Method and device for active noise reduction in a local area

• US 5,559,893 A
• Filed: 03/21/1995
• Issued: 09/24/1996
• Est. Priority Date: 07/22/1992
• Status: Expired due to Term

• Current Assignee: Sinvent AS
• Original Assignee: Sinvent AS
• Inventors: Pettersen, Odd K. .O slashed.., S.o slashed.rsdal, Svein, Krokstad, Asbj.o slashed.rn
• Primary Examiner(s): Kuntz, Curtis
• Assistant Examiner(s): Mei, Xu
• Application Number:
  US08/374,578
• Time in Patent Office: 553 Days
• Field of Search: 381/71, 381/94, 381/72, 381/73.1, 381/86, 381/83, 381/93
• US Class Current: 381/71.7
• CPC Class Codes: G10K 11/17854 the filter being an adaptive filter
• CPC Class Codes: G10K 11/17857 Geometric disposition, e.g. placement of microphones
• CPC Class Codes: G10K 11/17873 using a reference signal without an error signal, e.g. pure feedforward
• CPC Class Codes: G10K 2210/1282 Automobiles
• CPC Class Codes: G10K 2210/3045 Multiple acoustic inputs, single acoustic output
• CPC Class Codes: G10K 2210/3217 Collocated sensor and cancelling actuator, e.g. "virtual earth" designs
• CPC Class Codes: G10K 2210/3221 Headrests, seats or the like, for personal ANC systems

Abstract

A method for active noise reduction based on destructive interference of sound waves in order to reduce the energy in a sound field employs two omnidirectional microphones (M1, M2) provided in connection with a loudspeaker. The acoustic feedback of the microphones is eliminated by a closed loop consisting of the microphones and the loudspeaker. The loudspeaker used is an open loudspeaker with a dipole characteristic, thus causing one of the microphones to be more sensitive to the far field and thereby to the noise which has to be suppressed. The method is implemented by a device which comprises a digital signal processor (DSP) for processing the microphone signals and which transmits an output signal to the loudspeaker where the feedback component from the loudspeaker is substantially eliminated, while the output signal'"'"'s phase and amplitude are adjusted in such a manner that an effective cancellation of the noise is obtained in an area around the loudspeaker'"'"'s near field. The DSP can preferably be implemented in the form of software modules on an integrated circuit. With the method and the device an integrated reduction in noise level of almost 20 dB is achieved depending on how the filtering in the DSP is adapted. In practice a quiet zone can be obtained in the loudspeaker'"'"'s near field with an attenuation band which extends from approximately 100-500 Hz.

Claims

1. A method for active noise reduction in a local area, especially for generating a so-called quiet zone in the local area, wherein a loudspeaker and two microphones are used, the meshed being characterized in that the loudspeaker is provided adjacent to the local area where the quiet zone is to be generated, the loudspeaker being an open loudspeaker, that a first microphone is provided at a given first radial distance from the loudspeaker'"'"'s centre axis, that a second microphone is provided at a given second radial distance from the loudspeaker'"'"'s centre axis, the second radial distance being greater than the first radial distance and the microphones being located in the same radial direction, orthogonal to the loudspeaker'"'"'s centre axis, that the acoustic signal generated by the loudspeaker, overlaid on the sound field which exists in the local area, is detected with the first and the second microphone respectively, thus obtaining a first and second microphone signal respectively, that the first microphone signal is delayed by a value corresponding to the difference in running time between the first and second radial distance, that the first microphone signal is attenuated by a value corresponding to the difference in intensity between the detected microphone signals, thus obtaining a processed first microphone signal with the same intensity as the second microphone signal, whereafter the first processed microphone signal is inverted and summed with the second microphone signal to obtain a summed signal which, after filtering and amplification, is transmitted to the loudspeaker.

2. A method according to claim 1, characterized in that the two microphone signals are amplified after the output from the respective microphone, but before processing.

3. A method according to claim 2, characterized in that the amplified microphone signals before processing are converted to digital signals in an analog/digital converter.

4. A method according to claim 3, characterized in that the digital signals are processed in a digital signal processor, the first digital signal which corresponds to the first microphone signal being attenuated, delayed and inverted before being summed with the second digital signal which corresponds to the second microphone signal, whereafter the summed digital signal is filtered and converted to an analog output signal in a digital/analog converter, and amplified in a power amplifier and transmitted to the loudspeaker.

5. A method according to claim 4, characterized in the that for the filtering an FIR filter is used.

6. A method according to claim 1 characterized in the use of microphones with omnidirectional characteristic.

7. A method according to claim 1 characterized in the use of a loudspeaker with dipole characteristic.

8. A method according to claim 1 characterized in that optimum noise reduction is obtained in the space domain or the frequency domain through an adaptation of the filter.

12. A device according to claim 8 characterized in that between each microphone and the input on the analog/digital converter there is connected a microphone amplifier.

13. A device according to claim 8, characterized in that the filter in the digital signal processor is an FIR filter, preferably an adaptive FIR filter.

14. A device according to claim 8, characterized in that between the digital/analog converter and the loudspeaker there is connected a power amplifier.

9. A device for active noise reduction in a local area, especially for generating a so-called quiet zone in the local area, comprising a loudspeaker and two microphones, characterized in that the loudspeaker is provided adjacent to the local area in which the quiet zone is to be generated, the loudspeaker being an open loudspeaker, that there is provided a first microphone (M1) at a given first radial distance from the loudspeaker'"'"'s centre axis, that there is provided a second microphone (M2) close to the first at a given second and greater radial distance from the loudspeaker'"'"'s centre axis, the microphones (M1, M2) being located in the same radial direction, orthogonal to the loudspeaker'"'"'s centre axis, that the output from each of the microphones (M1, M2) is connected with respective inputs on an analog/digital converter, that the outputs on the analog/digital converter are connected with respective inputs on a digital signal processor, each input corresponding to a microphone channel, that the signal processor includes an attenuation stage connected to the input which corresponds to the first microphone channel, a delay stage connected to the output on the attenuation stage and an inverter stage connected to the output on the delay stage, that the output on the inverter stage is led to a first input on a summation stage whose second input is connected with the second microphone signal channel, that the output on the summation stage is connected with a filter stage connected in front of the output on the digital signal processor, and that the output on the digital signal processor is connected via a digital/analog converter with the input on a loudspeaker.

10. A device according to claim 9, characterized in the loudspeaker having a dipole characteristic.

11. A device according to claim 10, characterized in the microphones having omnidirectional characteristic.