Synthesis of room impulse responses for arbitrary source directivities using spherical harmonic decomposition

Abstract

The acoustics of rooms can be accurately described by the room impulse response, stating the temporal succession of the room reflections encountered for a specific transfer path in the room. The measurement is usually performed with a single channel spherical loudspeaker—ideally with an omnidirectional radiation pattern—providing an objective measure for a given combination of a source and receiver position in a room. The measurement result can be used to auralize sources in the measured room by convolution with a dry signal or to derive standardized room acoustical parameters. However, every acoustic source has a distinct frequency dependent radiation pattern, which is to some extent responsible for the perceived characteristics of the sound source. The conventional measurement of room impulse responses does not account for this effect. Hence, a method is proposed to obtain room impulse responses for specific radiation patterns by superposing measurement results of an electro-acoustic sound source with a known radiation pattern for different orientations. The limitations of the method are examined in terms of frequency range and spherical harmonic order. The assumption of an LTI system is studied by special measurements. Finally results of measured room impulse responses for a target source are compared to the synthesis results obtained by the proposed method.