Active Boom Noise Damping (242)

Due to the limited effectiveness of sound absorbing material at low frequencies, the first (or first few) normal mode(s) of an enclosure (approximately the size of an automobile cabin) are very difficult to damp out, resulting in the boom noise problem. This problem is more severe in sport utility vehicles and minivans than in smaller automobiles.

Two active boom noise damping techniques using a Helmholtz resonator-based compensator and a lead compensator called a positive pressure feedback have been developed at the University of Dayton. These techniques can add a substantial amount of damping to the first (or first few) normal modes of an enclosure. The two damping techniques are of the feedback type and their compensators can be implemented in software or hardware (using inexpensive operational amplifiers). Both techniques have been implemented in the laboratory on an enclosure. Numerical and experimental results indicate the addition of a substantial amount of damping to the first mode (more than a 12 decibel drop in the resonance peak) and a reasonable amount of damping to the second acoustic mode of that enclosure.

The use of one or a bank of Helmholtz resonators to increase the sound absorption in automobile cabins has been studied in the past. This approach is effective, but the large size of the resonators (due to the low frequencies they are tuned for) makes their use rather prohibitive. The active boom noise damping techniques developed at the University of Dayton can effectively address the boom noise problem. The control schemes can be implemented cost effectively in automotive applications by integrating them as part of the vehicle's sound system.

U.S. Patent No. 5,974,155 issued October 26, 1999.

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