Pawel/Ensemble PA-1 & Reference loudspeakers Fruit Flies & Impulse Responses
While on the subject of foam diaphragms, I recently ran across an interesting article, by Dr. H. C. Bennet-Clark, in the March 1969 issue of Wireless World. Bennet-Clark is a zoologist and an avid audiophile. At the time he was involved in a research project examining the courtship of fruit flies, in the course of which pulses of sound were played to the flies to simulate their courtship sounds. This "love song" is apparently produced when the fly flaps its wings and consists of a single cycle of pressure change lasting 3ms. It was this sort of pulse that he wished to reproduce. He discovered pretty quickly that commercial drive-units just did not have a good enough impulse response to do an adequate job for his experiment. Breakup modes in a driver translate into peaks and dips in the frequency response. And such a frequency response corresponds to poor behavior in the time domain. The impulse response for such a driver will exhibit an oscillatory buildup followed by an oscillatory wind-down or overhang.
So he took matters into his own hands and started modifying drive-units by building up the cone with a layer of expanded polystyrene followed by a skin of aluminum foil. The resultant impulse response was presumably adequate to make the flies happy. The moral of this story is that a tight well-damped impulse response is prima facie evidence of a well-behaved drive-unit with a reasonably flat frequency response.Dick Olsher