49 Fun Facts About Sonar

What does the acronym SONAR stand for?

Sound navigation and ranging was patterned on radar; Harvard's Frederick Hunt said he coined sonar around 1942 to make the Navy sound operator's job seem glamorous.

What turned out to cause the 'false bottom' that early sonar found in the deep ocean?

Swarms of small sea animals, often fish, made the echoes; the layer rose toward the surface each evening and sank at dawn, which first suggested it was alive.

Who wrote around 1490 that a long tube dipped in water lets you hear ships far away?

Leonardo da Vinci's instructions were to stop your own ship first, then put one end of a long tube in the water and hold the other to your ear.

Which maritime disaster spurred a wave of patents for detecting obstacles with underwater echoes?

The sinking of the Titanic in 1912 drove the idea that a pulse's echo off an iceberg's hidden bulk could warn a ship, and the same pulse aimed down could measure depth.

Which Canadian engineer detected an iceberg by echo ranging off Newfoundland in 1914?

Reginald Fessenden's employer marketed his oscillator for underwater telegraphy and was reluctant to exploit its potential for finding icebergs and submarines.

Which French physicist worked with Constantin Chilowsky on detecting submarines in 1915?

Paul Langevin's first patent relied on an electrostatic 'singing condenser', a design that piezoelectric transducers later replaced.

Which crystal did the first practical British echo-ranging apparatus use in 1917?

Quartz is piezoelectric: squeezing it makes a voltage and a voltage makes it flex, so it can turn electric signals into sound pulses and echoes back into signals.

What did the ASD in the British sonar name ASDIC actually stand for?

Anti-Submarine Division is almost certainly the origin; the popular 'Detection Investigation Committee' story has left no trace in the Admiralty's archives.

What is the pulse of sound emitted by an echo-ranging sonar commonly called?

A ping gave its name to the network tool that times how long a computer takes to echo a message back, because it works like sonar.

Sound travels over four times faster in seawater than in which of these?

Air carries sound at about 343 metres a second and seawater at about 1,500, so a sonar echo from a target 750 metres away returns in about a second.

How did two scientists on Lake Geneva time sound travelling through water in 1826?

Seeing a flash as a bell rang let Colladon and Sturm start the clock, because light crossed the lake almost at once while the bell's sound took several seconds.

Which ocean layer bends sonar beams so sharply that submarines can hide just beneath it?

The thermocline, where warm surface water gives way to cold deep water, bends sonar beams away, so ships tow their listening arrays below it to hear what hides there.

Which law describes how sound refracts as it crosses layers of different sound speed?

Snell's law is named for the Dutch astronomer Willebrord Snell, who found it in 1621 but never published it; sonar models use it to predict how beams bend.

Which airliner's wreckage on the deep ocean floor did sonar-equipped robot submarines finally find in 2011?

Air France Flight 447's flight recorders, raised weeks later, showed the crew kept pulling the nose up even after the jet had stalled.

Which anti-submarine weapon problem did the Hedgehog and Squid solve?

Losing contact when passing over the target let U-boats dodge unseen, so the new weapons threw charges ahead while the escort's sonar still held the submarine.

Which Royal Navy officer developed the two-ship 'creeping attack' using ASDIC?

Frederic John Walker's attacking ship crept in slowly with its ASDIC off, so the U-boat could not tell when the depth charges would fall.

Whose maps drawn from echo soundings revealed a rift valley running along the Mid-Atlantic Ridge?

Marie Tharp saw the rift in the 1950s, but her colleague Bruce Heezen dismissed it until earthquake records turned out to line up along the same valley.

What is a sonobuoy?

An expendable buoy dropped from aircraft or ships keeps a radio float at the surface to relay what it hears below back to the patrolling aircraft.

What really makes the crack of a snapping shrimp, loud enough to drown out sonar?

A collapsing bubble, formed as water jets from the closing claw, makes the noise; scientists had long believed it came from the claw halves hitting each other.

Whose long-lost plane did a sonar image seem to show in 2024, until a closer look found only rocks?

Amelia Earhart's supposed plane only turned up when the sonar data was reviewed months into the trip, too late to turn back, so a second voyage was needed for a closer look.

Why does a towed array sonar trail its hydrophones far behind the vessel?

To escape the ship's own noise, the array trails far astern, which also lets it cover the baffles, the blind spot behind a hull-mounted sonar.

What is a hydrophone?

A microphone designed for underwater use let a Navy engineer at a secret Bermuda station, listening for Russian submarines, record whale songs that Roger Payne later released on an album.

Which part of a fish reflects sonar most strongly?

Its swim bladder is full of gas, which reflects sound strongly, so sonar works best on fish such as herring and cod and poorly on sharks, which have none.

Which kind of sonar paints photo-like pictures of the seabed for wreck and mine hunters?

Side-scan sonar records how strongly the seabed echoes, so hard objects sticking up, like wrecks, stand out from soft mud and sand, which return weaker echoes.

Which Tudor warship's buried wreck did Harold Edgerton's sonar scans pick up in the late 1960s?

Mary Rose's timbers were sprayed with chilled water and later polyethylene glycol after her 1982 raising, because drying out could have shrunk them by up to half.

Which novelist's 1984 debut follows a Soviet submarine whose silent drive leaves sonar operators struggling to hear it?

Tom Clancy, then a Maryland insurance agent, sold the book for $5,000, and it became a bestseller after President Reagan praised it.

Before echo sounders, sailors measured depth in fathoms, a unit first based on what?

The span of outstretched arms gave the fathom its name; a leadsman's cry of 'mark twain' meant two fathoms, the phrase Samuel Clemens made famous as his pen name.

What do sonar designers gain by choosing a lower-pitched sound?

It travels farther before fading because seawater absorbs low notes only weakly; the trade-off is less detail and far larger equipment.

What is the SOFAR channel?

An ocean layer where sound speed is at its minimum traps sound so it travels great distances, and timing one blast's arrival at distant listening posts can pinpoint where it happened.

What was SOSUS?

A US system for tracking Soviet submarines, SOSUS was so secret that even its name was classified, and its arrays were laid under the cover name Project Caesar.

Which cetaceans most often strand after exposure to mid-frequency naval sonar?

Beaked whales stranded after naval exercises showed signs of decompression sickness, and a 2004 ban on such sonar around the Canary Islands stopped their mass strandings there.

Which zoologist coined the term echolocation and first demonstrated it in bats?

Donald Griffin worked with Robert Galambos at Harvard, using a physicist's new detector to show that bats call at pitches far above human hearing.

Glasgow doctor Ian Donald pioneered pregnancy scans with a machine first built to find what?

Cracks in welded metal were its first quarry; in 1957 a scan showed that a woman told she had inoperable stomach cancer actually had a removable ovarian cyst.

Which body part lets a dolphin focus its clicking sounds into a narrow beam?

Its melon, a fatty organ in the forehead, is built of fats of differing densities, so it bends the clicks into a beam much as a lens bends light.

What do fast-spinning propellers cause that gives a vessel away to listening sonar?

Cavitation is not only noisy: where water races over a propeller's blades, the pressure drops so low that the effect erodes the blades and wastes power.

Which phenomenon lets a single-frequency sonar measure a target's radial speed?

The Doppler effect was tested in 1845, when musicians played on a moving train while others beside the track wrote down how the note rose and fell as it passed.

What is sodar?

An in-air sonar aimed at the sky, sodar picks up sound scattered by patches of uneven air to chart wind speeds at different heights above the ground.

Which Japanese electronics giant's subsidiary sold the Soviets machines that made their submarine propellers quieter?

Toshiba Machine, part of the Toshiba group, made the sale, and in the uproar that followed, members of the US Congress publicly smashed a Toshiba radio.

Which company's 1978 Sonar OneStep camera judged distance by timing ultrasound echoes?

Polaroid's camera was the first autofocus SLR, and because it timed echoes instead of reading the image, it could focus even in the dark.

Why must a submerged submarine use sonar rather than radar to find other ships?

Radio waves fade fast in seawater, which conducts electricity, so the same water that hides a submarine also cuts it off from ordinary radio messages.

What did scientists eventually conclude made 'the Bloop', a mysterious loud ocean sound recorded in 1997?

An iceberg cracking apart made it; the recording NOAA released had been sped up 16 times, because the real rumble was too deep to hear well.

What did German engineers glue onto some WWII U-boats to help them hide from Allied sonar?

Rubber sheets, codenamed Alberich, worked on some boats, but the glue of the day could not hold them, so they tended to peel off at sea.

The ocean's deepest known point, measured by echo sounder in 1951, is named after which British ship?

HMS Challenger first sounded the trench in 1875, and a second HMS Challenger returned in 1951, when its echo sounder found a spot more than two kilometres deeper.

Why are sonar-equipped minehunters often built with wooden or fibreglass hulls?

To avoid triggering magnetic mines, and they are built to run quietly too, since other mines go off at the sound of a passing ship.

What was a hand-held imaging sonar for patrol divers designed to find in murky water?

Limpet mines, fixed to a hull by divers, were what French agents used to sink the Greenpeace ship Rainbow Warrior in Auckland Harbour in 1985.

In the early 1900s, what were bells rung underwater near coasts used to do?

Warn ships of hazards: in fog, a captain could turn his ship until a bell sounded equally loud on both sides of the bow, and then knew it lay dead ahead.

Which American company built the seabed listening arrays the US Navy laid in the 1950s?

AT&T worked through Bell Labs and Western Electric, and turned its sound spectrograph, invented to study human speech, into a tool for picking out submarine noises.

What name is given to a sonar built to detect and locate an enemy's own sonar transmissions?

Intercept sonar exploits a basic weakness: any vessel sending out sound can be heard, which is why submarines trying to stay hidden mostly just listen.

What did the US Navy hunt for off the coast of Spain in 1966, using sonar and deep-diving craft?

A lost hydrogen bomb fell into the sea when a B-52 bomber collided with a refuelling tanker, and it was finally raised intact after a search of about 80 days.