Method and apparatus for reducing repetitive noise entering the ear 



Apparatus for improving the audibility of incident sound (4) to a person (3) operating in an environment where there is a substantial background noise field (2) coming from a source (1) of repetitive noise, comprises an adaptable waveform generator (7), a first electro-acoustic transducer (10) receiving a synthesized cancelling waveform from the generator (7) and generating a cancelling noise to at least partly null the background noise, a second electro-acoustic transducer (11) to sense the partially nulled background noise, and adaptor (13) to modify the output of the generator (7) on the basis of the electrical output signal from the second transducer to minimize the nulled background noise, and sensor (5) to feed a triggering signal (6) derived from the source to the waveform generator. The electro-acoustic transducers (10, 11) can be mounted in headset (8) holding the transducers adjacent to an ear of said person without substantially impeding the arrival of said incident sound (4) to said ear. 

Apparatus and method for global noise reduction 



Apparatus is disclosed for attenuating sound radiated from a vibrating surface into a control volume as comprised of a source of a control signal indicative of the amplitude and frequency content of the sound transmitted from the vibrating surface. An actuator(s) or shaker is directly connected to the vibrating surface for further vibrating the vibrating surface to induce a cancelling sound into the control volume for combining with and attenuating the transmitted sound. A sensor(s) is also disposed within the control volume for detecting the resultant sound indicative of the combination of the cancelling and the transmitted sound to provide an error signal indicative thereof. A controller in the illustrative form of a computer executing a minimization algorithm, is responsive to the error signal for adaptively modifying the control signal as to phase and amplitude, which modified signal is applied to drive the actuator(s), whereby the error signal is driven to a minimum level and the sound within the control volume is similarly attenuated. One illustrative embodiment of this invention is particularly adapted to attenuate sound within the fuselage of an aircraft, wherein the principle source of noise is derived from the aircraft's engine and propeller and is introduced through the aircraft's fuselage into the aircraft's cabin. The actuator is coupled directly to the fuselage and is energized with the control signal adaptively modified as to phase and amplitude such that the cancelling sound emanating from the actuator(s) combines with and attenuates the engine and propeller noise within the aircraft cabin. 

Active noise control using blocked mode approach 



A method and apparatus for reducing noise across a broadband frequency range transmitted by a vibrating panel by blocking or otherwise opposing increasing non-resonant modes of vibration in the panel. The method and apparatus includes mounting an array of sensors on a surface of the vibrating panel. The sensors generate an input signal representing the incident vibration in the panel. The input signal is sent to an adaptive controller which generates an output signal. The output signal is essentially equivalent to the input signal but opposing in phase. The output signal is then sent and distributed to an array of actuators which are also mounted to a surface of the vibrating panel. The output signal forces each of the vibrating actuators to vibrate the panel in opposition to the incident vibration in the panel and thus reduce the transmitted noise. Continual readings of the vibration in the panel allows the sensors to update the input signal and thus adjust the vibration of the actuators. This approach requires only a simple adaptive controller to actively control broadband noise transmitted through the panel. 

Variable gain active noise canceling system with improved residual noise sensing 



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.