A Study On Sound Field Control Using Loudspeaker Array

Sound field control refers to reproducing a desired sound field or manipulating sound field distribution in a specific area using loudspeaker arrays, which can find various applications in audio systems, such as auditoriums, cinemas, home theatres, and audio systems on air planes or in cars. This thesis reviews the basic theories on sound field control, and investigates the method for generating higher sound energy inside a region with curve boundary while keeping the outside region relatively quiet.A method for generating sound beam propagating along arbitrary convex trajectories using a linear loudspeaker array is proposed first. The method treats the curving trajectory as an envelope of the tangent lines which can be produced by manipulating the spatial phase distribution of the loudspeaker array. The general derivation process of the spatial phase distribution for arbitrary convex trajectories is formulated. Then the mechanism of the proposed method is explored and it is found that the principle basis is the stationary phase method. Therefore, the sound pressure at a point on the trajectory is mainly produced by the sound source on the tangent line from the point, while the contribution from the other sound sources cancels out.The proposed tangent line based method is then validated through simulations and experiments. The simulation results show good agreement with the theoretical prediction. The relationship between the necessary length of the loudspeaker array and the desired propagating distance and the size of the trajectory is investigated, and the sound beam propagating along a parabolic trajectory, a quarter circle trajectory and a half circle trajectory is implemented through experiments with a 64-unit loudspeaker array in the anechoic room. The simulated and experimental results demonstrated that the proposed method shows strong robustness to the obstacle scattering effect when there exists a rigid sphere in front of the loudspeaker array.Finally, a potential application of the proposed tangent line based method in audio system is investigated through simulations. The performance of the proposed method on acoustic contrast control in an arc-shaped area is studied and compared with the acoustic energy difference maximization method. The simulation results demonstrate that the proposed tangent line based method can achieve a trade-off between the acoustic contrast and the acoustic uniformity inside the bright zone, and better robustness to the loudspeaker response mismatch and the loudspeaker position variation. The time domain simulation shows that the proposed method can produce flat frequency response inside an arc-shaped area, hence generates better speech quality than the acoustic energy difference maximization method.