Studying room acoustics using a monopole-dipole microphone array

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The use of a sound field microphone for examining the directional nature of a room impulse response was reported recently [R. Essert, J. Acoust. Soc. Am. 100, 2837(A) (1996)]. By cross-correlating monopole and colocated dipole microphone signals aligned with left–right, up–down, and front–back axes, a sense of signal direction of arrival is revealed. The current study is concerned with the arrays ability to detect individual reflections and directions of arrival, as a function of the cross-correlation window duration. If this window is too long, weak reflections are overlooked; if too short, spurious detections result. Guidelines are presented for setting the window width according to perceptual criteria. Formulas are presented describing the accuracy with which direction of arrival can be estimated as a function of room specifics and measurement noise. The direction of arrival of early reflections is more accurately determined than that of later reflections which are quieter and more numerous. The transition from a fairly directional sound field at the beginning of the room impulse response to a unidirectional diffuse field is examined. Finally, it is shown that measurements from additional dipole orientations can significantly improve the ability to detect reflections and estimate their directions of arrival. [Work supported by NASA.]