Theories and signal processing techniques for the implementation of sound ball in space using loudspeaker array

It has been known that we can make a certain region of more acoustic energy than others. This can be achieved by utilizing loudspeaker arrays and designing a multichannel filter that effectively controls interferences of sound waves in space and time. The concept of a bright and dark zone[Choi and Kim, “Generation of an acoustically bright zone with an illuminated region using multiple sources,” J. Acoust. Soc. Am., Vol. 111(4), 1695-1700, Apr. 2002] showed that this idea can be realized in practice. Then we attempted to make a “sound ball” that utilizes the concept of bright and dark zone for generating a small spatial region of concentrated sound energy inside. The 32 speaker system which surrounds the zone of interest tried to implement the ball that can be positioned and also allowed to be moved. However, it was also found that the solution based on the bright and dark zones control does not, in strict sense, guarantee an effective radiation of sound from the ball. Understanding this inherent limitation motivated us to design a novel mean to have a sound ball that can radiate effectively. This has to solve the well-known Kirchhoff-Helmholtz equation for the case of which the source or sources are surrounded by an array of speakers. The sound ball is implemented by using a 50-channel spherical loudspeaker array, in which the loudspeakers are positioned on the Lebedev quadrature grid.