Two approaches to tackling the speech-in-noise challenge
&NA; · The Hearing Journal · 2002
Outer hair cell damage results in a moderate degree of sensorineural hearing loss, which causes people to have more difficulty distinguishing among frequencies that are close together. Like a comb with fewer teeth, the fine-tuning is gone. No wonder people who wear hearing aids tell us that hearing aids + background noise = problems! Two primary solutions to the hearing-in-noise problem have emerged: directional microphones and digital noise-reduction algorithms. Both approaches will be discussed here in a seminar entitled “Headed in the Right Direction,” to be presented Thursday, September 12, from 8 to 10 am and again on Friday morning from 10 to noon. Ted Venema, PhD, the session moderator, points out that it is well known that making speech louder than background noise improves speech recognition. Directional mics have repeatedly been shown to accomplish this. Although they have been around for decades, they have been enjoying a resurgence in recent years because directional microphones that can be switched on and off are now common on ITEs. The benefits of directional mics have also been quantified better than ever before. A newer approach to the hearing-in-noise challenge is digital noise reduction, in which digital signal processing hearing aids statistically sense whether incoming sound waves resemble speech or noise. When noise is sensed, the gain is automatically reduced by 10 dB to 15 dB. However, distinguishing speech from noise is no easy matter, since the two are inextricably mixed. Imagine a single-channel digital hearing aid with noise reduction; if it senses noise, the gain is reduced for both noise and speech. This is like turning the volume control down, but it certainly won't increase speech recognition. Digital hearing aids with noise reduction routinely have multiple frequency bands. This allows gain to be reduced over narrow frequency ranges, thus compromising speech to a lesser degree. Venema says, “We are now at the threshold of actually increasing speech understanding in background noise with digital noise reduction; at this time, however, the challenge for digital hearing aids with noise reduction is to compromise speech as little as possible.” According to Venema, who is on the audiology faculty at the University of Western Ontario, “Conventional wisdom suggests that in background noise, directional microphones objectively improve speech recognition, while noise reduction subjectively enhances listening comfort. One can think of directional mics as the ‘brain’ or the ‘science,’ and digital noise reduction as the ‘heart’ or the ‘art.’ Together they make a good team.”Table: COMING UP THIS WEEK IN RENO