Hydrophones. What could they be useful for you?

Underwater microphone

We all know what a microphone is. It is a device that serves to receive, amplify or record sound on various media. But this is to record ordinary sounds, such as human speech or the barking of a dog.

Suppose we want to hear the “voices” of fish. If we set the microphone above the very surface of the water, we will not be able to record anything but irrelevant noise.

99% of the sounds you need will remain underwater – they won’t enter the air above it. For the microphone to work, we must lower it into the water. But wait a moment. An ordinary microphone will “choke” in water: the membrane will be saturated with water, the electrical circuit will fail, and you will ruin your device. What is needed is a specially designed microphone – an underwater microphone, or hydrophone: a hydroacoustic sound receiver.

What are hydrophones used for?

How do hydrophones work?

The oceans and many freshwater locations are “bubbling” with sounds. In seawater, sound travels five times faster than in the air: in water, its speed is 1500 m/s, while in the air, it is only 300 m/s.

The sounds of the sea are best heard through a hydrophone. It amplifies sounds and makes them clearer. A hydrophone can be suspended in the water column, placed on the sea floor, or fixed on the underwater part of a ship. With the help of a hydrophone, you can hear the splashing of water, splashes of the surf, sounds of engines and propellers of ships passing in the distance, and the noise from swimming fish, whales, dolphins and seals. So, this is what hydrophones are used in hydro-acoustics: to listen to underwater signals and noise, for measuring purposes, and also as components of directional receiving hydroacoustic antennas.

Hydrophone frequency range

Hydrophones have a rather diverse design and technical characteristics depending on the purpose. They generally cover the frequency range from 0.1 Hz (for perceiving and measuring infrasonic waves propagating hundreds and even thousands of kilometres in water) to 500 kHz (for high-precision ultrasonic location). They can operate at depths up to 1000 m or more, withstanding static pressures over 10 MPa.

Hydrophones. Underwater transducers

The mechanical action, or pressure, exerted by a sound wave on the material of the hydrophone sensing element leads to the occurrence of electric charges on the material’s surface and, consequently, a change in the charge distribution in the material, causing a voltage change.

One of the essential characteristics of hydrophones is sensitivity, which is the ratio of electrical voltage to sound pressure. The sensing element is hermetically sealed in a special sound-transparent synthetic rubber (for example, polychloroprene). The body is made of a strong metal resistant to corrosion and fouling in seawater, such as aluminum-bronze alloy. It is very important to have a hermetic connection of the hydrophone to the cable. Hydrophones with preamplifiers are used to increase the sensitivity (as well as to eliminate the shunting effect of the cable), which are mounted in the same housing with the receiver and lowered into the water together.

Hydrophones based on electrodynamic, piezoelectric, and magneto strictive effects are the most common. Electrodynamic hydrophones do not differ in operating from electrodynamic air microphones, except for the design features associated with isolation from water.

Piezoelectric hydrophones utilize the direct piezoelectric effect of some crystals when the variable deformation of the crystal causes the appearance of variable surface electric charges and, accordingly, a variable electromotive force on the electrode plates.

Hydrophone array

Two hydrophones are better than one, and more hydrophones are even better! If you place several hydrophones on the sea floor in line, crossing a projected path of a submarine, tow a streamer with a number of hydrophones, or moor them in a vertical line in water column. You will be able to use the time-of-arrival difference. This means that a sound from a distant source will reach each of your hydrophones at slightly different times, which can be used to calculate the direction of the sound (hence the location of the source) for more precision. This allows the detection and tracking of marine mammals and underwater objects such as submarines, locating vessels and the measurement of their noise signature, and even studying climate change!

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