Image-derived second-order differential microphones
Gary W. Elko, James Edward West, Robert A. Kubli, Jeffrey P. McAteer · The Journal of the Acoustical Society of America · 1994
Second-order differential toroidal and unidirectional microphones derived using a bipolar first-order differential sensor and a reflecting plane are described. The bipolar first-order differential microphone is positioned with its axis perpendicular to and suspended a few centimeters from a large acoustically reflecting surface. The resulting sensor image is phase reversed resulting in a transducer that is a linear quadrupole. The linear quadrupole can be described by two dimensions, the distance corresponding to the dipole distance and twice the distance from the reflecting plane. If the reflecting surface is large enough or if the wall of a room is used, the resulting microphone becomes a second-order differential unidirectional microphone. The match between the sensor and its image from a good acoustic reflector results in a second-order differential microphone with 3-dB beamwidth of ±33° and no grating lobes below 3 kHz for a 2.5-cm spacing between the sensor and reflecting surface. A wall-mounted toroid can be formed by using two bipolar first-order sensors at right angles to each other and with the axis of each sensor at 45° to the reflecting surface and a spacing between transducers that is twice the height of the transducers from the reflecting plane. A table-mounted toroid can be realized by properly combining a filtered version of a suspended first-order sensor and an omnidirectional sensor flush mounted to the reflecting table top.