SONAR

Podcast Transcript

One of the most significant developments in the history of naval warfare was the submarine.

The submarine offered a means of stealth and surprise that surface ships couldn’t compete with.

At first, navigating submarines was relatively simple, as they traveled just below the surface and used a snorkel and a periscope.

However, as submarines improved and could dive deeper, they encountered a problem. How could they see and navigate?

The answer came from nature.

Learn more about SONAR, how it was developed, and how it works on this episode of Everything Everywhere Daily.

SONAR, like RADAR, is an acronym. It stands for Sound Navigation and Ranging.

Essentially, it utilizes sound to navigate and detect objects underwater.

While that is a brief summary of what SONAR is, its operation is more complex.

It starts, of course, with sound.

Humans realized that sound had a speed for a very long time. The fact that sound wasn’t instantaneous can be seen with an echo, or via the difference in time between when you see a lightning bolt and hear the corresponding thunder.

The speed of sound was first measured with reasonable accuracy in 1635 by Marin Mersenne, a French scientist. He estimated it to be about 1,380 feet per second, remarkably close to the modern value.

He measured the speed of sound by observing the time delay between seeing the flash and hearing the sound of gunshots or striking objects at a known distance.

Despite the early measurement of the speed of sound, there was much that we still didn’t know about it.

In 1822, the French physicist Jean-Daniel Colladon measured the speed of sound in Lake Geneva, demonstrating that sound could travel long distances through water and that sound, in fact, traveled faster through water than air. This experiment hinted at the potential for using sound waves for detection beneath the surface.

In the early 20th century, research began on the nature of underwater acoustics.

The tragic sinking of the RMS Titanic in 1912 sparked a significant interest in underwater detection technology, as the disaster underscored the urgent need for effective iceberg detection systems. Shortly after the disaster, British scientist Lewis Richardson filed a patent for an echo-ranging device that could detect icebergs, though it remained largely theoretical.

With the advent of the First World War, submarines took on a new importance. This new weapon required changes in naval tactics and strategy. It also ushered in new technologies.

During World War I, the urgent need to detect enemy submarines spurred rapid innovation.

French physicist Paul Langevin, working with engineer Constantin Chilowski, developed one of the first active sonar systems in 1915. This early sonar used a quartz transducer to emit sound pulses and detect echoes—a precursor to what we now call active sonar. It could detect an enemy submarine as far as 1500 meters, or 0.9 of a mile away.

The British developed the ASDIC system. ASDIC stood for Anti-Submarine Detection Investigation Committee. It became their primary submarine detection method. This method used hydrophones for passive sound detection.

The technology was crude by modern standards, but it represented a crucial step forward in underwater warfare capabilities.

With the submarine cat now out of the bag, the interwar period became a period of rapid technological development.

In the UK, they continued with their ASDIC system. ASDIC systems used a rotating transducer to send out pings in multiple directions and were increasingly installed on warships and submarines.

The U.S. Navy also worked on sonar, especially through the Naval Research Laboratory and partnerships with companies like Western Electric. They explored different frequencies, propagation models, and methods of detecting and interpreting sonar returns.

Despite technical progress that was made, sonar in the interwar period was limited by weak signal processing technology, unreliable electronics, and a rudimentary understanding of sound propagation in varied ocean conditions.

Here, I want to break from the story of SONAR development for military use for a moment to mention a discovery made during the interwar period that had a profound impact on SONAR.

For centuries, humans knew that bats somehow could fly in pitch darkness. However, they had no clue how they were able to do this.

In 1793, Italian scientist Lazzaro Spallanzani conducted experiments showing that bats could navigate when blinded but not when their ears were plugged, suggesting they used sound rather than sight.

It wasn’t until 1938 that the true nature of echolocation was discovered by American physiologist Donald R. Griffin, working with physicist Robert Galambos. Using sound-detection equipment, they demonstrated that bats emitted ultrasonic pulses and used their echoes to orient themselves. Griffin coined the term “echolocation” in 1944.

Basically, bats use a natural form of SONAR.

World War II was a watershed moment in the development of sonar. Both Axis and Allied powers invested heavily in submarine warfare and, by extension, anti-submarine technology.

The Allies deployed improved ASDIC sets on most destroyers and escort ships. These systems were paired with depth charges and later hedgehog mortars to attack submerged submarines once detected. However, early sonar was limited in rough seas, and while the ship was moving quickly, it struggled with detecting submarines at depth or when lying still.

Germany, meanwhile, developed its own passive sonar systems, known as GHG (Gruppenhorchgerät), which allowed U-boats to detect enemy ships by their propeller noise. More ominously, the Germans developed acoustic torpedoes that could home in on the sound signatures of Allied ships.

The Americans were working on their own system known as bearing deviation indicator, or BDI. One of the researchers working on BDI, Frederick Hunt of Harvard University, suggested the word SONAR to refer not just to BDI, but all underwater acoustical ranging systems.

The term SONAR was purposely coined to be an analog of RADAR, and the acronym was developed after the fact.

In the Cold War era, sonar became a crucial component of naval strategy, particularly in submarine detection and nuclear deterrence.

The U.S. Navy launched the Sound Surveillance System (SOSUS) in the 1950s, a network of undersea hydrophone arrays laid on the ocean floor to passively detect Soviet submarines crossing the Atlantic. SOSUS played a vital role in tracking Soviet ballistic missile submarines throughout the Cold War.

The Cold War also saw the development of improved electronics. Transistors and later microprocessors improved acoustical signal processing and microphones.

With the advent of nuclear submarines, which can stay underwater indefinitely, and with submarines carrying intercontinental ballistic missiles, the need for improved SONAR became even more important.

As with any military technology, any advancement always brings about some countermeasure.

In the case of SONAR, there are many anti-SONAR countermeasures which have been adopted.

Submarines began using anechoic tiles, which rubber-like materials applied to the hull, to absorb sonar waves and reduce echoes.

They were first developed by Germany in WWII and later refined by the U.S. and Soviet navies.

Modern versions use layers that both absorb and scatter incoming sonar signals, making submarines harder to detect.

Another counter-SONAR measure has been the development of quiet submarine engines.

Modern submarines, especially nuclear-powered ones like the U.S. Navy’s Virginia-class or Russia’s Borei-class, are extremely quiet, often said to be quieter than the ambient noise of the ocean, such as waves or marine life.

Some are quieter than a whale’s heartbeat or the sound of a commercial ship’s propeller many miles away. This extreme stealth allows them to remain undetected even by advanced sonar systems, making them among the most elusive military platforms in the world.

Much of the noise level of a submarine is determined by the propeller. This is why submarine propeller designs are often kept a secret.

Submarines and ships can deploy towed sonar decoys, which emit false acoustic signals to confuse active sonar or lure away torpedoes.

Another active anti-SONAR measure is bubble curtains.

Bubble curtains work by releasing streams of air bubbles around a submarine or ship to disrupt and scatter incoming sonar waves, reducing the strength of echoes and masking the vessel’s acoustic signature.

The bubbles create a layer of varying density that reflects and absorbs sound, making it harder for sonar to detect or locate the target accurately.

After World War II, SONAR technology wasn’t only used for military purposes. A host of civilian uses also developed.

Side-scan sonar emerged during this period, providing detailed images of the seafloor and underwater objects. This technology proved invaluable for underwater archaeology, geological surveys, and search and recovery operations.

The famous discovery of the Titanic wreck in 1985 by Robert Ballard utilized advanced side-scan sonar technology.

Multi-beam sonar systems were also developed during this era, enabling comprehensive bathymetric mapping. These systems could survey large areas quickly and accurately, revolutionizing our understanding of ocean floor topography.

SONAR became essential for underwater construction, cable laying, pipeline inspection, and environmental monitoring. Recreational markets also developed, with fish finders and depth sounders becoming standard equipment on pleasure boats.

Medical ultrasound technology, derived from sonar principles, revolutionized diagnostic medicine. The same acoustic imaging principles used to detect submarines now enable doctors to examine internal organs and monitor fetal development.

Using SONAR isn’t just a matter of listening for sounds, aka passive SONAR, or making a sound and listening for an echo, aka active SONAR.

It is highly dependent on local conditions.

Sound travels through air at 20°C at approximately 343 meters per second.

In fresh water, it travels at approximately 1,480 meters per second.

In salt water, it travels even faster at approximately 1,500 meters per second.

The reason why sound travels through salt water faster than fresh water is that salt water is denser. It has water and salt as opposed to just water.

Both salinity and temperature can change considerably in seawater depending on depth and location, both of which will affect sounds.

Sound waves bend when they move through areas of differing temperature, salinity, or pressure.

A SONAR operator must take all of these factors into consideration when monitoring.

One question that some of you might be asking is, “Why is SONAR even necessary in the first place? Why can’t you just use RADAR like they use above the water to detect objects?”

RADAR works by emitting radio waves and detecting the reflections from objects.

Water is a poor medium for electromagnetic wave propagation, especially at the frequencies used by radar.

Water contains ions, especially seawater, which has salt. These ions interact with electromagnetic waves and convert the energy into heat, causing a rapid loss of signal strength.

Water molecules are also polar, meaning they have a positive and negative side, which means they absorb and re-radiate energy from electromagnetic fields—again, converting wave energy into heat.

Radar signals are absorbed within inches of water in most cases.

This makes radar useless for detecting submerged objects like submarines, fish, or the seafloor.

Radar can detect objects on the surface of water, such as ships and icebergs, but as soon as you try to detect below the surface, radar fails almost immediately due to absorption.

By the same token, you can’t really use SONAR above water. Bats and other animals are able to use it over very short distances.

Over longer distances, sound waves simply wouldn’t work.

SONAR in the air would be limited by the speed of sound and by the fact that the atmosphere can absorb and scatter sound. Think of how difficult it is to yell at someone if they are far enough away, especially if it’s windy outside.

That is why ultrasound imaging, which is a type of SONAR, is only used over very short distances, and usually when you want to see inside something solid or liquid.

From crude hydrophones to vast undersea surveillance networks, sonar has evolved into a cornerstone of maritime operations and ocean science.

Its development has paralleled advances in materials science, acoustics, computing, and electronics, shaping everything from undersea warfare and global security to deep-sea exploration and fishing.