Real‐world performance of directional microphone hearing aids

Difficulty understanding speech in the presence of background noise is a common complaint of persons with impaired hearing. Currently, directional microphones are the only option available in hearing aids that offer the potential of significantly improving the signal-to-noise ratio (SNR) for the wearer. In this issue, Andrew Dittberner has provided options for measuring the directivity of the microphone system, while Todd Ricketts has presented laboratory evidence on the effectiveness of the various microphone designs. In this article, we will address the issue of real-world benefit. The directivity of directional microphone hearing aids is typically measured in an anechoic space. The purpose of making these physical measurements in a non-reverberant enclosure is to optimize the influence of the angle of incidence of the signal. In a more reverberant environment, direct and reflected sounds from a given source could enter both microphones at comparable intensities, thereby defeating the directional processing. Similarly, behavioral measures of directionality are typically made in a sound-treated test booth. The speech signal is presented through a loudspeaker positioned at 0o azimuth and the background noise is presented from one or more additional loudspeakers that are often positioned at azimuths corresponding to the primary nulls in the polar response of the directional microphones. Again, such a testing arrangement tends to optimize the directional processing. It goes without saying that persons with impaired hearing almost never encounter listening environments in daily living that are as sound-treated as an audiometric test booth, much less that are anechoic. Hence, it is not surprising that the performance of directional microphone hearing aids in everyday listening situations generally falls short of what might be expected based on measures of the directivity index or the directional advantage. The discrepancy between the performance of directional microphones in the test booth and that typically observed in everyday listening is illustrated by the results of Walden et al.1 We obtained test booth measures of speech recognition in background noise and everyday ratings of speech intelligibility in noisy listening situations for each microphone mode of a switchable omnidirectional/directional hearing aid. To obtain the test-booth measures, we used the Connected Speech Test (CST).2,3 Test sentences were presented from a loudspeaker positioned at 0o azimuth, and a multitalker babble was presented from loudspeakers positioned at 90o, 180o, and 270o. Two test conditions were included: a 60-