On “light body” hydrophones and the directional properties of microphones

1.Ellipsoidal Directional Hydrophones . Prof. W. H. Bragg in 1916 suggested a simple form of directional hydrophone, consisting of an elongated body of rigid but light construction, in which was mounted a carbon granule microphone of the “button” type. The principle of action of this body was expected to be as follows:— If a body floats freely, wholly or partly submerged, in water through which sound waves (or other waves) are passing, it oscillates with the water and with the same amplitude as the water, provided its dimensions are small compared with the wave-length. This follows from the fact that both the mass of the body and the forces which act on it are the same as in the case of the water which it has displaced. If, however, the mass of the body is less than that of the water which it displaces (in which case, of course, constraint must be used to keep it down), the motion of the body is greater than that of the water, provided that the constraints do not interfere with the vibratory motion, and vice Since the body now moves with respect to the water, the amplitude of its motion will, from hydrodynamical considerations, depend upon the shape of the body. If, for example, the body is ellipsoidal, its motion will be greatest when the greatest axis coincides with the direction of the wave. The response of the enclosed microphone will vary, therefore, with the orientation of the ellipsoid ; hence the “light body” will constitute a directional hydrophone.