The Physics of Parabolic Microphones: Frequency Dependence of Gain

Summary

Introduction Parabolic microphones are known for their extreme sensitivity, and the origin of their acuity isn’t difficult to guess. It is the most obvious thing about them, which can also make them a liability for field use, namely, their considerable size. Just as a large amount of weak light is captured by a telescope’s parabolic mirror and made amenable to viewing with the much smaller human eye, so too are faint sounds harvested with a reflecting dish that far exceeds the dimensions of our native auditory tools. In both cases, it is a matter of intercepting more of the signal and bringing it to a focus, and the same basic wave physics is at play. A good overview of the early adoption of parabolic microphones for the field recording of bird sounds was provided by Wahlström (Wahlström, Sten. “The Parabolic Reflector as an Acoustical Amplifier.” Journal of The Audio Engineering Society 33 (1985): 418-429). He describes how Peter Paul Kellog and others first used a 32-inch dish to capture avian vocalizations, with a Yellow-breasted Chat having the honor to be the first species recorded with their novel device. That a large dish may be cumbersome to transport and use in the field isn’t the only caveat. Users of parabolic microphones soon learn that these devices have a bias that favors high frequencies. Where this is most evident is in the “tinny” quality to the captured sound, which some listeners find off-putting. Others find this drawback significantly outweighed by the benefits of hearing otherwise inaudible sounds, and the boost in signal strength that comes with the high directionality. The flip-side is that the performance drops off for low frequencies, with a cut-off point below which no benefit is derived. That is, the microphone will capture the same signal power whether the dish is present or not, which may seem curious. This cutoff frequency is given by , where V is the velocity of sound and a is the diameter of the dish. For a=32 inches, this works out ...