Active control of acoustic radiation due to discontinuities on thin beams

Two experiments were conducted to study the active control of acoustic radiation due to discontinuities on thin beams. One experiment investigated the radiation from a clamped end condition and the other investigated the radiation from a blocking mass. The beams were excited by subsonic flexural traveling waves which "scattered" (or produced reflected and transmitted traveling and near-field waves) when they encountered the discontinuity. This "scattering" produced supersonic wave number components in the beam vibrational response which were responsible for the acoustic radiation. The main purpose of these experiments was to control the acoustic radiation from discontinuities on beams by actively changing the characteristics of the "scattered" waves with control actuators.

In each experiment the system was disturbed by a harmonic, subsonic input from a point force shaker. Control actuator( s) (in the form of shakers and piezoelectric actuators) were attached to the beam near the discontinuity. Error microphone(s) were positioned in the acoustic field which supplied an error signal to the digital controller. The digital controller employed was the filtered-x version of the adaptive LMS algorithm programmed on a dedicated signal processing board in a personal computer.

An array of accelerometers was attached to the beam which were used to decompose the complex amplitudes of an assumed displacement equation. By applying a spatial Fourier transform to the displacement equation the wavenumber components present in the beam displacement were calculated. This aided in the investigation of the mechanism by which control of the acoustic field was affected.

Results from these experiments showed that large attenuations at the error microphones were possible (as much as 50dB) along with global attenuation of the acoustic field. The mechanism by which the control of the acoustic far-field was achieved was demonstrated as a decrease in the supersonic wavenumber components in the beam vibrational response.