The Physics and Testing of Hydrophones
Michael J. Moloney
Rose-Hulman Institute of Technology
Some properties of piezoelectric materials
Pressure creates a net dipole moment and a potential difference across the material
Cylindrical piezoelectric transducers
hollow cylinders are used, with an electrode on the inside curved surface, and on the outside curved surface
Hydrophones made from piezoelectric transducers
an array of 15 short, hollow piezoelectric transducers is built in a straight line
each transducer has two rubber grommets glued around it
five thin steel rods form the outside of the hydrophone
the hydrophone enclosure is filled with castor oil, to match the acoustic properties of sea water
Use of hydrophones on a submarine
a group of 50 or so hydrophones is placed in a circle of radius around 5 meters
this lets the submarine determine the direction of the incoming signals
Testing of Hydrophones
Test facility
large tank, 8m in diameter
one 'source' hydrophone, one 'reference' hydrophone, and one hydrophone being tested
these 3 are parallel to each other, equidistant, separated by less than 1 m
'Null Balance' test
the difference of the signals from the reference hydrophone and tested hydrophone is used
if the difference signal becomes too large, the tested hydrophone is rejected
the difference signal is always OK at low frequencies
Test results
At high frequencies, the difference signal may be too large, and the tested hydrophone fails
Why hydrophones fail the null balance test
Any path difference D in waves reaching the two detectors results in a phase difference F.
For a given D, the phase difference increases with decreasing wavelength: F = 2 pi D/ wavelength
The hydrophones fail a high frequencies (short wavelengths)
This results from a path difference D
The path difference D is present because the 'test' hydrophone is somewhat curved.
Hydrophones must be built exactly along a straight line to pass the null balance test