116 Lecture 6 
quencies. Directivity measurements are used to check off the identity of the 
different units of a split transducer. 
6.2.3.3. Transparency Measurements 
When designing sonar transducers, it is sometimes necessary to use for 
the construction of the radiating area a material having a high degree of trans- 
parency. Materials of this kind are tested in the tank by the pulse method. Tests 
are also carried out on sonar-dome materials. 
For this purpose, we use a directional transducer transmitting pulses re- 
ceived by an omnidirectional hydrophone located ata distance of two meters. The 
sample is placed halfway between transducer and hydrophone, and the com- 
parison of the signals obtained with and without interposition of the sample 
furnishes the transparency value. These measurements can be made at different 
angles of incidence, while the signal level is recorded as a function of the angle 
chosen. 
In this way, we measure rubber, plastic, and steel plates. A convenient size 
for the sample to be measured is 1 by 1 m. The accuracy of this kind of meas- 
urement is largely dependent on the way the inclination can be measured. 
As the results obtained are not very accurate, these measurements have only 
a relative value, but they are very convenient for comparing various materials. 
6.3. TEST-AND-CALIBRATION BARGE MOORED IN THE HARBOUR OF LE BRUSC 
6.3.1. Mission 
The mission of the field station of Le Brusc is to provide a test-and-calibra- 
tion facility for underwater sound transducers, arrays, and sonar domes under 
free-field conditions and by continuous wave measurements, on a large scale 
and more effectively than would be possible in an anechoic tank. Some of the 
measurements conducted in this barge are: 
Fig. 6.14. Test-and-calibration barge of Le Brusc. 
