How Sound Wave Work Underwater

Do You Know?

Acoustics is not only about sound propagation in the air, but also its propagation in the water. The study of sound propagation and how it behaves in water is called underwater acoustics. Underwater acoustics is a branch of science, and it has become a technology that has been used since World War I. Even before that, in 1490, Leonardo da Vinci has stated his theory in an article “if you stop your ship in the ocean and you put one side of a long tube into the water, then put your ear in the other side, you’ll listen the sound of the ship from great distance.” This indicates that underwater acoustics technology is already known for long time. 

In World War II, in military cases, underwater acoustics was used as a communication platform to channel information through the water. In 1925, underwater acoustics was used to measure ocean depth based on sound waves obtained — one of its usability is to find the plane crashed into the bottom of the sea. Time went by and many technologies developed and researches were performed.

One of the applications which can also be used for fisherman is fish-finder navigation tools. These tools can be used for fishers to find schools of fish in the ocean. We can also know the distance and the position of the school of fish from the ship based on the frequency range of the sound propagated.

In the industry, underwater acoustics has been applied to determine the presence of oil and gas in the sea. The method used is quite effective and efficient. In disaster management, early detection of a tsunami from the sea has been developed based on the propagation of infrasound detected from the seabed. In recent years, one technology that has attracted interest in many studies is the Autonomous Underwater Vehicle (AUV). AUV is an unmanned underwater vehicle, where the AUV can identify underwater biology and physics. The use of AUV can be the best choice in identifying the shape conditions of coastal waters because it can be operated in the long run. Besides, the use of AUV can also avoid damage to coral reefs and marine ecosystems.

The necessity for underwater research is quite high, especially for countries with vast oceans, such as Indonesia. Underwater acoustic research is needed in mining operations, observations on coral reefs, offshore oil exploration, and sea accidents.

The speed of a wave is the rate at which vibrations move through the medium. Sound moves at a faster pace in water and with long-distance than in air because the mechanical properties of water differ from the air. We know that the speed of sound wave propagation in the air is between 333 m/s and 340 m/s, the speed of sound waves in water is four times faster than the speed of sound in the air. The speed of sound waves in water ranges from 1500 m/s to 1520 m/s. We know that sound propagation occurs because of the ups and downs of particles in a medium. At sea, the deeper the depth of the sea, the higher the pressure. High-pressure water particles will be compressed so that they continue to propagate the sound without losing much energy. Besides, the density in water is higher than the density in the air. This causes the sound can travel fast and far away in the water. Unfortunately, the speed of sound in seawater is not a constant value. It varies by a small amount (a few percent) from place to place, season to season, morning to evening, and with water depth. Although the variations in the speed of sound are not large, they have important effects on how sound travels in the ocean. However, the temperature in seawater also affects the speed of sound waves, warm water travels faster and farther than colder water. 

There are three layers in the sea, based on its temperature, namely mixed water, thermocline, and deep water. In the thermocline, temperature decreases rapidly from the mixed upper layer of the ocean to much colder deep water. In the thermocline, the speed of sound waves decreases with the depth of the sea. In the layer below the thermocline, the temperature becomes constant again, and the pressure increases. In this layer, the speed of the sound waves again increases with the depth of the sea.

Temperature ⇢

As we know, wavelength is inversely proportional to frequency.

As can be seen in the equation above, the lower the frequency the longer the wavelength. Therefore, a 20 Hz sound wave is 75 m long in the water whereas a 20 Hz sound wave in air is only 17 m long in the air. Generally, the sensor used to capture underwater sound is a hydrophone or underwater microphone.

Decibels as a unit of sound pressure is the ratio between the pressure measurement and the reference pressure. Note that the reference pressure in the air with water is different. Therefore, 150 dB of sound in water is not the same as 150 dB of sound in air. In air, the reference pressure is 20μPa while in water the reference pressure is 1μPa. Based on the Sound Pressure Level equation, the conversion value of dB in air to water is

The characteristic impedance of water is about 3600 times that of air then

Therefore, the air to water conversion factor is

For example, if the sound of a jet engine in the air is 135 dB then the water is 197 dB in water.

Written by:

Adetia Alfadenata

Acoustic Engineer

Geonoise Indonesia


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