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 hea

The magnetic aid consists of a microphone, a processing unit and an electromagnetic coil, all of which are implanted. For amplification purposes, the aid will generate sound waves that match the "desired" sound or voice. Combining both signals in phase will have the constructive effect of directly increasing the amplitude of the wave travelling over the basilar membrane. To account for noise reduction, the system is analogous to a FlowMaster muffler (see figure). As shown, the signal processor

The BILL and TILL hearing aids are both effective with varying types of hearing losses, but this type of noise reduction is contingent on the fact that noise perception can be changed by varying the frequency response of an amplifier. For the majority of hearing aids, this is only a limited means of changing how well a client can understand speech in noise. Each client however, will have varying degrees of subjective opinions regarding the function of their aid. While acoustic modifications such as venting and mechanical filtering can be made in order to change sound quality, the BILL and TILL hearing aids, once chosen are adjustable only to the limits of their programming.

In conclusion, the advantage of noise reduction techniques in hearing aids is directly related to a reduction in hearing aid complaints, and an increase in client satisfaction. The directional microphone has been a valuable source for noise reduction since early Rock and Roll musicians wanted to reduce feedback during live performances as they got louder and louder. A hearing aid fitted with a directional microphone receives benefit from the assumption that a person will most likely want to attend to a person he is directly facing. In reality, a good portion of our daily communication occurs in less than ideal circumstances. A conversation partner may be just leaving the room when he remembers something important to say, and for normal hearing individuals it is quite commonplace to yell an instruction from one room to another. In some cases, this will defeat the purpose of directivity. Thankfully, manufacturers have responded to these complaints by offering the ability to switch the "sound focusing" on or off, sometimes with a remote control. (Phonak 1997)

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.

Although the system is currently being tested on animals with favorable results, human testing will be necessary. The human subject, with a control module, can adjust the aid to reduce undesired sounds and produce maximum benefit. With the advent of miniature computer chips, the magnetic aid allows for an adjustable frequency / intensity analyzer to be implanted in the skull. The noise reduction possibilities for this type of aid are substantial, even if combined with other digital processing techniques.

generates a sound wave that is out of phase with the incoming signal. The two signals then combine in the cochlea and cancel each other out.

The PILL circuit (Programmable Increases at Low Levels) affords the clinician more possibilities when adjusting f

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