Hyperacusis: case studies and evaluation of electronic loudness suppression devices as a treatment approach.

Hyperacusis, as defined here, is a relatively rare condition in which the patient, with or without hearing loss, experiences severe loudness discomfort to everyday environmental sound levels. The case studies of 14 patients with severe hyperacusis are described; all wore passive attenuators (earplugs and/or earmuffs) in an attempt to alleviate their discomfort, frequently producing communication difficulties. These subjects were fitted binaurally with experimental electronic loudness suppression devices housed in in-the-ear casings. The devices supplied low-level amplification followed by an extreme form of amplitude compression for moderate or high-level inputs in an attempt to reduce loudness discomfort without reducing audibility. Many of the subjects were found to function with a wider dynamic range with the active devices compared with passive attenuators or the unoccluded ear, and most reported that they benefited from the devices in at least some listening situations.

Field trial evaluations of a switched directional/omnidirectional in-the-ear hearing instrument.

The use of directional microphones is one of the few methods available for hearing aids to increase the signal-to-noise ratio. The smaller microphones available with today's technology have increased the viability of their application for in-the-ear (ITE) hearing aids. This study evaluated an ITE hearing aid containing two nondirectional microphones that provides wearer-selectable omnidirectional/directional operating modes. Ten sensorineural hearing-impaired patients were fitted binaurally. During the first trial period, the low-frequency gain decrease produced by the directional mode was not compensated for. The frequency responses were matched during the second trial period. For both trial periods, Hearing in Noise Test results using two uncorrelated noise sources indicated significant speech recognition improvements for the directional mode relative to the omnidirectional mode. Responses on Abbreviated Profile of Hearing Aid Benefit questionnaires, paired-comparison judgments, and interview data revealed that most subjects preferred the directional mode in noisier environments, but many also preferred the omnidirectional mode in quiet listening.

Acoustic and hearing aid circuit variables affecting the tolerability of aided impulsive-type sounds.

The tolerability of aided impulsive-type stimuli was investigated in a group of 13 hearing-impaired listeners. Two linear circuits (one with a class A and one with a class D output stage) and one adaptive frequency response (AFR) circuit (with a class D output stage) were investigated. In a three-way paired-comparison task, subjects chose the hearing aid that was most tolerable when 75 dB sound pressure level (SPL) impulsive-type sounds were presented. Real-ear measurements of rms SPL, peak SPL, crest factor, and spectral distribution were made to determine which of these variables was most closely associated with behavioral tolerability scores. Results indicated significant differences across hearing aids for tolerability scores, rms sound pressure levels, and spectral peak frequencies. Highest tolerability scores were associated with the hearing aid that produced the lowest rms sound pressure levels in the ear canal (class D AFR). Significant correlations were found between tolerability and both rms SPL and peak SPL. Results are discussed in terms of circuit algorithm and in terms of the possible effects of hearing aid saturation.

A new technique for quantifying temporal envelope contrasts.

A new technique has been developed for precisely quantifying the temporal contrasts that exist between two sound samples. This technique is based on envelope subtraction, and generates an Envelope Difference Index that may be used to help clarify whether alteration of the natural speech envelope via amplification improves or degrades speech intelligibility. The Envelope Difference Index method may also be used to assess hearing aid saturation, and may have other applications as well. The technique is applicable whenever a precise quantification of the difference between two temporal envelopes is required, regardless of stimulus duration.

Achieving prescribed gain/frequency responses with advances in hearing aid technology.

Technological limitations have restricted the capability of older generation in-the-ear (ITE) hearing aids to closely match prescribed real ear gain/frequency responses. Newer technology, widely available in currently marketed ITE hearing aids, has considerably improved this capability. Data for 60 ears are presented comparing the real ear insertion gain (REIG) actually achieved to the target REIG, using ITE hearing aids having: 1) older generation narrow-band receivers, and amplifiers with single-pole-filter low frequency tone control and a class A amplifier output stage (n = 30), and 2) newer generation amplifiers with a two- or four-pole-filter low frequency tone control, and wide band receivers, containing a class D amplifier output stage (n = 30). With the newer technology ITE hearing aids, the means and ranges of deviation from target gain were reduced. Capability for achieving prescription REIG with ITE hearing aids can be further improved with multichannel amplifiers. Examples of the latter are shown for several difficult-to-fit audiograms.

Flexibility in Frequency Response Shaping and Signal Processing With Analog Hearing Aids.

Mainly because of packaging size limitations, most amplifiers for in-the-ear (ITE) and in-the-canal (ITC) hearing aids have performed linear processing and had a class A output stage. Recent advances in the miniaturization of analog semiconductor technology have made it possible to package much more sophisticated signal processing circuitry and push-pull and class D output stages in ITE and ITC hearing aids.These advancements have been used in hearing aids to improve sound quality, enhance the speech signal to emphasize weak consonants, provide increased flexibility in frequency response shaping, and reduce the amplification of undesired noise. Although digital programmability offers increased flexibility in hearing aid fittings, in most programmable hearing aid designs it is the analog portion of the circuit rather than the digital portion that performs the signal processing functions.Although "true" digital signal processing holds promise for further dramatic improvements in hearing aid performance, the capabilities of analog electronics are just beginning to be exploited. Through advances in low-voltage CMOS circuitry, analog ITE and even ITC hearing aids are now being made with multiband amplifiers that have relatively steep filter slopes.These small, nonprogrammable hearing instruments are essentially master hearing aids for frequency response shaping that require only a few potentiometers and an ordinary screw-driver for adjustment. Consequently, analog circuitry should not be totally abandoned as yet in favor of digital circuitry.

Hearing aid saturation, coherence, and aided loudness discomfort.

Real-ear measurements of the aided Loudness Discomfort Level (LDL) were obtained from five hearing-impaired listeners who were fit with two Class A and two Class D linear hearing aids, each with a different saturation sound pressure level (HFA SSPL90). These measurements were obtained with 75 dB SPL continuous discourse to determine whether saturation-induced distortion contributes to the sensation of loudness. Real-ear coherence measurements made at LDL were used to determine the extent of saturation, and sound quality judgments were used to determine whether the distortion present at LDL affected sound quality. Results indicated that the SPL, coherence, and sound quality ratings obtained at LDL were all higher for Class D hearing aids with relatively high HFA SSPL90 than for Class A hearing aids with relatively low HFA SSPL90. Overall results were generally consistent with the hypothesis that distortion affects both sound quality and the perception of loudness.

Hearing aid saturation and aided loudness discomfort.

Clinical measurements of the loudness discomfort level (LDL) are generally performed while the subject listens to a particular stimulus presented from an audiometer through headphones (AUD-HP). The assumption in clinical practice has been that the sound pressure level (SPL) corresponding to the sensation of loudness discomfort under AUD-HP conditions will be the same as the corresponding to LDL with the hearing aid. This assumption ignores the fact that the distortion produced by a saturating hearing aid could have an influence on the sensation of loudness. To examine these issues, 5 hearing-impaired subjects were each fit with four linear hearing aids, each having a different saturation sound pressure level (SSPL90). Probe-tube microphone measurements of ear canal SPL at LDL were made while the subjects listened to continuous discourse in quiet under aided and AUD-HP conditions. Also using continuous discourse, real-ear coherence measures were made at various output sound pressure levels near LDL. All four hearing aid types produced mean LDLs that were lower than those obtained under AUD-HP conditions. Those hearing aids with higher SSPL90 produced significantly higher LDLs than hearing aids with lower SSPL90. A significant negative correlation was found between real-ear SPL and real-ear coherence. Quality judgments made at LDL indicated that sound quality of hearing aids with higher SSPL90 was preferred to that of hearing aids with lower SSPL90. Possible fitting implications regarding the setting of SSPL90 from AUD-HP LDL measures are discussed.

Strategies for enhancing the consonant to vowel intensity ratio with in the ear hearing aids.

Numerous investigators have suggested that increasing the consonant to vowel intensity ratio (CVR) may improve speech intelligibility. This investigation was designed to determine the extent to which analog circuits, small enough to fit into in the ear hearing aids, can increase the CVR, and whether CVR enhancement is of benefit to hearing-impaired listeners. Real ear CVRs, calculated from real ear recordings of nonsense syllables, were obtained from eight hearing-impaired listeners. Recordings from each listener were obtained through each of four hearing aid circuits: (1) an adaptive high-pass filter; (2) a faster acting adaptive high-pass filter; (3) the fast-acting adaptive high-pass filter with expansion; and (4) an infinite amplitude clipper. The amount of CVR enhancement was compared to performance of the subjects with a NST speech recognition task. Subjects also ranked the four circuits for amount of consonant emphasis provided. Results indicated that the four hearing aid circuits increased the real ear CVR by 4 to 6 dB, relative to unaided. Aided CVR varied, however, across circuits and between fricative and stop consonants. Performance on the NST recognition task was generally consistent with the amount of CVR increase provided. Rank ordering for consonant emphasis was consistent with aided CVR for stop consonants, but not for fricatives.

Input stimuli for obtaining frequency responses of automatic gain control hearing aids.

Developing a family of frequency response curves for AGC types of hearing instruments using swept pure tones at varying input levels often produces erroneous results. This problem is caused by exceeding the threshold for activating the AGC circuit at some frequencies but not at other frequencies during the pure-tone sweep, thereby producing a different frequency response from that which would be obtained with a complex input signal such as speech-shaped noise. This measurement artifact may be minimized by ensuring that the threshold for activating the AGC circuit is either always exceeded or never exceeded during the development of a frequency response curve. Three input signals are compared for developing a family of frequency responses for an AGC hearing aid: (1) swept pure tone, (2) swept pure tone with bias tone added, and (3) shaped broad-band noise. The shaped broad-band noise appears to be the input signal of choice.

Some issues in utilizing probe tube microphone systems.

Probe tube microphone systems are a useful validation tool for optimizing the hearing aid selection and fitting processes. To date, the terminology involved in using these systems has not been standardized, resulting in confusion in the measurements themselves and in associated parameters. Real time and stored procedures for sound field equalization have been adopted for real ear measurements from ANSI and IEC standards which were written for 2 cc coupler and manikin measurements. Suggestions are made for equivalent definitions pertaining to probe tube microphone measurements. Several of the equalization methods for probe tube microphone measurements are compared in the context of those ANSI and IEC standards from which they are derived. Tradeoffs in the selection of input stimulus type are examined. Of the probe tube microphone systems currently available, performance features and flexibility varies widely. Certain optional features associated with general purpose microcomputers, if incorporated in a probe tube microphone system would enhance the day-to-day operation of a hearing aid dispensary.