E. Meyer 143 
Platinum wire 
Glass capillary 
Fig. 9.3. Barium titanate probe 
conductive silvershield hydrophone, 
Platinum ball 
Sintered barium titanate 
One of the smallest microphones known today is the barium titanate "pearl" 
which was first developed by N. A. Roi and E. V. Romanenko [10] and is shown 
in Fig. 9.3. A thin platinum wire is drawn through a very thin glass tube. A 
small quantity of barium titanate powder is then made to adhere at the end of 
the platinum wire and is afterwards sintered in a Bunsen flame. The wire inside 
the pearl is used as one electrode andthe other electrode is produced by silver- 
plating the hydrophone on the outside. A polarization voltage is applied between 
the electrodes. The natural frequencies of such hydrophones are in the Mcps 
range; the frequency response curve of a pearl microphone is presented in 
Fig. 9.4. At 10 Mcps the sensitivity is reduced by about 30 per cent of its value 
in the midrange. 
According to a proposal by Eisenmenger the well-known condenser micro- 
phone may also be used. The design differs, however, from the types used in 
ordinary water- or air-borne sound technique. Since the sound pressure to be 
measured is very high, a thin plastic foil metalized on one side has the rear 
side glued to a solid back-plate such as a metal disk. The sensitive microphone 
surface of such an instrument is large compared to the wavelength. The com- 
pressibility of the plastic foil used as a solid dielectric is very small, but since 
the acoustic pressure is very high, sufficient changes of the thickness of the 
M: 10? uv / bar 
1 3 5 7 39 Mcps 
Fig. 9.4. Frequency response of miniature barium titanate hydrophone, 
