Active control of sound radiation from vibrating surfaces using arrays of discrete actuators

This paper reports an investigation of the active control of sound radiation from vibrating surfaces by using arrays of discrete actuators, or tiles, which cancel local volume velocity. The radiation of sound can be controlled by reducing the vibration levels on the structure and by changing its radiation efficiency. Reductions in vibration level are shown to be purely a function of the number of tiles per structural wavelength. Reductions in radiation efficiency an shown to be dependent on the relationship between the acoustic wavenumber in the fluid, the structural wavenumber on the vibrating surface, and the size of the tiles. It is also shown that there are three distinct regions of control. In the first region of control the acoustic wavelength is larger than the structural wavelength and large reductions in radiation efficiency are possible as long as there are at least two tiles per structural wavelength. In the second region of control the acoustic wavelength is smaller than the structural wavelength but is still more than twice as large as an individual tile. Control in this region is greatly improved if the tile size is reduced. In the third region half an acoustic wavelength is smaller than a tile and no reduction in radiation efficiency is possible. In the third region, attenuation is possible only by reducing the overall vibration level. The cancellation of local volume velocity by using small acoustic sources placed on or close to the vibrating surface is also considered and is shown to achieve even higher levels of attenuation in the radiated sound power than for tiles which cover the entire surface. (C) 1997 Academic Press Limited.