LECTURE 1 
TRANSDUCERS 
D. Schofield 
Naval Research Establishment 
Halifax, Nova Scotia, Canada 
1.1. INTRODUCTION 
Research into underwater acoustics could be carried out using purely acoustic 
links but the limitations would be many. In a purely acoustic system, there are 
very few practical instruments for detecting sound other than the ear. Although the 
ear is a beautiful instrument and in many ways surpasses electronic devices, it is 
not adequate when accurate quantitative measurements are required. The conver- 
sion of acoustic energy to electrical energy immediately allows the employment 
of a wide variety of electrical measuring and recording instruments. Similarly, 
electrical to acoustic conversion devices give a greater measure of control than 
the use of bells, explosive charges, etc., would permit in the generation of sound. 
Electroacoustic transducers for use in air, i.e., loudspeakers and micro- 
phones, are familar devices. However, waterproofed loudspeakers and micro- 
phones designed to couple to air are quite inadequate to couple to water. In 
technical terms, the acoustic mismatch is great and the loss in conversion ef- 
ficiency is high. The characteristic acoustic impedance of water is nearly 4000 
times the characteristic acoustic impedance ofair,anda simple calculation indi- 
cates that the efficiency of conversion of a transducer designed for operation in 
air would drop by a factor of about 10,000. An underwater projector is required 
to produce about 60 times the force and Veo the displacement of aa equivalent loud- 
speaker projecting the same energy into air. 
The other immediately obvious difference is the static pressure exerted by 
the environment. Greater interest is developing in the deep ocean, and a trans- 
ducer at 10,000 ft is subjectedtoa static pressure of 4457 psi. The problem posed 
by the hydrostatic pressure is not only one of mechanical strength. It also places 
limitations on the type of active component and on the acoustic design of the unit; 
for example, sponge rubbers cannot be used as pressure-release materials. 
Changes in the parameters of transducer materials may also be caused by the 
stresses produced by the hydrostatic pressure. 
This paper is restricted to electroacoustic transducers; i.e., projectors which 
convert electrical energy to acoustic energy and, vice versa, hydrophones which 
convert acoustic energy into electrical energy. Most of the units are reciprocal 
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