer (DP) with a bowl-shaped sensorineural hearing loss of about 40 dB between 1 and 2 kHz. In all stimulus conditions, interactions were found between the detection of amplitude modulations and frequency modulations. These interactions appear to correlate quantitatively with the effect of frequency on loudness (as given by the slope of the equal-loudness contour). When this effect of frequency on loudness if taken into account, the data support the hypothesis of independent detection of pitch and loudness modulations. The experimental data are also discussed in relation to the single-mechanism hypothesis for the detection of changes in amplitude and frequency (Zwicker, Maiwald). This model could not provide a satisfactory explanation of interactions between amplitude and frequency.