LECTURE 6 
UNDERWATER SOUND CALIBRATION STATIONS 
AT LE BRUSC LABORATORY 
M.P. Foache 
Laboratoire de Detection Sous Marine 
Le Brusc (Var.), France 
The Laboratoire du Brusc has at its disposal four different calibration sta- 
tions for hydrophones and underwater sound transducers, each one using a dif- 
ferent calibration technique. The joint operations of these calibration facilities 
permit measurements covering a range from 0.1 cps to 100 kcps. 
Depending on the frequency range to be covered, the following procedures are 
used: (1) electrodynamical calibration, (2) electrostatic calibration, (3) closed- 
tank alternating pressure technique, (4) calibration by comparison of filtered 
noise, (5) pulse technique, and (6) free-field reciprocity calibration. 
6.1, HYDROPHONE CALIBRATION 
The calibration of hydrophones at low frequencies is generally conducted in 
a range below 1 kc. The hydrophones being of different types, the procedure of 
measurement depends not only on the frequency range to be covered but also on 
the hydrophone design. 
The hydrophones used in the frequency range from 0.1 to 1000 cps have a 
rigid metal diaphragm vibrating in contact with a piezoelectric element, for 
instance, a slab of titanate or zirconate ceramic. Electronic calibration is 
particularly well adapted to this kind of design. The hydrophone used in higher 
frequency ranges is an assembly of mounted ADP crystals or ceramic elements 
sheathed in a rubber boot; the elements are either in direct contact or bathed 
in castor oil which fills the boot. Certaintechniques used with diaphragm -hydro- 
phones may not be applied to these structures. 
6.1.1. Electrodynamic Calibration by Discharge into a Ballistic Galvyanometer 
The approximate sensitivity of a hydrophone can be measured by the deflec- 
tion of a ballistic galvanometer when a sudden change of pressure is applied. 
This pressure variation is produced by applying a weight to the rigid diaphragm. 
For other types of hydrophones, a decompressional chamber may be used. 
This kind of measurement, when used for any of several types of production 
hydrophones, is liable to cause considerable errors, as the capacitance of some 
kinds of ceramics depends upon the frequency. We are using this method pri- 
marily for a rapid check of production hydrophones. 
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