Vib. and Acoust. Lab., Dept. of Mech. Eng., Virginia Tech., 203 Randolph Hall, Blacksburg, VA 24060
The presence of an air cavity behind a layer of absorptive material creates an impedance-release boundary condition at the interface between the material and the air cavity. This boundary condition results in high absorption coefficients over relatively narrow frequency bands. These high absorption values depend on the depth of the air cavity which in turn is a function of frequency. Therefore, the impedance-release boundary condition occurs only at a few discrete frequencies. The active control system uses an adaptive feed-forward filtered-x LMS control algorithm to maintain the impedance-release boundary condition over a broad frequency range. This is done by separating the incident and reflected vector intensity within the air cavity using two microphones and a wave deconvolution circuit. A speaker inside the cavity is used as an actuator to minimize the reflected intensity vector. The result is optimum absorption over a broad frequency range, including low frequencies. Very good agreement is found between numerical simulation and experimental results. The experimental results show high absorption coefficient (0.8--1.0) over a wide frequency band (100--2000 Hz).