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 h

Valente M. (1996). Amplifiers and Circuit Algorithms of Contemporary Hearing Aids. Hearing Aids: Standards, Options, and Limitations. New York: Thieme Medical Publishers, Inc; p. 189.

Automatic signal processing in hearing aids is also a valuable advantage for noise reduction. By determining which frequencies are responsible for noise at different intensities, BILL, TILL, or PILL circuits can compress or limit those frequencies from being amplified. Instead of a linear aid, in which all frequencies are amplified equally, these hearing aids reduce noise by limiting frequency bands. Once again, the question becomes a choice as to which frequency bands account for noise. If the hearing aid user is in a room with a noisy air conditioner, low frequency compression may eliminate that noise. If on the other hand the user works in an apiary, high frequency compression during conversation may be a better choice. Frequency range modification is also contingent upon the person's individual type of loss. For example, common sense dictates that a person with a high frequency hearing loss would probably not benefit from any circuit that provides treble reduction, unless it serves to prevent recruitment.

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.

Phonak. (1997). The New Standard in Noise-Reduction Technology. MicroZoom. Phonak advertisement.

Beck LB, Bess FH, Studebaker GA. (1991). Programmable and Automatic Noise Reduction in Existing Hearing Aids. The Vanderbilt Hearing Aid Report II. Parkton, Maryland: York Press; 66-67.

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 MicroZoom uses an electronic approach. Each MicroZoom 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.

In contrast, TILL processing or Treble Increases at Low Levels, provides a low frequency signal reduction at low intensities. "This type of device is based on the premise that hearing-impaired subjects have normal growth of loudness for high level stimuli; therefore, intense sounds should be "acoustically invisible," and thus not amplified." (Bess et. al. 1991) This type of circuit is employed by the K-Amp (Etymotic Research), which is used to decrease loudness discomfort associated with high level, high frequency sounds. "The K-Amp rationale assumes that most hearing-impaired listeners have greater hearing loss in the high frequencies than in the low frequencies and, therefore, require high-frequency emphasis amplification." (Valente 1996) Proponents of the TILL circuitry believe that by reducing low frequency sounds at low le

Some common words found in the essay are:
Phonak MicroZoom, BILL TILL, University Virginia, TILL PILL, , Rock Roll, Behind-the-ear BTE, Low Levels, Conversely TILL, hearing aid, Etymotic Research, hearing aids, noise reduction, low frequency, directional microphone, bill till, signal processing, low levels, sound quality, upward spread masking, magnetic hearing, magnetic hearing aid, bess et al, programmable hearing aids, insertion gain frequency,
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Approximate Pages = 11 (250 words per page double spaced)