The problem
Our lab conducts birdsong playback experiments with song sparrows. The design calls for songs to be played from the border of a subject neighbor territory. The problem is that the very neighbor being simulated often hears the playback and breaks into the experiment. We needed a speaker directional enough that the subject could hear the playback while the neighbor behind the speaker could not.
The solution
A Sony parabolic microphone, about 35 cm in diameter, was fitted with an enclosed Radio Shack midrange speaker facing inward, with the cone at the focal point of the parabola. A battery powered 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. The photograph below shows the finished unit on its tripod.
Results
The parabola is highly directional in the frequency range of our playbacks, 2 to 8 kHz. Directionality is reduced progressively at frequencies below 2 kHz. The graphs below show measurements at 0.5, 1, 2, 4, and 8 kHz. Each graph plots attenuation in dB from the measurement directly in front of the speaker at 0 degrees over a range of angles: 0, 45, 90, 135, and 180 degrees. All measurements were taken at 10 m distance. The shaded areas represent ambient noise levels at that frequency. The 0.5 and 1 kHz responses are nearly nondirectional.
Sizing a parabola
To find the minimum dish size needed to broadcast a desired frequency range, use the formula below.
Diameter (m) = (2 × 331.6) / minimum frequency (Hz)
The measurement approach follows the microphone array work reported in the Journal of the Acoustical Society of America, and the speaker was used in playback work at the Cornell Lab of Ornithology.