Pistonphones

The pistonphone is a very accurate, reliable, and simple device for calibrating a microphone that is convenient for use in the field. The principle of operation is quite simple. A small battery‐powered electric motor drives a shaft on which is mounted a cam disk. The cam disk drives two pistons symmetrically. The cam gives the pistons a sinusoidal motion at four times the shaft rotational speed. The stroke of the pistons (or peak amplitude from mean position) is thus one quarter of the difference in maximum and minimum diameters of the cam. The pistons vary the cavity volume sinusoidally in time and since for pistonphones a low frequency (e.g. 250 Hz) is normally chosen (for mechanical reasons), a corresponding sinusoidal variation in pressure occurs. (See Ref. [7] and Chapter 3 in Ref. [5] for the theory and further details of its operation.)

Provided the piston stroke is carefully controlled, the sound pressure level produced can be accurately predicted. Good sealing should be maintained when the microphone is fitted into the coupler opening. In principle, this type of pistonphone can be used to calibrate any microphone, provided that good sealing is maintained. If microphones of different diameters are to be calibrated with the pistonphone, and different coupler connections are used to keep the volume unchanged, the sound pressure level is unchanged. If a different volume results, the change in sound pressure level can be predicted. (See Ref. [5] for the theory.) A change in atmospheric pressure will alter the calibration; but a correction is simply made by the use of a barometer provided by the manufacturer.