Clinical evaluation of three different loudness scaling protocols.

Loudness scaling has recently attracted much attention as a valuable clinical tool for acquiring reliable knowledge about loudness perception. This information can be used for diagnostic and rehabilitative purposes. In this study a loudness scaling module implemented in a new PC-based audiological test system was comprehensively tested. The system has proven to be a reliable and useful tool in a clinical environment. The described 'Default' loudness scaling protocol seems to represent an appropriate set of parameters. A comparison between this protocol and two other protocols available for clinical use indicates that the 'Default' protocol presents the same consistency of subject response as the 'IHAFF' protocol, and a better consistency than the 'LGOB' protocol. Regarding time consumption, the 'Default' protocol is superior to the 'IHAFF' protocol and comparable to the 'LGOB' protocol.

Digital feedback suppression (DFS). Clinical experiences when fitting a DFS hearing instrument on children.

A new power behind-the-ear hearing instrument with digital feedback suppression (DFS) seems to be an important step towards solving the problems of acoustic feedback in high power instrument fittings. Previous experiences (Dyrlund & Bisgaard, 1991) with a DFS prototype are confirmed in the present work. Ten profoundly hearing-impaired children were fitted with the new DFS instrument and wore it over a trial period of approximately three weeks. with the new instruments the rationale was to supply equivalent low frequency (< 1 kHz) gain compared to the subjects' original hearing instruments, and extra 5-10 dB high frequency (> 1 kHz) gain. The hypothesis was that with the new DFS system it would be possible to provide extra high frequency gain for these children without the annoyance of acoustic feedback. The test included questionnaires before and after the trial period, comparative free-field audiometry between the subjects' original and DFS instruments. The free-field audiometric results show a typical improvement of 5-10 dB at 2000 Hz and even larger improvements at higher frequencies. The increased high frequency gain provided by the new instruments did not give rise to significant complaints of feedback or howling during the trial period, and was generally preferred by 9 of the 10 subjects after the trial period.

Digital feedback suppression (DFS). Characterization of feedback-margin improvements in a DFS hearing instrument.

The introduction of a new power behind-the-ear hearing instrument equipped with an integrated digital feedback suppression (DFS) system, based on adaptive, digital signal processing, creates the need for new methods for evaluating the characteristics of this new technology. A special measuring method based on determination of the complex loop gain of the DFS instrument and the associated feedback path is described. This method yields information about the static feedback-margin improvement due to the DFS system, and the method is usable especially in connection with measurements on real ears. It requires fairly advanced test facilities, including a dual-channel FFT analyzer and, by preference, an anechoic room. Loop gain measurements on the new DFS power behind-the-ear hearing instrument show encouraging results. Groups of profoundly hearing-impaired children and adults were tested, and static feedback-margin improvements in the order of 10 dB for the new DFS power hearing instrument were seen. Variations were largest for groups using own ear moulds and individually fitted instruments.

Hearing aid measurements with speech and noise signals.

An increasing number of hearing aid types include one or more features which are intentionally non-linear. In such devices measurement of frequency response and distortion using sweep tone measurements are typically of little relevance. Five different non-linear hearing aid types were used to evaluate three different broad-band measuring methods. The results revealed that these methods were meaningful in estimating average frequency response obtained with a specific input signal, but none of the three methods used in the study was able to evaluate separately the effects of the most important signal modifications: memoryless non-linearity like peak clipping, time-varying gain from AGC and additive internal noise.

Coherence measurements in hearing instruments, using different broad-band signals.

In this investigation the effect of different broad-band test signals on non-linear distortion in hearing instruments was examined using coherence measurements, and the interaction between different test signals and types of automatic signal processing is explored. It is concluded that the use of coherence measurements to quantify non-linear distortion in hearing instruments is only valid for instruments without automatic signal processing. This limitation arises because interaction between type of test signal and the automatic signal processing used causes system time-variations, which influences the measured coherence function in an uncontrollable way.

Acoustic feedback margin improvements in hearing instruments using a prototype DFS (digital feedback suppression) system.

The properties of a prototype DFS (digital feedback suppression) system have been investigated. 21 ears fitted with behind-the-ear (BTE) hearing instruments and hard acrylic ear-moulds and 4 ears fitted with vented in-the-ear (ITE) hearing instruments were selected for the investigation. Two ITE instruments with different venting were employed to one of the ears. Complex loop gain has been measured in an anechoic room, and from these measurements the improvements in acoustic feedback margin due to the DFS system have been determined. For the BTE group, median values of 13.1 and 10.0 dB of improvement were established for two sets of measurements introducing a 180 degrees phase shift in connection with the last set of measurements. For the ITE group, values from 9.8 to 16.1 dB and from 13.7 to 16.3 dB of improvement were observed for the normal and the 180 degrees phase shift conditions respectively. Beyond this the DFS system may improve the sound quality to some extent, because the amplitude distortion, caused by the external feedback signal, is almost completely eliminated.

Gain and feedback problems when fitting behind-the-ear hearing aids to profoundly hearing-impaired children.

Previous laboratory studies with severely and profoundly hearing-impaired persons aided with behind-the-ear (BTE) hearing aids have resulted in prediction rules for insertion gain and maximum gain without occurrence of acoustic feedback. The practicability of these findings was investigated in the present field trial with 21 profoundly deaf children fitted with power BTE hearing aids. In dialogue situations without background noise, the gain control settings were in accordance with the insertion gain prediction rule, whereas preferred gain may be 10 dB lower in the presence of noise. Consistent with the prediction rule for maximum gain without feedback and the gain response of the present test hearing aid, we observed oscillation in the high-frequency range in which the children had no remaining hearing. When the high-frequency gain was reduced, sufficient low-frequency gain could be provided without feedback problems.

Characterization of non-linear distortion in hearing aids using coherence analysis. A pilot study.

Coherence is a frequency-domain measure of linear dependence between input and output of a system, e.g. a hearing aid, and describes the cumulative effect of different forms of signal corruption, e.g. noise and non-linear distortion. From the coherence function, a general frequency-dependent signal-to-noise ratio can be derived. In this investigation, the applicability of this measuring technique is demonstrated in connection with non-linear distortion in hearing aids. The influence of hearing aid gain and automatic gain control is illustrated, with speech-shaped noise as input signal. For the three hearing aids tested. The gain setting influences the signal-to-noise ratio heavily due to non-linear distortion, especially near maximum gain. The introduction of automatic gain control reduces the effect of non-linear distortion somewhat at high gain settings.

Acoustical feedback associated with the use of post aural hearing aids for profoundly deaf children.

The feedback properties of 29 ears fitted with post aural hearing aids and hard acrylic earmoulds, have been investigated for a group of profoundly deaf children. Complex loop gain has been measured, and maximum hearing aid gain before instability has been calculated from the measurement results. Guidelines for prediction of maximum hearing loss, which can be managed, are stated, and a suitable hearing aid frequency response for profoundly deaf children is proposed. This response ensures that acoustical feedback above approximately 1 kHz does not limit the low frequency gain, which is assumed to be very important for the speech recognition.

Some relationships between hearing aid frequency response and speech discrimination of profoundly deaf children. Pilot study.

A significant improvement in auditory-visual speech discrimination has been established for a group of profoundly deaf children with hearing losses in the range from 85 to 120 dB HL (pure tone average (500, 1,000, 2,000 Hz] by increasing the low frequency hearing aid gain by approximately 10 dB and reducing the high frequency gain by approximately the same magnitude. The improvements were concentrated in the hearing loss range from 95 to 115 dB HL. A tendency for improvement in auditory discrimination was indicated as well.