On the Frequency of Vibration of Circular Diaphragms
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The Paper describes measurements of the natural frequency of diaphragms of various sizes having a rigid rim and a central boss for attachment of a microphone or receiver. In air the frequency was found by means of a monochord, but under water the resonance-frequency was noted in the neighbourhood of a subaqueous transmitter of variable pitch. The results are in good agreement with the theoretical conclusions of Prof. H. Lamb. In many cases harmonies were observed round about a fifth or an octave above the fundamental, but their occurrence was capricious, and their pitch inconsistent with theory. The resonance peaks of the frequency curves are more or less of the same area, being high and narrow or low and broad. The effect of increasing the pressure on one side of the diaphragm was studied, and the pressure-displacement curve was found to be linear up to the elastic limit, while the pressure-frequency curve is of the saturation type. A large diaphragm is less affected by pressure (and therefore by iminersion in deep water) than a small one of the same natural frequency, in consequence of its greater thickness.