Re: Parabolic versus pressure-zone microphone?

Submitted by Bill Evans (December 7, 2006)

Andy,

A typical parabolic microphone setup will pick up a narrower cross-section of sky than the pressure-zone "flowerpot" microphone described on oldbird.org. When mounted for night flight call monitoring, the latter has roughly a 60 degree cone of enhanced sensitivity that expands up into the sky. A typical parabolic will have a 10 degree or less cone. The frequencies for which this generalization applies depends on the width of the parabolic dish and the dinner plate (or boundary) of the flowerpot mic.

From what I understand, parabolic microphones do not amplify all frequencies equally, so when recording bird calls you get what is called a "colored" frequency response. In othe words, the quarterbacks voice will sound more tinny as the higher portions are amplified more than the lower portions. This same phenomenon occurs when more than one boundary is used in pressure-zone microphones (PZMs). This "coloring" phenomenon can lead to smeared spectrograms in some broad frequency, modulated flight calls like that those of Common Yellowthroat. Yet, relatively monotonal calls like those from Swainson's Thrush would not be affected much. I would expect parabolics to be quite good at detecting calls from high flying Swainson's Thrushes.

A typical parabolic will produce much more signal gain compared to the PZM "flowerpot" design on the oldbird website, but the latter will pick up a wider cross-section of sky. You will likely pick up many more calls with the "flowerpot" than with a parabolic in most regions of eastern North America. But in situations, where the majority of the bird are calling high in the sky (>600m agl), such as in Israel or the desert areas of western North America, the parabolic would be the better choice.

Bill Evans