Sound Scattering and Its Reduction by a Janus Sphere Type

Sound scattering by a Janus sphere type is considered. The sphere has two surface zones: a soft surface of zero acoustic impedance and a hard surface of infinite acoustic impedance. The zones are arranged such that axisymmetry of the sound field is preserved. The equivalent source method is used to compute the sound field. It is shown that, by varying the sizes of the soft and hard zones on the sphere, a significant reduction can be achieved in the scattered acoustic power and upstream directivity when the sphere is near a free surface and its soft zone faces the incoming wave and vice versa for a hard ground. In both cases the size of the sphere’s hard zone is much larger than that of its soft zone. The boundary location between the two zones coincides with the location of a zero pressure line of the incoming standing sound wave, thus masking the sphere within the sound field reflected by the free surface or the hard ground. The reduction in the scattered acoustic power diminishes when the sphere is placed in free space. Variations of the scattered acoustic power and directivity with the sound frequency are also given and discussed. 1. Introduction Sound scattering is a fundamental problem in acoustics. It affects a wide variety of acoustic analysis and design ranging from issues concerning room acoustics and noise pollution to acoustic detection of objects. In this study we look at the fundamental problem of sound scattering by a sphere and more particularly by a Janus sphere type; that is, a sphere that has two zones that are not necessarily of the same size. One zone is a soft surface of zero acoustic impedance and the other one is a hard surface of an infinite acoustic impedance. Sound scattering by a hard sphere placed in free space and subject to an incident planar monochromatic sound wave was already investigated by Rayleigh [1]. Both the incoming and scattered waves were expressed as Fourier-Legendre series, where the amplitudes of the scattered wave modes’ were determined by fulfilling the boundary condition on the sphere for each mode. A similar derivation can be applied for a soft sphere [2]. Both cases of sound scattering by a hard or a soft sphere can be approximated using the sound fields of a monopole and a dipole placed at the centre of the sphere for the low frequency limit. Reduction of sound scattering is of great interest for a range of reasons from better audio communication of speech and music to reduction of noise pollution or avoiding acoustic detection. Commonly reduction in sound scattering is achieved by coating the object