A plurality of fixed arms projecting to the inside of the frame, a plurality of movable arms that are longer than the fixed arms and are detachable, , And a number of microphones (14). A small-sized microphone array is constructed as shown in FIG. 8 (A) by radially storing a plurality of movable arms (8) inside the frame (2). On the other hand, by deploying the plurality of movable arms (8) radially outwardly of the frame (2), a large size microphone array is constructed as shown in FIG. 8 (B). In this manner, when constructing a circular two-dimensional microphone array, it is possible to cope with acoustic exploration at a wide frequency by making it easy to change its size.
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