EXPERIMENTS ON ACTIVE CONTROL OF ACOUSTIC RADIATION DUE TO A CLAMPED EDGE ON A SEMIINFINITE BEAM

Active control of acoustic radiation from a “semi-infinite” beam clamped at one end is studied experimentally. The beam is excited by subsonic flexural travelling waves which “scatter" (or produce reflected travelling and near-field waves) when they encounter the clamped discontinuity. The main purpose of these experiments is to demonstrate control of the acoustic radiation from the beam by actively changing the characteristics of the “scattered" waves with control actuators. The beam is disturbed by a harmonic input from a point force shaker. Control actuators, in the form of shakers or piezoelectric actuators, are attached to the beam near the clamped discontinuity. An error microphone is positioned in the acoustic field which supplies an error signal to a digital controller. The digital controller employed is the Filtered-X version of the adaptive LMS algorithm. An array of accelerometers is attached to the beam in order to obtain the out-of-plane displacement amplitude of the vibrating system. Applying a spatial Fourier transform, the wavenumber spectrum is calculated and used to describe the far-field radiated pressure. Experimental results are compared with related theoretical predictions. Experimental and theoretical results show that large attenuation at the error microphone is possible along with global attenuation of the acoustic field. Control is shown to be associated with reductions and changes in the shape of the vibration distribution near the discontinuity; the vibrational magnitudes away from the discontinuity are left largely unchanged. © 1994 Academic Press Limited.