M. P. Foach 105 
V=1000 v e=1 mm 
v=100v s=80cm? 
The signal output voltage developed across the terminals of the hydrophone 
by this pressure is measured withthe same voltmeter used for measuring v, with 
the help ofa precision attenuator, inorderto have the same deflection for v as for 
the signal output voltage. The errors due to the instrument are thus eliminated. 
The hydrophone response is given by 
S, = 20 log & — 20 log i attenuation 
Accuracy of measurement may be affected by fringe effects, but classical cor- 
rection factors will reduce this effect to a negligible value. 
The distance between the electrode and the diaphragm may prove to be a 
limiting factor for the accuracy of the method. This distance being of the order 
of 1 mm, it will have to be measured with an accuracy better than 0.015 mm in 
order to avoid a resulting error greater than all the other errors involved. 
Provided that the diaphragm be absolutely plane, good precision can be obtained 
by using a comparator. 
The total error involved in one measurement is less than 1 db. Taking the 
average of a series of measurements we may assume the error to be less than 
0.5 db. 
The holding device for hydrophone and electrode has to be completely rigid, 
free from vibration and natural resonance, and it must be set up on resilient 
mountings. We have mounted this device on aconcrete pillar erected on the rock 
foundation and isolated from the laboratory building. 
The upper limit of the frequency range covered by this calibration method is 
given by the natural resonance of the holding device or of the diaphragm. In this 
case, free-field calibration will be called for. 
Basically, this method has no limits in the low frequency region, but in fact 
ac voltages below 1 cps are not easy to produce. On the other hand, it is not 
necessary in this range that the driving force be distributed evenly over the 
whole area of the rigid diaphragm; a drive in the center is sufficient. In this case, 
simpler mechanical methods can be applied; for instance, stretching a spring 
periodically by means of a crank. Another method consists of gluing to the 
diaphragm a coil which is placed in a magnetic field. The driving force being 
Proportional to the current in the coil, the calibration is easy to achieve. 
6.1.3. Relative Electrostatic Calibration 
In the case of a diaphragm that is not rigid and plane, the absolute electro- 
static calibration technique does not work but may be applied in a relative way 
for measurements up to 10 kcps. It then works on the following principle. The 
electrode surrounds the diaphragm, the latter being made of rubber and having 
a cylindrical or any other convenient shape. The conductivity of the rubber is 
assured by a thin coating of silver paint. Although the value of the distance e be- 
tween the diaphragm and the electrode is not known with an accuracy adequate 
for absolute measurements, the shape of the frequency curve can be plotted with 
sufficient accuracy. We use this technique for achieving calibration of the hydro- 
