122 Lecture 6 
6.4.3.4. Acoustical Test-and-Calibration Facilities 
The facilities currently provided allow the acoustical testing and calibrating 
of transducers weighing less than 5 tons, by means of a bridge with rolling 
carriage and a hand-operated rotating shaft mounted across the well. 
The standard projectors used are of the types TP8E, Ik4O, and P3OP, per- 
mitting a continuous wave calibration down to 1.5 kcps at depths attaining 30 m. 
The following improvements are already provided for in the budget: 
1. Remote control for the training of the shaft 
2. Bridge and rolling carriage for the handling of 8-ton transducers 
3. A set of two concentric shafts allowing the testing of sonar domes. 
For the lowering of 8-ton transducers into the well, the structure around the 
point of suspension will have to be stiffened. 
The installation of the test-and-calibration barge on the lake of Castillon, at 
a cost of nearly 300,000 NFr, has provided a research facility where continuous 
wave measurements and ranging downto1.5kcpsare possible under very accept- 
able free-field conditions. Systematic studies are under way to improve the 
measuring conditions, such as transducer depth, influence of the water level, 
and so forth. 
Improvement of the test facilities will have to proceed with a view to making 
the equipment suitable for the handling of very large transducers and capable of 
testing them completely and rapidly. 
DISCUSSION 
DR. H. A. J. RYNJA commented, regarding the electrostatic method of deter- 
mining sensitivity, on the measurement of the distance between the hydrophone 
membrane and the capacitor plate using an interferometer. He thought that a 
more straightforward method was to measure the actual capacitance of the air 
gap because the thickness of the gap occurs in the formula for the calculation 
of the capacitance. This latter method offers some advantages when calibrating 
hydrophones having a rubber-covered face or a curved sensitive surface. 
DR. FOACHE: It is certainly possible to measure the distance between the 
hydrophone membrane and the capacitor plate by a capacitance measurement, 
but this method does not allow one to check the parallelism of the plates. As the 
capacitance is a linear function of this distance and the attraction strength a 
quadratic function, the result may be aslight error if the plates are not parallel. 
DR. D. SCHOFIELD said that the reciprocity calibration of hydrophones at 
low frequencies can be made in water-filled rigid tanks. The only measurement 
required, other than what is normal for the reciprocity technique, is the com- 
pressibility of the tank, pipes, and contents. This procedure provides a simple 
means of determining the sensitivity of the hydrophone as a function of the hydro- 
static pressure. 
Dr. FOACHE: The reciprocity method for low-frequency measurements is 
an excellent one, but further work still needs to be done, and it has, therefore, 
not yet been possible for us to use it. 
