Noise Reduction In Hearing Aids Essay Research

Noise Reduction In Hearing Aids Essay, Research Paper

Recently in a local hearing clinic, a client.s concerns were discussed. .I.m afraid I won.t like them. My brother in law bought two hearing aids, and he keeps them in a drawer in the kitchen.. While the number of people dissatisfied with their hearing aids hovers around 50%, the hearing aid industry is hard pressed to decrease the number of returns, and increase the average daily use of each aid. In order to accomplish this, hearing aid manufacturers must answer the most often heard complaint: .It doesn.t work well in noise..

Unfortunately, a hearing aid will never be able to accomplish the sifting and sorting that is carried out in the human brain. While a person with normal hearing sits in a restaurant, he can distinguish a conversational speech signal that is as little as three decibels greater than the ambient noise. On the other hand, a person with a 30-decibel sensorineural loss might need the speech signal to be 15 or more decibels greater than the ambient noise. The hearing aid.s task is to acoustically or electronically compensate for both the neurological shortcomings of the hearing impaired person and the wide band increase inherent in any basic amplifier.

Acoustic compensation can be carried out in a hearing aid microphone. Most hearing aids today utilize omnidirectional microphones, which pick up sound equally from all directions. This may be beneficial and practical in some cases, as in the completely in the canal (CIC) aid. The CIC aid uses the natural funneling of the auricle in order to focus sound directly into the instrument. Behind-the-ear (BTE) and full concha in-the-ear (ITE) aids lose this anatomical feature, and may benefit from a directional microphone. .The purpose of using a directional microphone is to focus its sensitivity toward the front of the listener, thereby attenuating or reducing unwanted .noise. or competition emanating from behind the listener.. (Stach 1998)

Microphone directionality can be accomplished by using a single microphone with two sound inlets. In this mechanical method, the time lag created by a sound entering each inlet is precisely calibrated to cancel out sounds from the sides and back of the microphone as they strike the diaphragm. The Phonak MicroZoomä uses an electronic approach. Each MicroZoomä contains two omnidirectional microphones. According to the flyer, .One picks up sound in front of you while a second picks up sound from the sides and rear. A tiny computer chip inside the aid analyzes both sounds. It then enhances the sound from the front, and reduces background sounds from the sides and rear.. (Phonak 1997) In reality, the .tiny computer. is analyzing the time it takes for a sound to reach each microphone and mathematically decreasing ALL sounds from the sides and rear, speech included.

Directional microphone technology is fallible in one regard: it assumes that all .noise. is spatially related. Unfortunately, much of the noise present in our environment is omnidirectional. Even though significant benefit can be achieved by .sound focusing., a directional aid also must perform in an environment with multiple speakers situated around a listener, combined with noise from high priority directions.

Signal processing, or digital signal processing (DSP), is partly utilized in amplification in order to narrow the focus from .all. sounds to .speech. sounds. A linear aid will amplify all frequencies equally, limited to the characteristics of the input / output transducers and the coupler type. On the other hand DSP aids can alter the frequency response of an aid based on their design and/or programming. There are many techniques that can be used which are based on type of loss, but since speech encompasses such a wide frequency range, audiologists need to choose wisely which type of processing to use. Minimally, signal processing for noise reduction is analogous to a continually adjusting graphic equalizer for a home stereo system. Level dependent frequency response (LDFR) hearing aids can also adjust their .equalizer. settings based on input volume. Along with standard fitting techniques, subjective client opinions are one of the most important factors in decreasing hearing aid returns. In the past several years, technological advances have given audiologists several options.

One such LDFR hearing aid incorporates BILL (bass increase at low levels) circuitry. If it were assumed that most detrimental noise occurs in the low frequencies, then decreasing the low frequency response of a hearing aid at high levels would reduce noise and improve a person.s speech discrimination performance score. This concept seems natural based on the upward spread of masking principle. Since a low frequency sound must travel farther up the basilar membrane, it will mask out a competing higher frequency sound that does not have to travel as far. By reducing low frequency sounds at high levels, a hearing aid could minimize the upward spread of masking, and allow for increased speech reception.

According to one study, while it is reasonable to assume that BILL circuitry would be beneficial in the presence of low frequency noise, .there is no evidence that listeners with hearing loss exhibit increasing amounts of upward spread of masking with increasing level.. (Bacon, et. al. 1997) Another study shows that while there i