High-frequency audibility: the effects of audiometric configuration, stimulus type, and device.

BACKGROUND: For the last decade, the importance of providing amplification up to 9-10 kHz has been supported by multiple studies involving children and adults. The extent to which a listener with hearing loss can benefit from bandwidth expansion is dependent on the audibility of high-frequency cues. The American National Standards Institute (ANSI) devised a standard method for measuring and reporting hearing aid bandwidth for quality-control purposes. However, ANSI bandwidth measurements were never intended to reflect the true frequency range that is audible for a speech stimulus for a person with hearing loss.

PURPOSE: The purpose of this study was to (1) determine the maximum audible frequency of conventional hearing aids using a speech signal as the input through the hearing aid microphone for different degrees of hearing loss, (2) examine how the maximum audible frequency changes when the input stimulus is presented through hearing assistance technology (HAT) systems with cross-coupling of manufacturers' transmitters and receivers, and (3) evaluate how the maximum audible frequency compares with the upper limit of the ANSI bandwidth measure.

RESEARCH DESIGN: Eight behind-the-ear hearing aids from five hearing aid manufacturers were selected based on a range of ANSI bandwidth upper frequency limits. Three audiometric configurations with varied degrees of high-frequency hearing loss were programmed into each hearing aid. Hearing aid responses were measured with the International Speech Test Signal (ISTS), broadband noise, and a short speech token (/asa/) as stimuli presented through a loudspeaker. HAT devices from three manufacturers were used to create five HAT scenarios. These instruments were coupled to the hearing aid programmed for the audiogram that provided the highest maximum audible frequency in the hearing aid analysis. The response from each HAT scenario was obtained using the same three stimuli as during the hearing aid analysis.

STUDY SAMPLE: All measurements were collected in an audiometric sound booth on a Knowles Electronic Manikin for Acoustic Research (KEMAR).

DATA COLLECTION AND ANALYSIS: A custom computer program was used to record responses from KEMAR. Maximum audible frequency was defined as the highest point where the Long-Term Average Speech Spectrum (LTASS) intersected the audiogram.

RESULTS: The average maximum audible frequency measured through KEMAR ranged from 3.5 kHz to beyond 8 kHz and varied significantly across devices, audiograms, and stimuli. The specified upper limit of the ANSI bandwidth was not predictive of the maximum audible frequency across conditions. For most HAT systems, the maximum audible frequency for the hearing aid plus HAT condition was equivalent to the hearing aid for the same measurement configuration. In some cases, however, the HAT system imposed a lower maximum audible frequency than the hearing aid-only condition.

CONCLUSIONS: The maximum audible frequency of behind-the-ear hearing aids is dependent on the degree of hearing loss, amplification device, and stimulus input. Estimating the maximum audible frequency by estimating the frequency where the speech spectrum intersects the audiogram in the high frequencies can assist clinicians in making decisions about which device or configuration of devices provides the greatest access to high-frequency information, as well as whether frequency-lowering technology should be used.