Broadband active acoustic absorption
Abstract
A noise suppression system that is both broadband and stable in a guide in the presence of a flow is described. Behavior of an acoustic field in a rectilinear channel is explored analytically and a system of 2nd order linear differential equations is obtained. A general solution of these equations yields a physical interpretation of an infinite number of plane waves propagating downstream and driven by a piston, either above or below a reference frequency. Parasitic noise is considered cancellable by electroacoustic transducers operating under the constraint of not altering the channel flow. Difficulties of realization are noted as time lag between microphone and transducer, signal attenuation in supersonic flow (ameliorated by the presence of not less than 2 sources), and, most importantly, control of the propagation speed of the acoustic damping signals, whose recommended solution is an electrodynamic speaker emitting VHF signals centered around the resonance frequency and including 2 or 3 octaves around it. Designs for the corrector circuit are given.
- Publication:
-
Association Aeronautique et Astronautique de France
- Pub Date:
- November 1980
- Bibcode:
- 1980acae.collQ....R
- Keywords:
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- Acoustic Attenuation;
- Broadband;
- Channel Flow;
- Noise Reduction;
- Differential Equations;
- Electroacoustic Transducers;
- Linear Equations;
- Plane Waves;
- Sound Propagation;
- Supersonic Flow;
- Acoustics