Sonar

French F70 type frigates (here, ) are fitted with VDS (Variable Depth Sonar) type DUBV43 or DUBV43C towed sonars
Sonar image of the Soviet Navy minesweeper T-297, formerly the Latvian Virsaitis, which was shipwrecked on 3 December 1941 in the Gulf of Finland
ASDIC display unit from around 1944
Principle of an active sonar
Bubble clouds shown under the sea. From ref.
Comparison of Standard Sonar and TWIPS in finding a target in bubbly water. Adapted from ref.
Variable depth sonar and its winch
AN/AQS-13 Dipping sonar deployed from an H-3 Sea King
Lofargram writers, one for each array beam, on a NAVFAC watch floor.
AN/PQS-2A handheld sonar, shown with detachable flotation collar and magnetic compass
Cabin display of a fish finder sonar
Graphic depicting hydrographic survey ship conducting multibeam and side-scan sonar operations
Active (red) and passive (yellow) sonar detection of bubbles from seabed (natural seeps and CCSF leaks) and gas pipelines, taken from ref.
A humpback whale

For other uses, see Sonar (disambiguation).

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Reginald Fessenden

Canadian-born inventor, who did a majority of his work in the United States and also claimed U.S. citizenship through his American-born father.

Portrait photograph of Reginald Fessenden from Harper's Weekly Magazine, 1903
Whitney Institute in Bermuda, founded in 1881, of which Fessenden was headmaster
Cobb Island on the Potomac River, scene of the first successful radio transmission of speech in the fall of 1900.
April 1904 company advertisement
Gravesite in St. Mark's church cemetery in Bermuda

During his life he received hundreds of patents in various fields, most notably ones related to radio and sonar.

Underwater acoustics

Study of the propagation of sound in water and the interaction of the mechanical waves that constitute sound with the water, its contents and its boundaries.

Output of a computer model of underwater acoustic propagation in a simplified ocean environment.
A seafloor map produced by multibeam sonar

The field of underwater acoustics is closely related to a number of other fields of acoustic study, including sonar, transduction, signal processing, acoustical oceanography, bioacoustics, and physical acoustics.

Hydroacoustics

Study and application of sound in water.

Collecting Multibeam Sonar Data
A 38 kHz hydroacoustic tow fin used to conduct acoustic surveys by NOAA. Alaska, Southeast.

Hydroacoustics, using sonar technology, is most commonly used for monitoring of underwater physical and biological characteristics.

Hydrophone

Microphone designed to be used underwater for recording or listening to underwater sound.

A hydrophone being lowered into the North Atlantic
Hydrophones and directional hydrophones using a baffle.

From late in World War I until the introduction of active sonar in the early 1920s, hydrophones were the sole method for submarines to detect targets while submerged; they remain useful today.

Piezoelectricity

Electric charge that accumulates in certain solid materials—such as crystals, certain ceramics, and biological matter such as bone, DNA, and various proteins—in response to applied mechanical stress.

Piezoelectric balance presented by Pierre Curie to Lord Kelvin, Hunterian Museum, Glasgow
View of piezo crystal in the top of a Curie compensator in the Museum of Scotland.
A piezoelectric disk generates a voltage when deformed (change in shape is greatly exaggerated).
Piezoelectric plate used to convert audio signal to sound waves
Any spatially separated charge will result in an electric field, and therefore an electric potential. Shown here is a standard dielectric in a capacitor. In a piezoelectric device, mechanical stress, instead of an externally applied voltage, causes the charge separation in the individual atoms of the material.
Tetragonal unit cell of lead titanate
Piezoelectric disk used as a guitar pickup
Many rocket-propelled grenades used a piezoelectric fuse. Pictured, a Russian RPG-7
Metal disk with piezoelectric disk attached, used in a buzzer
A stick-slip actuator

The first practical application for piezoelectric devices was sonar, first developed during World War I.

Squid (weapon)

British World War II ship-mounted anti-submarine weapon.

Squid anti-submarine mortar on display at the Devonport Naval Base

The weapons were automatically fired from the sonar range recorder at the proper moment.

Thermocline

Thin but distinct layer in a large body of fluid (e.g. water, as in an ocean or lake; or air, e.g. an atmosphere) in which temperature changes more drastically with depth than it does in the layers above or below.

Graph showing a tropical ocean thermocline (depth vs. temperature). Note the rapid change between 100 and 1000 meters. The temperature is nearly constant after 1500 meters depth.
Graph of different thermoclines (depth versus temperature) based on seasons and latitude
Two moon jellyfish disturbing a thermocline in the top water layer of Gullmarn fjord, Sweden
Lakes are stratified into three separate layers: the epilimnion (I), metalimnion (II), and (III) hypolimnion. The scales are used to associate each section of the stratification to their corresponding depths and temperatures. The arrow is used to show the movement of wind over the surface of the water, which initiates the turnover in the epilimnion and hypolimnion.

In the open ocean, the thermocline is characterized by a negative sound speed gradient, making the thermocline important in submarine warfare because it can reflect active sonar and other acoustic signals.

Grand Banks of Newfoundland

The Grand Banks of Newfoundland are a series of underwater plateaus south-east of the island of Newfoundland on the North American continental shelf.

Map showing the Grand Banks
Historic chart including the Grand Banks.

Technological advances in fishing (such as using large factory-ships and sonar), as well as geopolitical disputes over territorial sea and exclusive economic zone (EEZ) boundaries, led to overfishing and a serious decline in the fish stocks of the Grand Banks from around 1990.

Hedgehog (weapon)

Forward-throwing anti-submarine weapon that was used primarily during the Second World War.

On, November 1945
A Mark-15 Hedgehog launcher on display at the USS Silversides (SS-236) museum in Muskegon, Michigan
Live and practice projectiles – note the protective fuze caps (22) shown removed in the picture at the top of page.
{{USS|Moberly|PF-63|6}} makes a Hedgehog attack against {{GS|U-853||2}} in 1945.|alt=Sailors observe a ring of small circular waves from the impact of Hedgehog bombs into the water
A large white upwelling of water from an underwater explosion just ahead of Moberly{{'}}s bow following Hedgehog launch<ref>USS Moberly, PF-63, U.S. Coast Guard, p. 4</ref>
{{USS|Sarsfield|DD-837|6}} after firing dual Hedgehogs, 1950|alt=A destroyer moving to the left; on the water just ahead of her are two rings of small circular waves, one to either side of her path. The rings nearly meet directly ahead of her and are each about half her length in diameter.

The system was developed to solve the problem of the target submarine disappearing from the attacking ship's ASDIC when closer than the sonar's minimum range.

Ultrasound

Sound waves with frequencies higher than the upper audible limit of human hearing.

Ultrasound image (sonogram) of a fetus in the womb, viewed at 12 weeks of pregnancy (bidimensional scan)
An ultrasonic examination
Fetal ultrasound
Galton whistle, one of the first devices to produce ultrasound
Approximate frequency ranges corresponding to ultrasound, with rough guide of some applications
A medical ultrasound result on a piece of paper
Bats use ultrasounds to navigate in the darkness.
A dog whistle, a whistle which emits sound in the ultrasonic range, used to train dogs and other animals
Principle of flaw detection with ultrasound. A void in the solid material reflects some energy back to the transducer, which is detected and displayed.
Non-destructive testing of a swing shaft showing spline cracking
Principle of an active sonar
Sonogram of a fetus at 14 weeks (profile)
Head of a fetus, aged 29 weeks, in a "3D ultrasound"
Schematic of bench and industrial-scale ultrasonic liquid processors

A common use of ultrasound is in underwater range finding; this use is also called Sonar.