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Compact Spherical Loudspeaker Array for Variable Sound-Radiation

Authors Kerscher, M.
Year 2010
Thesis Type Diploma thesis
Topic Spatial Audio
Keywords technical acoustics, audio recording, audio reproduction, Ambisonics
Abstract In this work a new spherical loudspeaker array is developed that allows variable sound- radiation in all directions, so called spherical beamforming. In addition to the realization of musical and artistic performances, it can be used for room-acoustical measurements. From a mathematical point of view, radiating sound fields can be decomposed into spherical harmonics, which can be played back by the loudspeaker array and hereby reproduce the desired sound field. The few existing spherical loudspeaker arrays in this field of application use a regular design with Platonic solids, which on the one hand show convenient behavior due to their mathematical decomposition, but on the other hand use an inefficiently large number of loudspeakers. In contrast to Platonic solids, this work uses a spherical geometry, which allows irregular arrangements in favor of an optimal arrangement of the loudspeakers. The spherical cabinet developed in this work mounts 16 loudspeakers, which are uniformly distributed over the surface. This arrangement provides controlled playback of the spherical harmonics up to the third order. As the array is designed for mid-frequency range, small dimensions of the sphere and loudspeakers are desirable. Therefore a com- promise between compact dimensions and upper and lower cut-off frequencies must be achieved. In particular spatial aliasing should be avoided and a feasible dynamic range shall be enforced by appropiate filtering. The work starts with the design and construction of the spherical loudspeaker array including amplification, wiring and mounting. Simulations using the so called sphere cap model, which models the distribution of the loudspeakers on the sphere analytically, enable a pre-estimation of the radiation properties. Finally, acoustical measurements with the complete prototype of the array are made to determine the transfer properties of the loudspeakers and their mutual interaction in the all-over system. For system control, an appropriate algorithm using this data is implemented. This control system contains angular and radial filters. As the latter can use unfeasibly high bass-boosts, an appropriate filterbank for limitation is developed.
Supervisors Zotter, F.