What are the uses of a hydrophone

Hydrophone - Hydrophones

Not to be confused with hydraulics, a musical instrument.

A Hydrophone (Ancient Greek: ὕδωρ + φωνή, literally "Water + Sound") is a microphone that can be used underwater to record or listen to underwater sound. Most hydrophones are based on a piezoelectric transducer that generates an electrical potential when the pressure changes, such as a sound wave. Some piezoelectric transducers can also function as a sound projector, but not all have this capability and some can be destroyed if used in this way.

A hydrophone can detect airborne sound, but is insensitive because it is adapted to the acoustic impedance of water, a liquid that is denser than air. Sound moves 4.3 times faster in water than air, and a sound wave in water exerts 60 times the pressure that is exerted by a wave of equal amplitude in air. Similarly, a standard microphone can be buried in the ground or submerged in water if placed in a waterproof container, but will perform poorly due to the similarly poor acoustic impedance match.


A hydrophone is lowered into the North Atlantic

The first hydrophones consisted of a tube with a thin membrane that covered the submerged end and the observer's ear at the other end. The design of effective hydrophones must take into account the sound resistance of water, which is 3750 times that of air. therefore the pressure exerted by a wave of equal intensity in air is increased by a factor of 3750 in water. The American Submarine Signaling Company developed a hydrophone to detect underwater bells being rung by lighthouses and lightships. The case was a thick, hollow brass disc with a diameter of 35 centimeters. On one side was a 1 millimeter (0.039 inch) thick brass diaphragm connected to a carbon microphone by a short brass rod.

First World War

At the beginning of the war, French President Raymond Poincaré provided Paul Langevin with the facilities necessary to work on a procedure to search submarines through the echoes of sound pulses. They developed a piezoelectric hydrophone by increasing the power of the signal with a vacuum tube amplifier. The high acoustic impedance of piezoelectric materials made them easy to use as underwater transducers. The same piezoelectric plate could be vibrated by an electrical oscillator to generate the sound pulses.

The first submarine to be discovered and sunk using a primitive hydrophone was the German submarine UC-3 on April 23, 1916. UC-3 was used by the submarine trawler Cheerio discovers since that Cheerio located directly above the UC-3, the UC-. 3 was then caught in a steel net pulled by the trawler and sank after a major underwater explosion.

Hydrophones and directional hydrophones using a baffle.

Later in the war, the British Admiralty belatedly convened a scientific panel to advise on how to combat submarines. These included the Australian physicist William Henry Bragg and the New Zealand physicist Sir Ernest Rutherford. They concluded that the best hope was to use hydrophones to search for submarines. Rutherford's research resulted in his only patent for a hydrophone. Bragg took over the helm in July 1916 and moved to the Admiralty Hydrophone Research Facility at Hawkcraig on the Firth of Forth.

The scientists have set themselves two goals: to develop a hydrophone that can hear a submarine despite the bat of a patrol ship with the hydrophone, and to develop a hydrophone that shows the bearing of the submarine. A bidirectional hydrophone was invented at East London College. They mounted a microphone on each side of a diaphragm in a cylindrical housing; If the sounds heard by both microphones are of the same intensity, the microphone is the same as the sound source.

Bragg's laboratory pointed such a hydrophone towards by placing a baffle in front of one side of the diaphragm. It took months to establish that effective baffles must contain a layer of air. In 1918, Royal Naval Air Service airships experimented with anti-submarine combat by towing submerged hydrophones. Bragg tested a hydrophone on a captured German submarine and found that it was inferior to British models. By the end of the war the British had 38 hydrophone officers and 200 qualified listeners, paid an additional 4 d per day.

From the late First World War to the advent of active sonar in the early 1920s, hydrophones were the only method submarines could use to detect targets in the underwater. They remain useful today.

Directional hydrophones

A small single cylindrical ceramic transducer can achieve near perfect omnidirectional reception. Directional hydrophones increase unidirectional sensitivity using two basic techniques:

Focused transducers

This device uses a single transducer element with a bowl or cone-shaped sound reflector to focus the signals in a manner similar to a reflective telescope. This type of hydrophone can be made of an inexpensive omnidirectional type, but must be used when stationary because the reflector hinders its movement through water. A new way of steering is to use a spherical body around the hydrophone. The advantage of directional balls is that the hydrophone can be moved within the water to free it from the interference created by a conical-shaped element.


Multiple hydrophones can be arranged in an array so that signals from the desired direction are added while signals from other directions are subtracted. The array can be controlled using a beamformer. Most often, hydrophones are arranged in a "line array" but can be in many different arrangements depending on what is being measured. For example, in the article, measuring the propeller noise of fleet ships required complex hydrophone array systems to produce actionable measurements.

SOSUS hydrophones, laid on the ocean floor and connected by underwater cables, were used by the U.S. Navy beginning in the 1950s to guide the movement of Soviet submarines along a line known as GIUK from Greenland, Iceland and the UK pursue loophole. These are able to clearly record extremely low frequency infrasound, including many unexplained ocean sounds.

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