Acoustic directivity control by point mass attachment

The available passive techniques for noise and vibration control do not have a good performance at low and medium frequency range. In various applications the need is to reduce audible noise at specific locations. In this work, we propose a novel concept of local control of the radiated noise at the target location. This is in contrast to the existing concept of global control wherein the aim is to reduce the acoustic output over the entire surroundings of the source. The idea is to exploit the directivity of acoustic radiation and redirect the acoustic energy in directions away from the target location. To demonstrate the concept, we analyze a simply-supported square plate placed on a rigid planar infinite baffle subjected to harmonic point excitation. We seek to control the directivity of the radiated noise through point mass attachments. The magnitude of attached point mass is kept constant for analysis. The effect of locations of the point masses on the vibration and acoustic characteristics is presented. Analytical expression to find structural response to the harmonic excitation is derived using mode-summation technique and validated through finite element simulation in ANSYS. Taking the computed structural velocities as input, Rayleigh integral is used to find the sound pressure on a hemispherical grid over the plate which gives the directivity pattern. The complete procedure is non-dimensionalized for generalization. Optimal mass location to minimize sound pressure at a receiver location is obtained by using numerical optimization. The possibility of local noise control at any receiver location is demonstrated. It is shown that reduction of sound pressure at receiver location is much more than by mere uniform distribution of the additional mass over the plate. It is also observed that attachment of point mass at arbitrary location over the structure does not always reduce the sound radiation.