Wing vibration is a very common method of sound production in insects. Such sounds are characteristic of a lower frequency than those produced by stridulation or percussion. The attenuation of such low-frequency sounds in air is considerable, and consequently these sounds are used mainly for communication at close range. In some cases, the sound produced is of such a low amplitude as to approach the limits of sensitivity of conventional microphones. For example, in the genus Drosophila, the signal does not exceed ten-billionths of a Watt, in terms of power output. A further complication is that the small acoustic source can easily be drowned out by other sounds: a relatively high noise/sound ratio is produced by environmental noise and alternating currents (e.g. light sources, recording equipment). Sound insulation is necessary to achieve high quality recordings. In order to insulate the signal from interfering sounds, researchers in the domain have used different "acoustic chambers". Nevertheless, despite these precautions of isolation, it is often necessary to make recordings when conditions are particularly quiet (night, weekend, when the extraneous sound level is less obtrusive) or in special sound-attenuated rooms. These conditions for recording are highly constraining. The aim of this paper is to present a very simple and efficient method of recording acoustic signals of low amplitude without using the constraining methods described above. The principle of the method is based on the use of two simultaneous recordings, one of the ambient noise and the other of the signal to be analysed plus the ambient noise. Then, a simple subtraction of these two recordings allows the possibility to isolate the signal with a good signal to noise ratio. The different steps of the method can be achieved either analogically or digitally. Some examples of results obtained with this method are provided. They concern the courtship song of the fly Drosophila melanogaster recorded in a room without any particular conditions of insulation.
T. Aubin (1996). New methods for recording low-amplitude signals. Application to the analysis of the courtship song of Drosophila genus [abstract]. Bioacoustics 6(4): 297-298