D. Schofield 25 
ferroelectric transducer materials. Not only will information pertinent to the 
design of transducers be produced, but also data which must be explained by 
any comprehensive theory of ferroelectricity. 
To date great use has been made ofpressure release materials, e.g., sponge 
rubber, or air, in transducers. These conventional pressure-release materials 
are not suitable for use at great depths: sponge rubber collapses and the use of 
air requires cases which are mechanically very strong, and hence heavy and 
bulky. It may, of course, be possible to design elements which will only radiate 
unidirectionally. Techniques using spatially separated units and time delays can 
be used to obtain unidirectional hydrophone arrays. 
Knowledge of cavitation applicable to transducers is limitedand further work 
is desirable. 
Although in the past decade with the introduction of ferroelectric ceramics, 
there has been much improvement intransducer materials, further improvements 
in coupling factor and loss are desirable. 
1.8. ACKNOWLEDGMENT 
This work is published by permission of the Defence Research Board. 
DISCUSSION 
DR. W.N. ENGLISH asked the lecturer how closely the performance of a low- 
frequency transducer followed the design performance figures and, secondly, 
whether the production of a transducer operating at 1 to 2 kcps and giving, say, 
110 db above 1d/cm” at 1 yard was now feasible without having a weight of 
several tons. 
DR. SCHOFIELD: The physical principles of low-frequency transducers are 
the same as for transducers at high frequencies. Therefore, provided that all the 
important parameters are taken into account, the design performance of low- 
frequency transducers can be predicted to the same order of accuracy as trans- 
ducers at higher frequencies. 
Low-frequency resonant projectors are large and heavy for two reasons: 
1. Low-frequency mechanically resonant systems are inherently larger than 
for frequencies in the range of tens of kilocycles 
2. To obtain a good transfer of energy from a vibrating piston to the water, 
the dimensions of the radiating face must be at least the order of one 
wavelength 
For a highly efficient projector at 1 kcps, the dimensions of the radiating 
face must be abuut 5 ft. To keep the weight of the projector low, care must be 
taken in designing the mechanically resonant system, and some compromise in 
characteristics such as frequency bandwidth and depth of operation may be 
necessary. I think that a 1 keps projector with a weight of less than one ton is 
practical. 
DR. S. WENNERBERG suggested that the reduction of the dynamic tensile 
strength in a composite transducer could originate inthe excess stresses caused 
by differences between the Poisson coefficient of the different materials, and 
inquired whether any investigation had been made of this possibility. 
