User Manual

Page 56
Figure 65. Hydrophone
environmental stability is critical. Waterproof microphones are being supplied for divers, withstanding
total immersion in salt water without damage.
SONAR
Piezo ceramic materials have been used in hydrophones for
SONAR since the 1940's. Ceramics have many desirable
features as hydrophones including high hydrostatic
sensitivity, high capacitance, ability to fabricate into shapes,
and availability in thick cross-section (sensitivity is
proportional to piezo element thickness). However, the
weight of ceramic is much greater than water, so buoyancy
must often be added to structures as compensation.
The current trend in SONAR is toward higher numbers of
hydrophone sensors per vehicle. Longer towed arrays and
larger and more numerous hull mounted hydrophone panels
are being employed by the fleet. This trend requires the
advancement of much lower weight hydrophones making
piezo film an excellent choice for these applications.
New piezo polymer hydrophones of thick film (1200µm)
have been made into sheet and cylinder form (Figure 65).
The cylinders, about 2-3 cm in length and <1 cm in diameter, are strung together into a continuous
hydrophone line array. The hydrophones are very light weight (0.60 g/cm
3
), yet have excellent
hydrostatic sensitivity.
Piezo polymer hull mounted hydrophones in panels, of about one square meter in area and greater than
1000µm in piezo polymer thickness, are used by the British and French Navies for submarines. Called
"Flank Arrays", these units work in combination with towed arrays in advanced SONAR systems. Piezo
film sheets are not fragile, conform to the radius of curvature of the hull, and are low cost. Typical
sensitivities for the piezo film thick sheet hydrophones are similar to ceramic type (-192 to -195 dB re 1
V/µ Pa). The lower capacitance of the piezo film hydrophones is compensated by the large area and
number of parallel hydrophones used.
New hydrophone technologies, including underwater ultrasound imaging for divers and related
applications are also in development.
FUTURE APPLICATIONS
Piezo film research is underway into an exciting new array of applications. A sampling of this R&D
activity is highlighted below:
Active Vibration Damping
Piezo film sensor and actuator pairs are in development for active vibration damping. In this
application, a piece of piezo film is employed as a strain gauge to detect vibration and another piece of
piezo film is employed as an actuator to dampen the vibrational noise by applying a 180 degree phase-