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A hydrophone, from the Greek words ‘hydro’ (water) and ‘phone’ (sound), is a sonar sound receiver. Hydrophones are electroacoustic sensors that are used 1) in hydroacoustics to listen to underwater signals and noise for measurement purposes, and 2) as elements of directed hydroacoustic receiving antennas. The most common type of hydrophone is based on electrodynamic, piezoelectric and magnetostrictive effects. Electrodynamic hydrophones work according to the same principle as air electrodynamic microphones except for the design features associated with waterproofing.

Nominal sensitivity 0.25 mV/Pa Frequency range 20–10,000 Hz Self-resonant frequency > 50 kHz Self noise, RMS < 2 mPa Amplitude range < 1,000 Pa Output impedance < 500 Ohm Temperature range 0…+70 °Ð Â Ð Ð‹ ICP powering Voltage +(18 - 30) V Supply current 3.0 mA Level of DC voltage at the output 12.5 V Weight (without cable) 40 g Cable length 1.5* m Explosion-proof design The hydrophone can be connected directly to the ZET 017 spectrum analyzer *the maximum cable length without loss of signal quality is 200 m and is determined when ordering

The BC 312 hydrophones support Transducer Electronic Data Sheet (TEDS) technology, which enables the spectrum analyzer to automatically determine the type of sensor and its sensitivity in accordance with the IEEE p1451.4 standard.

At the customer’s request, hydrophones can be included with the spectrum analyzer.

In piezoelectric hydrophones, the direct piezoelectric effect of certain crystals (Rochelle salt, quartz, ammonium dihydrophosphate, lithium sulfate and others) is used, in which variable deformation of the crystal causes variable surface electrical charges to appear, thus giving rise to variable electromotive force in the electrode liners. Piezoelectric ceramic materials such as ceramic barium titanate, lead zirconate-titanate and others are widely used. The sensitive elements of piezoelectric hydrophones are made in the form of rectangular or cylindrical packets.

Hydrophone without housing

Hydrophone without housing

These magnetostrictive hydrophones are based on the reverse magnetostriction effect of some ferromagnetic metals (mostly nickel and its alloys), where the deformation causes a variable magnetic induction in the magnetic core and, as a consequence, variable emf on the coil. The sensitive hydrophone elements (cores) are usually made of thin plates in order to avoid losses due to eddy currents.

Hydrophones designed for measurement purposes must be unfocused and have a flat frequency response over the entire range of the frequencies being studied. For this purpose it is convenient to use small (compared with the wavelength) hollow spherical receivers made from piezoceramics that complete spherical symmetrical vibrations.

One of the most important characteristics of hydrophones is their sensitivity, which is the ratio of the voltage to the sound pressure in microvolts/bar. It lies in the range from fractions of a mV/bar for small (diameter of several mm) ceramic spherical receivers to hundreds of microvolts/bar for packages made from piezoelectric crystals. To increase the sensitivity (as well as to eliminate the shunting effect of the cable) it is common to use hydrophones with preamps, which are mounted in a single housing along with the receiver and submersed together.

Hydrophones are available in two versions:

• For submersion by a cable
• For inserting into the wall of a sealed container or pipe with 20 x 1.5 thread

The hydrophone can be delivered without a built-in amplifier, in which case it can be used as a sonar emitter.

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