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Audio Signal Processing

Automatic-noise-eliminator-for-hearing-aids

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Automatic noise eliminator for hearing aids

Details

Inventors: Heide, Jorgen;
Assignee: Richards Medical Company (Memphis, TN)
Primary Examiner: Ng; Jin F.
Assistant Examiner: Byrd; Danita R.
Attorney, Agent or Firm: Pravel, Gambrell, Hewitt, Kimball & Krieger

A noise elimination circuit, especially for a hearing aid, which removes all sound below a predetermined level and transmits a compressed sound range for all sounds above a predetermined level. The circuit uses a field effect transistor as a voltage variable resistor to transmit a signal when the sound signal is being compressed or is over a predetermined level and block the signal when the sound signal is not being compressed or is below the predetermined level. The field effect transistor drain to source resistance is varied by transmitting a signal indicative of the compression or signal level to the gate of the transistor, such that the gate voltage is higher than the source voltage when signal transfer is desired and the gate voltage is less than the source voltage when no signal transfer is desired. A switch is provided to defeat the noise elimination circuitry.

DETAILED DESCRIPTION The circuit of the present invention uses a novel design that is adapted for use in a small space and with relatively low power requirements and, at the same time, provides an effective noise elimination circuit which greatly reduces the fatigue of a wearer.
The sound is received by a microphone located in the hearing aid.
The microphone produces a signal which is amplified using a preamplifier having an adjustable gain and transmitted to a middle and high frequency compression amplifier.
The compression amplifier produces a signal which is approximately linearly amplified for input signal levels below approximately 60 db and which is compressed for input signal levels greater than approximately 60 db.
The compression amplifier output signal includes a direct current component in addition to the alternating current component comprising the audio signal.
Additionally, the compression amplifier produces a direct current compression level signal which is proportional to the compression level being applied by the compression amplifier.
This compression level signal is applied to the gate of a metal-oxide-semiconductor field effect transistor (MOSFET) while the compression amplifier output signal is applied to the source of the MOSFET.
The drain of the MOSFET is connected through a potentiometer and filtering network to ground.
The potentiometer is used as a voltage divider to supply a signal of the proper voltage to the input of an output amplifier which drives the hearing aid speaker.
Alternatively, the filtering network and the potentiometer can be connected between the compression amplifier and the MOSFET source terminal, with the MOSFET drain terminal being connected to the input of the output amplifier.
The MOSFET acts as a voltage variable resistor so that when the compression portions of the compression amplifier are active and the compression level signal is a higher voltage signal, the drain to source resistance of the MOSFET is low, causing an effectively complete transmission of the compression output signal to the output circuitry and thereby an adequate signal to the wearer

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