144 Lecture 9 
dc voltage 
pay 
electrical signal incident pressure pulse 
Fig. 9.5. Electrostatic hydrophone 
with rigid dielectric. 
/) 
brass back plate 
BS SSESSSSSSSSSSSSESE SS 
VILLLLLLL LLL LLL. 
LB 
conductive shield 
dielectric foil 
foil are obtained. With a dc voltage applied, an alternating signal voltage is 
obtained across a load resistance as in conventional condenser microphones. 
The highly damped resonant frequencies occur in the Mcps-range. Figure 9.5 
shows the configuration of such a microphone. 
If the sound wave to be measured hits a free water surface, it is possible 
to measure directly the elongation of the surface, for example, with an electro- 
static pick-up arrangement. At higher frequencies it is, however, more ex- 
pedient to measure the particle velocity. According to Eisenmenger, this is 
achieved by covering the free water surface with a thin light plate with a con- 
ductive strip of silver paint on the rear side. This conductive strip is placed 
in a homogeneous magnetic field. The arrangement is similar to a ribbon 
microphone measuring the particle velocity of the free surface. The frequency 
response of this microphone depends more upon the electric circuit charac- 
teristics than upon the mechanical design which is illustrated in Fig. 9.6. It is 
obvious that this microphone operates only at high sound energy levels and that 
it will always be relatively large compared to the wavelength. 
Due to the high sound energy levels, it is also possible to measure the 
density variation of the water in the sound field or, more exactly, to determine 
the density gradient with a photoelectric means. For this purpose, a beam of 
light is sent through the water at agiven test point and Schlieren optics are used 
to mask out the beam when there is no applied sound field. When the sound field 
perspex 
incident wave 
/ 
ff 
a ~ 
permanent magnet 
Fig. 9.6. Ribbon-type hydrophone. 
