UNDERWATER ACOUSTICS
Underwater acoustics studies the sound and its behavior under water. When objects underwater vibrate, they produce sound-pressure waves, which compress and then decompress the water molecules as it moves through the water. Sound waves radiate away from the source much in the way ripples radiate on the surface of a pond or lake.
Typically, aquatic acoustics is a study applies to oceans; however, the laws are applied to rivers, lakes, or water held in tanks. The frequencies generally measure between 10 Hz and 1 MHz, and the discipline is sometimes called hydroacoustics.
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Hearing Underwater – The History of Underwater Acoustics
Leonardo Da Vinci noted in 1490 that if a ship stops and an individual on board places a long tube into the water with the other end held to the ear, that person will hear other ships at a significant distance through the water. Even earlier, Aristotle (384-322 BCE) noted that sound could be heard through water in addition to air. Still, advancement in understanding underwater acoustics did not take place until at least 200 years after Da Vinci’s observations. Galileo and Marin Mersenne discovered the laws surrounding vibrating strings. Mersenne published his findings in the late 1620s. Indeed, Mersenne’s work and experimentation with measuring the speed of sound in air is recognized as the foundational work for the study of acoustics. In 1687, Sir Isaac Newton wrote and published his mathematical theory explaining how sound travels. This work focused on sound through air, although the same primary mathematical theory applies to aquatic acoustics.
Later in 1743, Abbe Nollet conducted experiments to determine if sound travels through water. Holding his head underwater, Nollet reported hearing a bell, a pistol shot, a whistle, and shouts. He also observed that an alarm clock ringing underwater could be heard by an individual underwater, but not above through the air. This settled the debate about whether sound travels through water.
It is now well-established science that sound waves do travel long distances, especially in relation to the distance that electromagnetic waves can travel. Therefore, in the ocean in contrast to air or a vacuum, one utilizes sound navigation and SONAR rather than radar. In addition, aquatic acoustics now studies the masking of sound underwater by interference and extracting the sound from that interference.
Faster Than the Speed of Sound
The speed of a sound wave is measured as the rate its vibrations move through the water or any other substance. Sound moves faster through water – about 1,481 meters
per second at 20ËšC – than through air (340 meters per second) due to the differing properties between them. Sound travels faster through warmer waters; thus, temperature also impacts the speed of sound, which makes this dynamic highly significant in some parts of the oceans.
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The Noisy Oceans
One important application of underwater acoustics is oceanography. The ocean conducts sound very efficiently due to it acting as a “waveguide”, reflecting sound back and forth between the surface and the ocean floor.
At a particular depth range, this efficiency is even greater, allowing frequencies lower than a few hundred hertz to travel thousands of miles. This range is what is known as the “Deep Sound Channel”, or SOFAR. The SOund Fixing And Ranging Channel, and sits at depths of approximately 500-1000 meters. It was discovered when a downed aviator had set off this small explosive charge, and rescue teams used hydrophones to identify the source of the charge. Sound through the Deep Sound Channel can travel these great distances with little loss of signal, and thus it is here that deep ocean acoustic monitoring is optimal.
The Impact of Man
The changes in sounds underwater have been driven in large part by man-made activities, including wind farm development, maritime shipping, naval-sonar systems, and seismic exploration. In addition, some natural phenomena have had an impact on aquatic acoustics. These include ocean acidification and climate change.
In a broader sense, aquatic acoustics concerns the entire underwater soundscape, which includes natural and man-made sounds. It allows scientists to quantitatively observe and predict the impact of natural and anthropogenic noise pollution in the oceans.
Some governmental agencies have responded by regulating man-made activities in consideration of the negative effect the growth of underwater noise can have on marine life. Scientists use advanced modeling techniques to mitigate the potential for damage to marine mammals, such as non-intrusive measuring, and integrated modeling approaches. Indeed, these developments are geared towards protecting marine life and the ocean environment.
The Importance of Aquatic Acoustics
Aquatic acoustics is a prime foundational technology that underpins technology in environmental and oceanographic studies, as well as offshore oil and gas activities. In addition, the discipline plays a critical role in national defense.
Applications of underwater acoustics include: tomographic measurement of ocean temperature and currents, weapons systems, sonar, water quality measurement, geophysical surveying, echo-sounding, navigation, positioning, and communications. In addition, the discipline is used to accurately measure anthropogenic noise and its impact on marine life.
“Echo sounding” is a technique used to determine water depths.
Today’s professional underwater acoustics experts offer a wide array of skills and technology, such as:
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Underwater characterization and acoustics testing by transducers using a variety of test facilities, which can include both open water sites and laboratory tanks.
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Acoustic testing and measurement of materials and transducers using simulated oceanic conditions
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Developing accurate standards that are necessary for an evolving industry, such as assessment and measurement of underwater radiated sound
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Key free field measurement and dissemination across a broad range of frequencies
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Using acoustic near-field methods and innovative scanning techniques for sonar characterization
Instrumentation
Aquatic acoustics scientists work closely with both academia and industries. They frequently possess a wide array of capabilities that enable them to formulate precise, unique solutions for acoustic measurement and problematic instruments. Underwater acoustics uses sophisticated instruments for measuring and calibration, and scientists offer expertise in acoustic measurement research and development in fields such as ultrasonics for industrial and medical use.
The science of aquatic acoustics gives practitioners the necessary tools to quantitatively measure and describe underwater sounds. By quantifying elements such as amplitude and frequency, scientists have added a great deal of knowledge about the oceans and marine life. In fact, this discipline has enabled scientists to study such things as whale migration, volcanic activities, and earthquakes. Aquatic acoustics has a wide range of applications across a variety of sciences, and it continues to grow in importance.
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