16 Lecture ] 
All but the 120 kcps unit have aluminum heads with a head-to-shank area ratio of 
4 to 1 and a brass tail. The components of an individual unit are cemented to- 
gether with a strong epoxy adhesive, electrical contact being made to the elec- 
trodes of the barium titanate discs by copper gauze in the joints. 
The elements are usually mounted in air-filledcases, radiation being admitted 
through a neoprene window which has the same characteristic impedance as sea 
water and is bonded to the radiating surface of the heads. Figure 1.7 is a typical 
box and Fig. 1.8 shows an array of boxes in a towed body for use in propagation 
studies. 
The calibration results of a 4-element, 5-kcps transducer having a radiating 
area of about one wavelength by one wavelength are shown in Fig. 1.9. 
1.6.4. Power-Handling Capabilities 
Transducers such as the ones just described are capable of radiating powers 
of several watts per square centimeter of radiating area, and with the develop- 
ment of such projectors the factors which limit the power-handling capabilities 
of transducers become important. 
The onset of cavitation determines the limit imposed by the medium, i.e., the 
maximum power that can be transmitted by a projector of a certain radiating 
area. Cavitation occurs when the instantaneous acoustic pressure exceeds the 
sum of the static pressure and any inherent cohesive pressure of the liquid. 
Under this condition small cavities filled with water vapor are formed. When 
applied to transducers cavitation can be described physically as the rupture of 
the water in front of the projector face caused by the negative-pressure excur- 
sions exceeding the sum of the hydrostatic pressure and the tensile strength of 
Fig. 1.7. Five-kcps trans- 
ducer. 
