Variable gain active noise cancelling system with improved residual noise sensing

• US 7,103,188 B1
• Filed: 03/08/1999
• Issued: 09/05/2006
• Est. Priority Date: 06/23/1993
• Status: Expired due to Fees

• Current Assignee: NCT Group, Inc.
• Original Assignee: NCT Group, Inc.
• Inventors: Jones, Owen
• Primary Examiner(s): Mei, Xu
• Application Number:
  US09/595,558
• Time in Patent Office: 2,738 Days
• Field of Search: 381/87, 381/71.1, 381/71.2, 381/71.6, 381/71.8, 381/71.9, 381/71.13, 381/71.14, 381/72, 381/73.1, 381/FOR.123
• US Class Current: 381/71.9
• CPC Class Codes: G10K 11/17823 Reference signals, e.g. ambient acoustic environment
• CPC Class Codes: G10K 11/17825 Error signals
• CPC Class Codes: G10K 11/17833 by using a self-diagnostic function or a malfunction prevention function, e.g. detecting abnormal output levels
• CPC Class Codes: G10K 11/17853 of the filter
• CPC Class Codes: G10K 11/17855 for improving speed or power requirements
• CPC Class Codes: G10K 11/17857 Geometric disposition, e.g. placement of microphones
• CPC Class Codes: G10K 11/17881 the reference signal being an acoustic signal, e.g. recorded with a microphone
• CPC Class Codes: G10K 11/17885 additionally using a desired external signal, e.g. pass-through audio such as music or speech
• CPC Class Codes: H04R 1/1083 Reduction of ambient noise active noise reduction per se G10K11/175; protective devices for the ear, e.g. providing acoustic protection A61F11/06
• CPC Class Codes: H04R 2460/01 Hearing devices using active noise cancellation

Abstract

An active noise cancellation system includes a series of features for more effective cancellation, greater reliability, and improved stability. A particular feature adapted for headset systems includes locating a residual microphone radially offset from the center of a sound generator to detect a signal more similar to that incident upon the eardrum of the user. In addition, an open back headset design includes perforations on the side of the headset instead of the back, so that the perforations are less susceptible to inadvertent blockage. The system also includes a mechanism for detecting changes in the acoustic characteristics of the environment that may be caused, for example, by pressure exerted upon the earpieces, and that may destabilize the cancellation system. The system automatically responds to such changes, for example, by reducing the gain or the frequency response of the system to preserve stability. The system further includes other methods for detecting imminent instability and compensating, such as detecting the onset of signals within enhancement frequencies characteristic of the onset of instability, and adjusting the gain or frequency response of the system or suppressing the enhanced signals. The system further includes a mechanism for conserving battery life by turning the system off when sound levels are low, or adjusting the power supply to the system to correspond to the current power requirements of the system.

Claims

1. An active noise canceling system comprising:

a sound generator, responsive to drive signals applied thereto, for generating an anti-noise field;

a noise cancellation processor, for generating the drive signals to the sound generator;

a first sound sensor disposed within said anti-noise field to generate a residual signal indicative of the sum of ambient sounds and anti-noise impinging on the sensor, wherein said noise cancellation processor processes the residual signal to form a component of the anti-noise field; and

means for generating indicia of the level of ambient noise and responsively varying a transfer function of the system in response to said indicia of amplitude of ambient noise such that a gain of the transfer function of the system is decreased in response to a decrease in said indicia of amplitude of ambient noise, wherein the transfer function comprises a ratio of an output signal of the noise cancellation processor to at least the residual signal.

2. A method for increasing the stability of an active noise cancelling system comprising a noise cancellation circuit, a sound sensor and sound generator cooperating in a feedback loop, the feedback loop having an associated transfer function, the method including the steps of:

generating, in accordance with drive signals, an anti-noise field;

sensing the residual noise resulting from interaction of the anti-noise and ambient noise;

generating the drive signals in accordance with said sensed residual noise;

sensing ambient noise outside of the anti-noise field; and

feeding forward a first range of frequencies that includes at least the high frequency components of the ambient noise to effect feedforward cancellation thereof;

wherein a feedback signal, generated by the feedback loop that processes a second range of frequencies, is processed by a noise cancellation processor without affecting the transfer function of the noise cancellation processor to form a component of said anti-noise field with the transfer function comprising a ratio of an output signal of the noise cancellation processor to at least said sensed residual noise, and

wherein the first range of frequencies and the second range frequencies substantially overlap in a cancellation band below an enhancement frequency range.

3. An active noise canceling system, comprising:

a sound generator for generating an anti-noise field in response to drive signals applied thereto;

a first sound sensor, disposed within an anti-noise field, for generating a residual signal indicative of the sum of ambient noise and anti-noise impinging on the sensor;

a second sound sensor disposed a predetermined distance from said sound generator and output of said anti-noise field to generate a signal indicative of ambient noise; and

a processor, for generating the drive signals to the sound generator in accordance with said residual signal and said signal indicative of said ambient noise;

wherein said first sound sensor is connected to cooperate with said processor and said sound generator to form a feedback loop that processes a first range of frequencies to produce a feedback signal, and said second sound sensor is connected to cooperate with said processor and said sound generator to provide feedforward signal noise cancellation of a second range of frequencies, said feedforward and feedback signals being processed by said processor to form a component of said anti-noise field, and

wherein the first range of frequencies and the second range of frequencies substantially overlap in a cancellation band below an enhancement frequency range.