An objective method for measuring the attenuation of hearing protection devices using otoacoustic emissions

Abstract

An objective human subject test technique for measuring the attenuation performance of hearing protection devices is presented. The technique is similar to the real-ear attenuation at threshold (REAT) technique, except distortion product otoacoustic emission (DPOAE) measurements in the ear canal replace the standard audiometric threshold measurements. The primary advantages of the new technique are that the measurement is objective, relatively fast, and can be applied at supra-threshold sound levels. Microphone in real ear (MIRE) data is also available from the DPOAE probe in the ear canal. Disadvantages are the restricted frequency range in which DPOAE signals may be reliably recorded (2–5 kHz) and the need for an in-ear probe. Attenuation data for a set of earmuffs was measured on a pool of seven subjects using DPOAE, MIRE, and threshold methods. The DPOAE and threshold measurements were in agreement, validating the DPOAE method. The MIRE data were significantly higher than the DPOAE or threshold data. The difference in the attenuation levels represents sound reaching the cochlea that is bypassing the ear canal. This difference was smaller for measurements of the attenuation performance of a helmet, which provides increased shielding from bypass transmission paths through the head.