This explains why, if you hear an explosion underwater and quickly surface, you will hear it again as the slower traveling sound reaches you through the air.

The name comes from active sonar terminology which sends a pulse of sound and listens for the echo to detect objects underwater.

In underwater acoustics, refraction is the bending or curving of a sound ray that results when the ray passes through a sound speed gradient from a region of one sound speed to a region of a different speed.

Sonar ( originally an acronym for SOund Navigation And Ranging ) is a technique that uses sound propagation ( usually underwater, as in submarine navigation ) to navigate, communicate with or detect objects on or under the surface of the water, such as other vessels.

The study of underwater sound is known as underwater acoustics or hydroacoustics.

The use of sound to ' echo locate ' underwater in the same way as bats use sound for aerial navigation seems to have been prompted by the Titanic disaster of 1912.

The British made early use of underwater hydrophones, while the French physicist Paul Langevin, working with a Russian immigrant electrical engineer, Constantin Chilowski, worked on the development of active sound devices for detecting submarines in 1915 using quartz.

This work, for the Anti-Submarine Division of the British Naval Staff, was undertaken in utmost secrecy, and used quartz piezoelectric crystals to produce the world's first practical underwater active sound detection apparatus.

Research on ASDIC and underwater sound was expanded in the UK and in the US.

During the 1930s American engineers developed their own underwater sound detection technology and important discoveries were made, such as thermoclines, that would help future development.

The major source of sound when a droplet hits a liquid surface is the resonance of excited bubbles trapped underwater.

Piezoelectric speakers can have extended high frequency output, and this is useful in some specialized circumstances ; for instance, sonar applications in which piezoelectric variants are used as both output devices ( generating underwater sound ) and as input devices ( acting as the sensing components of underwater microphones ).

" sound ") is a microphone designed to be used underwater for recording or listening to underwater sound.

Sailboats and windsurfers are especially worrisome as their quiet propulsion systems means the snorkeler may be unaware of their presence, unlike any motor-driven craft, as sound travels farther underwater.

The project used an air gun that sent sound waves underwater to determine the underlying structure of the ocean floor and the submersible Alvin, which was used to find and recover a sample from the bedrock.

* Astral Propagation Model a US Navy underwater sound progation loss model

For these reasons divers find it difficult to determine the direction of sound underwater.

Dynamics offers ground combat weapons, missile systems, torpedoes, sensor systems, unmanned underwater vehicles and signature management systems, remotely operated vehicles for armed forces as well as civil security applications.

The fish Limpet, complete with his signature spectacles, discovers a new-found ability during some of his initial misadventures, a powerful underwater roar, his " thrum.

The first attempt at underwater exploration of the North Pole was made by a Russian firefighter and diver Andrei Rozhkov with a support of Diving Club of Moscow State University on April 22, 1998 but ended in fatality.

Malta stands on an underwater ridge that extends from North Africa to Sicily.

In the spring semester of 2013, North Carolina State University will offer an underwater basket weaving course as a part of their College of Design curriculum.

The Grand Banks of Newfoundland are a group of underwater plateaus southeast of Newfoundland on the North American continental shelf.

Other suggestions for the bay have included a proposal to build giant underwater gas storage tanks, and to infill the near-lagoon behind North Bull Island to form a leisure park.

The Haymarket North Extension rerouted the Orange Line through an underwater crossing of the Charles River.

The structure will boast the longest underwater tunnel in North America and is scheduled to open in 2013.

It consists of twin tubes, carrying four traffic lanes, and at 9, 117 feet ( 2, 779 m ) is the longest continuous underwater vehicular tunnel in North America.

The Board has examined virtually every important joint defence measure undertaken since the end of the World War II, including construction of the Distant Early Warning Line of radars, the creation of the North American Aerospace Defense Command ( NORAD ) in 1958, the bi-national operation of the underwater acoustic surveillance system and high-frequency direction finding network, and the decision to proceed with the North American Air Defence Modernization program in 1985.

The Bimini Road, sometimes called the Bimini Wall, is an underwater rock formation near North Bimini island in the Bahamas.

For 10 years this ship completed numerous underwater pipe joints ( hyperbaric tie-ins ) and other maintenance work in and around the Shell operated Brent oilfield in the northern North Sea.

Like oil, natural gas is often found underwater in offshore gas fields such as the North Sea, Corrib Gas Field off Ireland, and the Sable Offshore Energy Project in the Scotian Shelf near Sable Island.

The Gully is a large underwater canyon in the Atlantic Ocean near the edge of the eastern continental shelf of North America.

* Photo Album and underwater images of North Minerva Reef ( 2009 )

The Navy does not believe his story and it appears his career is over until he is recruited to command a covert mission to tap an underwater cable lying between North Korea and mainland China.

Although it has been known for a while that the Black Sea and the Sea of Marmara flow into each other in an example of a density flow, findings of a study by the University of Leeds in August 2010 reveal that there is in fact an underwater channel of high density water flowing across the floor of the Bosphorus ( caused by the difference in density of the two seas ), which would be the sixth largest river on Earth if it were to be on land.

* In Jules Verne's novel 20, 000 Leagues Under the Sea, the Nautilus travels through an underwater passage beneath the Suez Canal.

*" The Dance of the Sea ", an " underwater " dance performed by Buster Keaton

Underwater the Nereo Cave, considered the biggest and spectacular marine underwater cave of the Mediterranean Sea and Europe, is the most visited by scuba-divers.

Forty days later, the Soviet Union made its first successful underwater launch of a submarine ballistic missile in the White Sea on 10 September 1960 from the same converted Project 611 ( Zulu Class ) submarine that first launched the R-11FM ( SS-N-1 Scud-A, naval modification of SS-1 Scud ) on 16 September 1955.

In the Great Sea and in the northern part of the Little Sea, there are some underwater springs called citri, which carry undrinkable freshwater together with salt water.

Sea lion swimming underwater

Sea lions have fairly acute underwater hearing, with a hearing range of 0. 4 – 32 kHz.

Sea lions rely on their whiskers or vibrissae for touch and detection of vibrations underwater.

In 2000, these projectiles were used to successfully destroy a range of live underwater mines when fired from a hovering Sea Cobra gunship at Aberdeen Proving Grounds.

* Sea Research Society, a non-profit research institution for diving and underwater archaeology

He has attributed his early interest in underwater exploration to reading the novel Twenty Thousand Leagues Under the Sea, living by the ocean in San Diego, and his fascination with the groundbreaking expeditions of the bathyscaphe Trieste.

* Pacifica, a fictional underwater city in the 1971 TV movie City Beneath The Sea

Sea Trek is an underwater diving experience where guests can view the ocean through a diving helmet without ever leaving the park.

* The Kolumbo underwater volcano in the Aegean Sea is discovered when it bursts from the sea and erupts, killing 70 people on a nearby island.

Cussler and NUMA have helped produce a television series on underwater exploration called The Sea Hunters, which chronicles the discovery and subsequent removal and conservation of the CSS H. L. Hunley in 1995.

* HMS Superb is a nuclear-powered hunter killer submarine launched in 1974, and decommissioned on 26 September 2008 after sustaining damage in an underwater grounding in the Red Sea.

Clu Clu Lands story starts with a type of Sea Urchin, the Unira, stealing all of the treasures in the underwater kingdom of Clu Clu Land.

* Sea Urchin-An amphibious boy who had underwater adventures with his friend Roger, the flying fish.

The ocean water in the Sargasso Sea is distinctive for its deep blue color and exceptional clarity, with underwater visibility of up to.

Macclesfield Bank, also called Zhongsha Islands (​), is an elongated sunken atoll of underwater reefs and shoals in the South China Sea.

In 1942 Hass completed the filming of his second underwater movie called Menschen unter Haien ( i. e. Men among Sharks ) which lasted for 84 minutes and was produced during several months in the Aegean Sea and the Sea of Crete.