TABLE OF CONTENTS The ring, or short thin-walled cylinder, is one of the most common forms of underwater transducer which is used for both projectors and hydrophones (see Chapter 4). It may be stacked and formed into a line array, end-capped and air-backed, or free-flooded and operated in either the piezoelectric 31 or 33 mode. The shape provides omnidirectionality in the plane perpendicular to the axis and near omnidirectionality in planes through the axis, if the height is small compared to the acoustic wavelength in the medium. A small height compared to the wavelength in the active material also prevents the excitation of length-extensional and bending modes of vibration. In the fundamental circumferential mode the ring vibrates in a radial direction as a single mass as a result of the expansion and contraction of the circumference induced by an electric or magnetic field and can be represented by a simple lumped equivalent circuit.
The 31-mode piezoelectric ring is the most commonly used ring transducer. It is usually inexpensive when compared to other transducers and achieves an effective coupling coefficient that is nearly equal to k 31. Usually, adding two leads, two end caps and waterproof encapsulation to the ceramic ring is all that is needed to fabricate this transducer. In addition, the equivalent circuit takes on the simplest form of any transducer. A sketch of a 31-mode ring with electrodes on the inside and outside cylindrical surfaces and piezoelectric coordinates 1, 2, 3...
