J. E. West
G. W. Elko
R. A. Kubil
AT&T Bell Labs, Acoust. Res. Dept., 600 Mountain Ave., Murray Hill, NJ 07974
J. P. McAteer
Consumer Products Lab., Indianapolis, IN
Second-order differential (SOD) toroidal and uniaxial microphones derived using a first-order differential (FOD) sensor and a reflecting plane are described. For a uniaxial implementation a dipole FOD sensor 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 FOD sensor dipole distance and the distance from the reflecting plane. If the reflecting surface is large enough, or if the wall of an enclosure is used, the resulting microphone becomes a SOD axial microphone. The close match between the sensor and its image from a good acoustic reflector results in a uniaxial SOD microphone with a 3-dB beamwidth of 66(degrees). The sensor will operate as a differential microphone at frequencies 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 FOD sensors at right angles to each other and with the axis of each sensor at 45(degrees) to the reflecting surface. Also, the spacing between sensors is twice the height of the sensors from the reflecting plane. Measurements show that these transducers can be realized with existing commercially available electret condenser dipole sensors.