Measurement of spatial impulse responses with a Soundfield Microphone

Abstract

Concert hall designers have a keen interest in measuring the spatial evolution of sound fields. So far, 3-D impulse responses have been measured with sound intensity methods, with tetrahedral microphone arrays, and with large microphone arrays. These methods are very sensitive to phase, and therefore to particulars of microphone construction and analysis bandwidth. In this paper a method is proposed for capturing the spatial impulse response of a room by means of orthogonal cosine directivity microphones. Tests have been done with a simple figure-eight/omni microphone pair oriented in three directions, one after the other, as well as with a purpose-built Soundfield microphone. Impulse responses are captured for each of the X, Y, Z, and omni (Soundfield B-format) channels. The directional impulse responses are then cross correlated with the omni impulse response to construct ‘‘direction cosines’’ of strength with respect to the listener position, over time. Data can be mapped graphically in 3-D space to visualize the evolution of the sound field; the degree of scatter in the data represents diffuseness. In addition to graphics, convincing three-dimensional auralizations have been performed with soundfield impulse responses.