Electrical audio signal processing systems and devices – Acoustical noise or sound cancellation – Counterwave generation control path
Patent
1997-05-06
1998-12-08
Isen, Forester W.
Electrical audio signal processing systems and devices
Acoustical noise or sound cancellation
Counterwave generation control path
381 717, G10K 1116
Patent
active
058481699
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
Sound control or attenuation techniques fall into two general categories: feedback and feedforward. Olson and May first developed a feedback system based upon the virtual acoustical earth principle. "Electronic Sound Absorber", Journal of the Acoustical Society of America, 25(6) (1953). Later, Chaplin, et al., in U.S. Pat. No. 4,527,282, disclosed a similar device designed primarily to control engine exhaust gases. Both of these systems are implemented by positioning an acoustic microphone a small distance from an acoustic loudspeaker. The output of the microphone is passed through an inverting amplifier, which is then used to drive the loudspeaker. The primary application for this approach has been to create a local quiet zone, or region of reduced sound pressure in front of the speaker.
Feedforward acoustic disturbance rejection relies on the availability of an uncontrollable reference signal that is correlated to the disturbance. An adaptive filter receives this reference signal along with usually an acoustic error signal and regulates the driving of a speaker to minimize the error signal. As a result, feedforward solutions tend to be ideal for harmonic input disturbances but less appropriate for stationary random or impulsive disturbances.
SUMMARY OF THE INVENTION
Feedback-based sound control systems better handle the random disturbances. The primary limitation of conventional feedback configurations, however, is that the frequency bandwidth of operation is severely limited by the transduction device, i.e., microphone and loudspeaker, dynamics. The stability and useful bandwidth of the system are defined by the gain margin and phase margin obtained from the Bode plot of the open-loop response. The microphone has a zero at the origin in the s-plane, a real pole typically somewhere between 2 and 8 Hz and a complex conjugate pair of poles at some higher frequency, typically in the kHz range, which dictates the bandwidth of the device. The acoustic loudspeaker has a complex conjugate pair of poles at some low-frequency (i.e., between 20 and 60 Hz) and a real pole due to the electrical dynamics. These additional dynamics, when coupled to an enclosed sound field, impose finite gain margins, limiting the useful bandwidth of operation. Thus, for direct output feedback control proposed in the prior art, an upper limit to the feedback gain results. Due to this limited bandwidth of operation, the prior art finds limited practical use.
The present invention concerns a system for dissipating or changing the characteristics of acoustic energy of a region, such as an enclosure. This system includes at least one acoustic sensor, for example a microphone, that is positioned close to an associated acoustic driver, such as a loudspeaker. An inverting amplifier is used to drive the acoustic driver in response to the sensor. A series electrical signal conditioning circuit, i.e., a compensator, is electrically interposed between the sensor and driver to modify the open loop response of the system.
The present invention finds particular application in the context of reverberant sound fields. When the control system is placed in the corner of a reverberant enclosure, or at the position of maximum response to the acoustic modes, the acoustic response generally in the lower frequencies of the reverberant sound field can be attenuated globally, i.e., at every point within the enclosure. The series compensator increases the robustness of the system while also extending the bandwidth of operation and degree of attenuation.
In specific embodiments, the compensator increases a gain margin of the system. In most situations, to increase the gain margin, the compensator must compensate for transduction device dynamics associated with the acoustic driver and/or the acoustic sensor. The compensator constrains the phase response to alternate between +90 degrees and -90 degrees for each alternating complex conjugate pair of poles and zeros for an operational bandwidth of the system.
In other embodiments, an
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Clark, Jr. Robert L.
Cole Daniel G.
Duke University
Isen Forester W.
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