Underwater acoustics

Underwater acoustics, also known as hydroacoustics, is the study of how sound travels through water and how it interacts with the water, its contents, and its boundaries. This can be water in the ocean, a lake, a river, or even a tank. The sounds studied usually have frequencies between 10 Hz and 1 MHz. At very low frequencies, below 10 Hz, sound cannot travel far without going deep into the seabed, and at very high frequencies, above 1 MHz, sound is absorbed quickly and does not travel far.

Hydroacoustics is often used with sonar technology to learn about underwater environments. It helps scientists measure the depth of water bodies and find out about plants and animals living underwater, including their numbers, where they are, their sizes, and how they behave. This can be done using passive acoustics, which means just listening for sounds, or active acoustics, where a sound is made and the echo is listened for. This method is commonly called using an echo sounder or echosounder.

Ships can also create noise underwater from different sources, such as their propellers, machinery, or the movement of the ship through the water. This noise comes from turbulent flow around parts of the ship and creates changes in pressure and flow that produce sound. The study of underwater acoustics connects closely with other areas of science, including sonar, transduction, signal processing, acoustical oceanography, bioacoustics, and physical acoustics.

History

Underwater sound has likely been used by sea animals for millions of years. The study of underwater sound began in 1490 when Leonardo da Vinci described how placing a tube in the water could let people hear distant ships.

Important progress happened in 1826 when Daniel Colladon, a Swiss scientist, and Charles Sturm, a French mathematician, measured how fast sound travels in water on Lake Geneva. They found it travels about 1,435 meters per second, which is very close to what we know today.

The sinking of the Titanic in 1912 and World War I led to new inventions for detecting icebergs and underwater ships. During these years, scientists developed ways to send and receive sound underwater, which helped both submarines and ships detect each other. These tools were improved during World War II and the Cold War, leading to many new discoveries about sound in water.

Theory

Sound waves in water move by squeezing and stretching the water. These changes can be picked up by things like our ears or special devices called hydrophones as shifts in pressure. The waves can come from human-made sources or happen naturally.

The speed at which sound travels depends on factors like frequency and wavelength. It also relates to how the water molecules move and the pressure of the wave. In water, sound moves about four times faster than in air and the water is much denser. At 1 kHz, a sound wave in water has a wavelength of roughly 1.5 meters.

Low-frequency sounds don't get absorbed much in water. Higher frequencies are more affected by water’s thickness and by tiny particles like salt or tiny air bubbles. Sounds can also get reflected or scattered by the surface, the ocean floor, or objects in the water. The way sound travels far through water can change due to temperature and pressure, creating paths where sound can travel long distances without losing much strength.

Measurements

Sound in water is measured using a hydrophone, which works like an underwater microphone called a microphone. The hydrophone records changes in water pressure, which are turned into something called sound pressure level.

We can describe these sound measurements in two main ways: using RMS acoustic pressure in pascals, or using spectral density in pascals squared per hertz.

The way we measure sound in water is different from how we measure sound in air. For the same number on the scale, sound in air is much stronger than sound in water. A special rule from 2017, called ISO 18405, explains these measurements for underwater sounds.

Sound speed

The speed at which sound travels in fresh water is about 1450 meters per second, and in seawater, it is about 1500 meters per second. This speed changes with pressure, temperature, and salinity.

Absorption

Scientists have studied how sound weakens in lakes and the ocean.

Ambient noise

To hear sounds in water, they need to be louder than the background noise, which comes from many places. At very low frequencies, things like ocean movement and small waves create noise. Ship traffic makes noise around 100 Hz, while wind on the surface makes noise between 1 kHz and 30 kHz. At very high frequencies, the movement of water molecules themselves creates noise. Other sounds, like whales, certain fish, and even rain, also add to this background noise.

Reverberation

Studies have looked at how sound bounces off the sea surface, the ocean floor, and within the water. These reflections can change how we hear sounds underwater.

Bottom loss

How much sound is lost when it hits the ocean floor depends on the angle at which the sound hits and the materials on the floor. This loss is important for understanding how far sounds can travel in shallow water.

Underwater hearing

Sound in water is measured in the same way as sound in air, using units called decibels. However, water and air are different, so comparing the two can be tricky. Water uses a different reference point for pressure, and the way we understand sound in water varies between people and animals.

Humans can hear sounds underwater, especially around a frequency of 1,000 vibrations per second (1 kHz). Very loud underwater sounds can be unsafe for divers, and guidelines help protect them. Animals like dolphins and whales can also hear very well underwater, especially at higher frequencies. Even fish and lobsters can sense sounds, and some birds that swim can react to underwater sounds too.

Applications of underwater acoustics

Sonar

Main article: Sonar

Sonar is like the acoustic version of radar. It uses sound pulses to explore the sea. By sending out sound and listening for the echoes, we can learn about the sea, its boundaries, and objects hidden beneath the water. There is also a method called passive sonar, which listens to the sounds made by underwater objects instead of sending out its own sound.

Underwater communication

Main article: Underwater acoustic communication

We need ways to send information underwater, such as collecting data about the environment, talking to underwater vehicles, or letting divers speak. One way to do this is by using sound waves, since radio waves do not travel well in water. This is called underwater acoustic communication. It can be tricky because sound travels slowly and can get mixed up, but scientists are working on ways to make it better.

Underwater navigation and tracking

Main article: Underwater acoustic positioning system

Underwater navigation and tracking help divers, underwater robots, and submarines find their way. Unlike radio waves, sound can travel far in water and can be used to measure distances very accurately. This helps these vehicles know exactly where they are.

Seismic exploration

Main article: Reflection seismology

Scientists use low-frequency sound to study the sea and its floor. This began after a big ship sank in 1912, leading to the development of tools to measure the depth of the ocean and map its floor.

Marine biology

Main article: Bioacoustics

Sound travels well underwater, making it useful for studying sea creatures, from tiny plants to large whales. Scientists use sound to learn where animals are, how many there are, and how they behave. Special devices can even track individual fish by attaching tiny sound transmitters to them.

Particle physics

Neutrinos are tiny particles that are hard to detect. Sometimes, scientists use the ocean to look for high-energy neutrinos by listening for the sounds they might make.

Other applications

Other uses of underwater sound include measuring rain, wind, and ocean temperature, studying how gases move between the ocean and air, and monitoring water movement.