Michael A. Smith
Gary W. Elko
J. E. West
R. A. Kubli
Acoust. Res. Dept., AT&T Bell Lab., Murray Hill, NJ 07974
Second-order differential microphones provide an excellent means for obtaining a superdirectional response with frequency-independent directivity and small size. By subtracting the output of two closely spaced first-order differential dipole [cos((theta))] microphones, a linear quadrupole is obtained whose directivity pattern is cos[sup 2]((theta)). This particular directivity pattern is realized only when the microphones are in an acoustic free-field. When differential systems are placed on or near a finite surface, the effects of diffraction and scattering can significantly modify the directional response relative to the free-field conditions. Since most differential microphones in teleconferencing and business environments are placed on a desktop surface, it is important to understand the effects of diffraction on these systems. A second-order differential microphone was constructed that has been tested for directivity fluctuation in the presence of a large reflecting surface. These experimental results match predicted theoretical patterns for diffraction on a hard disk. Further experimentation with varying surface treatment and different reflector geometries have given valuable insight into the effects of diffraction and scattering on differential microphones placed on a finite reflecting surface.