Acoustic & Undersea Sensing

His notebooks recorded that a tube inserted below the surface carried the sound of a distant vessel to the ear, an early written observation of the principle that sound travels efficiently through water.

The Italian scientist's experiments on blinded bats suggested they used an unknown non-visual sense to avoid obstacles, a phenomenon not understood as echolocation until acoustic research explained it more than a century later.

An underwater bell was employed as an ancillary to lighthouses and lightships to signal navigational hazards. This was an early practical application of deliberate underwater sound transmission.

Using a submerged bell and a flash of light to mark the moment of the strike, the two scientists timed sound travelling across Lake Geneva, the first accurate measurement underpinning later underwater acoustic ranging.

Within weeks of the sinking of the Titanic, the English meteorologist Lewis Fry Richardson filed at the British Patent Office the world's first patent for an underwater echo-ranging device, proposing to detect icebergs by timing the echo of a sound pulse. It is the first patent for the active sonar principle, later refined into practical sonar.

Working for the Submarine Signal Company in Boston, Fessenden built an experimental system from 1912, tested in Boston Harbor and in 1914 from the US Revenue Cutter Miami on the Grand Banks. The trial demonstrated depth sounding, Morse-code underwater communication and detection of an iceberg at two miles, though the 500 Hz oscillator could not determine bearing.

The German physicist patented an echo sounder for measuring depth by acoustic echo. It was among the first protected designs for practical acoustic depth measurement.

The ten Montreal-built British H-class submarines launched that year carried Fessenden oscillators. This marked one of the first operational naval deployments of underwater acoustic equipment.

The French physicist and Russian-immigrant engineer worked on active acoustic detection of submarines using electrostatic transducers. Although later superseded by piezoelectric and magnetostrictive transducers, their work shaped future active sonar designs.

Under the British Board of Invention and Research, Boyle took on the active sound detection project in 1916, delivering a prototype in mid-1917 for the Anti-Submarine Division of the Naval Staff. The device used quartz piezoelectric crystals and was developed in secrecy, giving rise to the codename ASDIC.

Working with Constantin Chilowsky in France during the First World War, Langevin used the piezoelectric effect to generate and detect underwater ultrasound, creating a working system for locating submarines by their echo.

On leave from Bell Labs, Horton worked at Nahant, Massachusetts and later US Naval Headquarters in London. His use of the newly developed vacuum tube replaced the carbon button microphone with the precursor of the modern hydrophone, and he also experimented with towed detection whose principles persist in modern towed arrays.

By 1918 both nations had working prototypes of active underwater detection, alongside an operational passive sonar system in British use. Wartime submarine warfare had driven acoustic detection from concept to fielded apparatus in four years.

ASDIC was trialled aboard HMS Antrim in 1920 and production started in 1922. This moved active sonar from prototype to standard naval fit.

Vessels of the 6th Destroyer Flotilla carried ASDIC sets, the first flotilla-scale operational deployment. It marked the integration of sonar into fleet anti-submarine practice.

An anti-submarine school and a training flotilla of four vessels were set up on Portland. This institutionalised sonar operator training and anti-submarine doctrine.

During the 1930s American engineers built their own detection systems and discovered the existence of thermoclines and their effect on sound propagation. Frederick Hunt coined SONAR as the acoustic equivalent of RADAR.

Active sonar, developed after the First World War, became decisive in the Battle of the Atlantic, letting escort ships detect submerged submarines and protect convoys carrying supplies across the ocean.

Walker's tactic used a directing ship to hold ASDIC contact while a second ship attacked slowly with its own set switched off. It compensated for the depth charge's blind spot, in which contact was lost just before an attack.

Hedgehog and Squid projected warheads ahead of the attacker while contact was maintained, and wartime sets such as the Type 144 Q attachment and the articulated-transducer Type 147B gave deeper and wider beam coverage. The FIDO homing torpedo used passive sonar to steer itself onto its target.

Japan entered the war with large, heavy quartz-based projectors such as the 9 kHz Type 91 and the spherical Type 93. These were later replaced by the Type 3, which followed German practice with magnetostrictive nickel and iron-aluminium cores.

Heavy losses to merchant shipping triggered large-scale high-priority US research into transducer materials. ADP proved superior and made Rochelle salt obsolete within nine months, with the manufacturing workforce growing from dozens in 1940 to thousands by 1942.

Early in the Second World War British ASDIC technology was handed to the US, expanding research on both sides of the Atlantic. It underpinned wartime and post-war Allied anti-submarine capability.

Researchers Maurice Ewing and J. Lamar Worzel found that sound generated at a certain depth could travel thousands of kilometres through the SOFAR channel with little loss, a discovery later exploited to locate objects and downed aircrew at long range.

Sonobuoys were among the many new military sound detection types created during the war, alongside dipping sonar and mine-detection sonar. Such work formed the basis of post-war systems for countering nuclear submarines.

The mission followed a series of incidents in which power and data cables on the Baltic seabed were damaged, reportedly by ships dragging anchors, and combined patrol ships, aircraft and underwater surveillance to deter further damage.

Acoustic surveys helped estimate fish populations and monitor illegal fishing activity, part of a wider regional effort to balance the economic importance of fisheries against signs of overfishing in South-East Asian waters.

Updated non-binding guidance addressed underwater radiated noise from commercial shipping, which can interfere with the acoustic senses whales and other marine mammals rely on for navigation, feeding and communication.

The US company's wind and solar-powered surface vehicles carried sonar for seabed mapping and, under contracts including with the US Navy, for maritime surveillance, part of a wider shift toward uncrewed platforms for underwater sensing.

P-1 patrol aircraft and destroyers continued tracking submarine activity around Japanese waters, reflecting sustained concern over Chinese and Russian submarine movements near Japan and through nearby straits.

The DRDO laboratory advanced sonar systems intended for the Indian Navy's Kalvari-class submarines, part of a broader push to reduce reliance on imported sonar and strengthen domestic underwater sensing expertise.

The South Korean defence electronics firm advanced indigenous sonar suites intended to reduce reliance on foreign-supplied systems, part of a wider effort to build a domestic submarine and undersea sensor industry alongside the Korean navy's expansion.

China continued to expand a network of seabed hydrophones in the South China Sea, part of a longer-running effort to improve detection of foreign submarines approaching Chinese waters and add to shore-based sonar coverage.

Cables linking the Matsu Islands and other outlying territory to the mainland were damaged multiple times, generally attributed to vessels dragging anchors, prompting Taiwan to invest in surveillance, backup satellite links and faster repair capacity.

Work continued on training Australian submariners, expanding shipyard capacity and developing the trilateral SSN-AUKUS design, part of a programme intended to give Australia nuclear-powered submarines equipped with advanced sonar and combat systems from the 2030s.

The review assembled evidence on mass strandings of beaked whales coinciding with naval exercises using sonar. It sharpened scientific scrutiny of sonar's ecological impact.

The suit combined ultrasonic sensors with haptic feedback to alert the wearer to incoming threats, allowing response even when blindfolded. It illustrated the migration of acoustic sensing into wearable civilian devices.

The ruling addressed the US Navy's use of sonar and observed that no case had conclusively shown sonar to have harmed or killed a marine mammal. It shaped the legal environment for sonar operations amid environmental objections.

The mission of IUSS was declassified, opening the network's existence and purpose to public knowledge. Shore stations were meanwhile consolidated and arrays rerouted to central processing centres through the 1990s.

A division of the Japanese company was reported to have supplied machinery allowing Soviet submarine propeller blades to be milled far more quietly. The resulting generation of quieter submarines was substantially harder to detect by passive sonar.

With new mobile arrays and other systems becoming operational, the collective system name changed from SOSUS to IUSS. The change reflected a shift from purely fixed arrays to a mixed fixed and mobile architecture.

Compounds of rare earths and iron with superior magnetomechanical properties were found, enabling new transducer designs such as hybrid magnetostrictive-piezoelectric projectors. Galfenol is the most recent of these improved materials.

Underwater recordings revealed complex, repeating patterns in whale vocalisation, released as a widely distributed album that helped turn passive acoustic monitoring into a tool for marine conservation as well as naval detection.

The Scottish physician adapted industrial ultrasonic flaw-detection equipment to image inside the human body, founding obstetric ultrasound and extending acoustic sensing from naval and industrial use into everyday medicine.

Horton wrote the work as chief research consultant at the US Navy Underwater Sound Laboratory, where he later became technical director from 1959 until 1963. The book codified sonar theory and practice for a generation of engineers.

A test array was followed by a full-scale forty-element prototype operational array, then wider installation under the unclassified Project Caesar name. This built out the first ocean-wide fixed surveillance network.

Project Artemis was an experimental research and development effort examining acoustic propagation and signal processing for a low-frequency active sonar suited to ocean surveillance. It explored the feasibility of active, as opposed to passive, wide-area detection.

ADP crystals were too small for the lower frequencies needed for longer range, so magnetostrictive and barium titanate piezoelectric systems were developed. Barium titanate was then replaced by more stable lead zirconate titanate and the operating frequency lowered to 5 kHz, material used for decades in the AN/SQS-23.

The US Navy began the classified Sound Surveillance System (SOSUS), contracting it to AT&T with Bell Laboratories doing the research and Western Electric manufacturing and installing the arrays. Fixed arrays of underwater microphones laid across ocean basins tracked Soviet submarines by the noise of their engines and machinery, exploiting how far sound travels through deep water, and gave a strategic ocean-basin-scale passive acoustic detection capability.

At the end of the Second World War the standard US scanning sonar operated at 18 kHz with an array of ADP crystals. Demand for longer range subsequently forced a move to lower frequencies and larger transducers than ADP could support.