By John Burt
Our lab conducts birdsong playback experiments with song sparrows. Our current design calls for us to play songs to our subjects from their neighbor's territory border. The problem is that the very neighbors we are simulating often hear the playback and break into the experiment. We needed a speaker that was directional enough that subjects could hear the playback, but the neighbor behind the speaker could not.
I took a Sony parabolic microphone (~35cm diameter) and mounted an enclosed mid-range speaker (Radio Shack M#40-1289A) so that it was facing inward with the cone at the parabola's focal point. A battery powered speaker amplifier was mounted on the back, and sound baffling material was wrapped around the outer rim of the dish to reduce side 'leakage' of sound. (see picture below)
The parabola speaker is highly directional in the frequency range of our playbacks (2-8kHz). Directionality is reduced progressively at frequencies below 2kHz. The figures below are from measurements I took of directionality at different frequencies (.5, 1, 2, 4, 8kHz). Each graph plots dB attenuation from the measurement directly in front of the speaker (ie; 0 deg) over a range of angles (0,45,90,135,180 deg). All measurements were taken at 10m distance. The filled areas represent ambient noise levels at that frequency. Directionality can be seen to decrease with frequency with .5 and 1kHz being almost non-directional. Click on each graph to see a bigger version. A formula to determine what size parabola to use for a given frequency range is given below.
The formula for figuring out the minimum size of parabola needed to broadcast a desired frequency range is as follows:
Diameter (m) = (2 * 331.6)/min frequency (Hz)