Room noise filtering by shunted loudspeaker

Abstract

There are a variety of needs in room acoustics, such as the control of ventilation noise, adjustment of reverberation time and selective enhancement of echos over some frequency bands. All these call for a tunable sound absorber. This work describes an electro-acoustic method to design such an absorber, which is here described as an acoustic filter. The absorber consists of a moving-coil loudspeaker diaphragm backed by a cavity. When incident noise pushes the diaphragm, it moves and its structural damping consumes some sound energy. An RLC shunt circuit is attached to the moving coil giving an electrically induced mechanical impedance that alters the acoustic behaviour of the shunted loudspeaker. By manipulating component values of the shunt RLC circuit, the absorber drains noise at flexibly chosen frequency bands and at any assigned absorption levels, much like designing an electronic filter. The essential part of the theory is validated by experiment.