Toroidal Microphones
Gerhard Martin Sessler, James E. West, M. R. Schroeder · The Journal of the Acoustical Society of America · 1969
“Toroidal” microphones are defined as microphones having toroid-shaped directional characteristics, i.e., they have minimum sensitivity along the rotational axis of the toroid and maximum sensitivity in the plane perpendicular to that axis. Several practical designs of toroidal microphones based on second-order gradient receivers are proposed and analyzed as to directional characteristics and frequency response. Experimental microphones are presented, some of which use a Helmholtz resonator tuned to the lower end of the frequency range to compensate the ω2 dependence of the second-order gradient receivers and to improve the over-all signal-to-noise ratio of the microphones. Measurements show that the microphone sensitivity in a plane through the rotational axis obeys approximately the expected cosine-squared law. In the direction of the rotational axis, the sensitivity at midfrequencies was found to be typically 20 dB lower than the sensitivity in the plane normal to that axis. The equalized frequency response in this plane is within ±3 dB from 0.3 to 2.5 kHz. Sensitivities are about −80 dBV/μbar. Using existing low-noise pre-amplifiers, this corresponds to a signal-to-noise ratio of about 35 dB for speech at conversational levels.