20 Lecture ] 
place either in the barium titanate or inthe joints. Since with present techniques 
the joint strength can be made at least equal to the tensile strength of the barium 
titanate ceramic and since it is unlikely that the strength of the ceramic can be 
increased significantly, a method was sought which would overcome the inherent 
tensile weakness of the elements. Use was made of the high compressive strength 
of ceramics: a mechanical bias or compressional prestressing was applied to the 
ceramic sandwich so that the dynamic tensile forces would be eliminated or at 
least held within the dynamic tensile strength of the ceramic [17]. 
Figure 1.12 is a cross-section sketch of a typical element. The stress is 
applied by means ofa bolt which passes through a hole along the center line of the 
element and is threaded into a tapped hole in the head piece. The bolt is then 
tightened with a torque wrench to apply compressional stress to the ceramic. 
Calibration of stress-rod elements showed that the introduction of the stress - 
rod had little effect on the performance of the elements; the electroacoustic 
efficiency remains sensibly the same regardless of stress. Power outputs of 
7 w/cm? were achieved but tests at higher powers were eliminated by the incep- 
tion of cavitation. Driving these units ina highly cavitating state at shallow depths 
or in air did not change their characteristics. 
The principles of prestressing are, of course, applicable to any transducer 
in which the amplitude of vibration of the element is limited by the mechanical 
strength of the active material or by the joint strength. Since all ferroelectric 
ceramics and magnetostrictive ferrite ceramics are weak intension but inherently 
strong in compression, prestressing should allow transducers to be built with 
higher power-handling capability and with greater resistance to mechanical shock. 
1.6.5. Deep Hydrophones 
To illustrate the problems associated with the design of transducers for 
operation at great depths, a hydrophone designed to operate at depths down to 
15,000 ft will be described. The first requirement of a deep transducer is that it 
be mechanically capable of withstanding the hydrostatic pressure. A spherical 
shell is an excellent shape to withstand hydrostatic pressure since the stress is 
evenly distributed and no stress concentrations occur. 
Consider a thin spherical shell of a ferroelectric material which is polarized 
in the radial thickness. The low-frequency sensitivity of the unit is given by 
VE tes Dees N=m=p) Gn Gs p+ o) (19) 
P aaa ees Be . | 
NODAL PLATE STRESS BOLT 
Fig. 1.12. Sketch of stress-rod 
element. 
HEAD ee TAIL 
