Feasibility Assessment of a New Method for Measurement of Hearing Protector Attenuation: Bone Conduction Loudness Balance

Abstract

In response to the need for a field-usable method for the measurement of hearing protector attenuation, a new procedure called bone conduction loudness balance has been developed. This procedure measures attenuation as the difference caused by the hearing protector in the levels of an air-conducted sound required to balance the loudness of a bone-conducted reference sound. A feasibility assessment was carried out in which the attenuation of an expandable foam earplug was measured in four subjects for frequencies from 250 through 4000 Hz using two variations of the loudness balance procedure and a pure-tone, threshold-difference assessment for comparison. All test sounds were produced by a personal computer with an analog-digital sound processor, with the level being controlled by subject response. Standard audiometric headphones were used for the air conduction sounds and the bone conduction sound was produced from a vibrator held to the subjects' foreheads. The bone conduction sound was either a pure tone of the same frequency as the air conduction sound or a 1/3 octave noise band at 2000 Hz. The mean difference in attenuation measurement over all frequencies between the loudness balance and threshold difference methods was 3.1 dB with a standard deviation of 6.5 dB, and with the noise-band loudness balance method showing a trend toward greater error as the frequency decreased. In terms of repeatability, administration time, and subjects' acceptance, all the methods were approximately equal.