Sound Reflections from Concave Spherical Surfaces. Part II: Geometrical Acoustics and Engineering Approach

Abstract

Focussing arising from concave surfaces has long been a well-known problem in room acoustics. The focussing can cause high sound pressure levels, colouration or an echo. Part I of this paper [1] provides a mathematical approximation based on wave extrapolation of the sound field in and around the focal point due to reflections by a spherical surface. This second part of the paper provides some practical methods for calculating the sound field. A geometrical approach is presented, the limitations of image source methods are described and an engineering method is presented. Computer models based on image source methods are not capable of describing the pressure in or out of the focal. The sound field outside the focal area can be calculated within reasonable accuracy with a geometrical approach. The results show that for a small wavelength the focussing effect is fairly strong. Generally the possible reduction of the focussing effect by absorbers or diffusers is not enough to eliminate the focussing effect by spherical surfaces.