104 Lecture 6 
Générateur 
THT = 
Fig. 6.1. Electrostatic calibration. 
Générateur 
Générateur 
Pont dilviseur 
Mesure HT 
atténuative 
rapport] en 
tension] =1000 
Voltmétre 
a lampes 
Ampli Préampli 
Oscillo 
Mesure THT 
continue 
Boite d'atténuation 
a2 
Sh=20 log = -att-k 
6.1.2, Absolute Electrostatic Calibration* 
This is our basic technique adopted forthe low frequency range, and we con- 
stantly endeavor to improve it. Covering a range from 1 to 1000 cps, it may be 
used only for hydrophones having a rigid and absolutely plane diaphragm. It works 
on the following principle: a driving force generated by an ac field is applied 
between the diaphragm and another plate representing the electrode. This tech- 
nique is currently used for small standard microphones; it may also be used for 
hydrophones, but the technological conditions are notthe same. For instance, the 
diaphragms of our hydrophones have a diameter of 640mm and are 8mm thick. 
The value of the pressure applied to the diaphragm can be found as follows: 
if the voltage applied to the plates of a plane capacitor contains a dc component 
v and an ac component v, we have for the electrostatic force driving the elec- 
trode, e being the distance between the plates, 
en 1D) 
If V is much higher than y, andif the electrode displacements are negligible, 
we can write ac 
FE = Fe Vv 
Denoting the electrode area by s, and the area of the diaphragm by S, the pres- 
sure due to the electrostatic force is 
p=—— = Vv 
47e? S 
In practice, we have adopted the following values: 
