24 Lecture 1 | 
effect of pressure on the dielectric constant of 3-in. diameter, '/-in. wall thick- 
ness spheres of barium titanate and PZT-4, Each experimental point was taken 
10 min after the application of stress. In additionto the pressure effect, there is 
a time effect shown in Fig. 1.17. The capacity decays linearly with the logarithm 
of time after the application of the stress. This is an aging phenomenon similar 
to the aging obtained following the poling of ferroelectric ceramics. Figure 1.18 
shows the effect of pressure on the dielectric constant, with d constant and the 
coupling coefficient of NRE-4 (BaCaCoTi0s); the material was in the form of a 
3-in.-diameter, '/,-in.-wall thickness sphere. 
It should be emphasized that all the parameters (e.g., dielectric constant, 
coupling), need to be examined when considering the effect of pressure on trans ~ 
ducers. It should also be noted that these results are only applicable to trans - 
ducers subjected to two-dimensional stresses, the components of which are 
approximately equal. 
1.7. CONCLUSION 
It has not been possible to cover even superficially the whole of the field of 
electroacoustic transducers and no attempt has been made to do so. High-power 
projectors, highly efficient over a relatively narrow bandwidth, can be designed 
for the frequency region of a few kcps to over 100 kcps. These longitudinally 
resonant projectors are not satisfactory at low frequencies, say less than 1 kcps, 
because of their physical size and consequent difficulties in handling. Flexural 
resonances and designs incorporating relatively soft mechanical springs can be 
used and are effective in reducing the physical size. However, low-frequency 
Projectors, particularly for deep operation, remain a problem. 
Hydrophones for particular applications at shallow depth are fairly readily 
designed. 
Both projectors and hydrophones for operation at great depth present inter- 
esting problems. At N.R.E. a study has been undertaken of the effect of stresses, 
unidirectional, bidirectional, andthree-dimensional, onthe parameters of ceramic 
= 
“ T T T T T T T T 
i 60 
, Bike nu = | 
Fig. 1.18. Variation of die- — = ~= ~_ 
lectric constant, piezoelec- 0 
tric constant, and planar = 50 1 1 1 i i 
, ns ; rs) 
coupling coefficient with hy- 
drostatic pressure for NRE- 
4 in the form of a 3-in.-di- 35 
ameter sphere. 
it ESE | 
fo) I 2 3 4 5 6 7 8 
PRESSURE (102 PS!) 
