Exploring the Science of Underwater Acoustics: How Sound Travels Beneath the Waves?

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Exploring the Science of Underwater Acoustics: How Sound Travels Beneath the Waves?

For centuries, humanity has been fascinated by the mysteries concealed within the ocean’s expansive depths, which hold countless enigmatic secrets. Beneath the surface lies a world of sound, where the physics of underwater acoustics influence the intricacies of marine life and maritime operations. Underwater acoustics also known as hydroacoustics investigates how sound waves travel through water and interact with its surroundings, including marine life, substances, and boundaries.

In the maritime industry, understanding the behavior of sound waves at the bottom is vital for protecting aquatic life as well as for communication and navigation in submarine operations.

How does the science of shallow water acoustics elucidate the journey of sound beneath the waves? 

Overview of sound propagation in water

The origins of underwater acoustics trace back to 1490, with Leonardo da Vinci being among the pioneers who wrote: “If you cause your ship to stop and place the head of a long tube in the water and place the outer extremity to your ear, you will hear ships at a great distance from you.” In 1912, following the tragic sinking of the Titanic and the outbreak of World War I, there was a surge in innovation within the field of shallow acoustics.

This period saw the development of systems designed to detect icebergs and U-boats. During 1912-1914, several echolocation patents were awarded in Europe and the U.S., leading to Reginald A. Fessenden’s echo-ranger invention in 1914.

How do the physics of sound waves govern their behavior in underwater environments?

Temperature

The speed of sound in water is significantly influenced by temperature. A higher temperature of the medium facilitates faster sound travel. When salinity and pressure remain unchanged, sound travels about 15 feet per second faster with each 1.8-degree Fahrenheit increment.

Pressure

Underwater acoustic transmission is greatly affected by pressure, which varies with depth. As depth increases, so does water pressure, compressing the molecules and increasing the speed of sound. This phenomenon, known as Adiabatic Compression, contributes to the complex dynamics of sound propagation in deep-sea environments. For example, when descending deeper into the sea, the ears start to feel a slight pressure. This pressure sensation occurs because the water above exerts its weight, compressing the air in the ears.

Salinity

Salinity, the concentration of dissolved salts in water, also impacts acoustic speed. The higher the salinity levels, the denser the water medium, resulting in even faster sound propagation. Conversely, low salinity conditions, such as those found in areas affected by freshwater influx, can lead to slower sound speeds. While the average salinity of the earth’s ocean stands at 34.7 psu (practical salinity unit) resembling the salt content commonly found in deep waters, surface salinity often fluctuates between 30 and 38 psu, depending on freshwater flows.

Comparing Sound Speed in Water and Air

Whether in water or air, the principles governing sound waves remain consistent. These waves can traverse through different substances, spanning from gas to liquids such as water and even solids like the ocean floor.

How does sound speed in air differ from sound in water?

Differences and Implications

Although sounds exhibit comparable characteristics whether traveling through water or air, the conventions for measuring and expressing acoustic levels vary between the two mediums. Water’s high density offers a degree of compensation for its extreme incompressibility, facilitating the rapid transmission of sound at a speed of 1,493 meters per second (m/s), which is four times faster than it travels through the air.

This fundamental difference has significant implications across multiple fields, from seafaring navigation to underwater communication systems.

Marine Exploration Through Sound

How Do Medium Properties Influence Sound Speed?

Properties such as density, elasticity, and temperature profoundly impact the rate at which sound travels.

For example, in denser mediums like water, molecules are closely packed, facilitating faster transmission compared to the less dense air. Elasticity, which determines how easily a medium can be compressed or stretched, also affects sound speed, with more elastic mediums transmitting sound more efficiently.

Moreover, temperature alterations in a medium can significantly affect its density, thus influencing the speed at which acoustic waves travel.

How Sound Speed Variations Impact Marine Technology?

The speed of sound typically ranges from 1,450 to 1,570 meters per second in the oceans. It increases by approximately 4.5 meters per second for each degree Celsius rise in temperature and 1.3 meters per second for every 1 psu increase. Variations in sound speed wield profound impacts on diverse facets of marine technology:

  • The difference in acoustic speed between layers of water, known as sound refraction, complicates the interpretation of sonar data, leading to inaccuracies in in-depth measurements.
  • Assists in the design of efficient underwater communication frameworks for submarines and remotely operated vehicles (ROVs), optimizing data transmission rates.
  • The accurate prediction of sound speed variations enables the development of advanced marine navigation technologies, enhancing the safety and efficiency of maritime operations.

Marine Exploration Through Sound

In ancient times, sailors pioneered the use of simple tools like submerged bells to navigate the seas and gauge water depths, marking the earliest instances of using sound for marine exploration. Today, modern sonar technology allows scientists to explore the depths, communicate with submarines, and study oceanic life, opening new frontiers in oceanic research.

The Role of Sonar

Sonar, short for Sound Navigation and Ranging is crucial for ocean exploration and mapping because sound travels longer distances in water compared to radar or light waves. The National Oceanic and Atmospheric Administration (NOAA) scientists used the main tool to create nautical charts, identify underwater navigational hazards, explore and map seafloor artifacts like shipwrecks, and survey the ocean floor. Two main types of sonar exist in the marine society: active and passive.

Active Sonar

In active sonar systems, transducers generate and transmit an acoustic signal or sound pulse into the water. When the pulse encounters an object, it reflects off the object and returns as an ‘echo’ to the sonar transducer. When the transducer can receive signals, it evaluates the signal’s intensity.

Passive Sonar

In contrast to active sonar, passive sonar refrains from emitting its signal but detects only incoming acoustic waves. This feature benefits military vessels to evade detection or scientific missions prioritizing silent observation of oceanic sounds. The main purpose of passive sonar mechanisms is to detect acoustic signals generated by marine objects, including submarines, ships, and mammals such as whales.

Submarine Communication

Unlike terrestrial communication, which relies heavily on radio waves, submarine communication predominantly uses sound waves due to the unique properties of water. Understanding the angle at which sound waves propagate in water helps scientists gauge the energy they carry. Here are the technologies and techniques that facilitate effective communication beneath the wave.

Telephone Systems

Submarines use specialized underwater telephone systems that convert electrical signals into acoustic waves. These processes enable direct communication between submarines, surface vessels, and command centers by modulating the frequency, amplitude, or phase of the acoustic waves.

Acoustic Modems

In April 2017, NATO’s Centre for Maritime Research and Experimentation announced the approval of JANUS, a standardized protocol to transmit digital information underwater using acoustic sound. Acoustic modems provide a means of digital transmission between submerged submarines and surface platforms. These devices encode digital data into acoustic waveforms, allowing submarines to exchange encrypted messages, sensor data, and tactical information with other vessels and shore-based facilities.

Sound Signals Among Marine Mammals

Marine mammals, including dolphins, whales, and seals, inhabit the Earth’s oceans and possess remarkable communication abilities. Their sophisticated use of sound signals has long fascinated scientists, prompting investigations into the intricacies of marine mammal language.

Sound serves as the primary medium of communication for mammals, allowing them to navigate, locate prey, and communicate with conspecifics over great distances. These animals produce a wide range of sounds, including clicks, whistles, moans, and songs, each carrying distinct meanings and functions within their social and ecological contexts. Researchers use a range of methods to study mammal communication including passive acoustic monitoring and controlled experiments.

Acoustic monitoring: hydrophones record underwater sounds continuously, offering insights into natural communication patterns.

Controlled experiments: conducted in controlled environments or captivity, enable researchers to manipulate variables and study specific aspects of marine mammal communication in detail.

Key Concepts in Underwater Acoustics

Hydroacoustics examines the behavior of sound as it travels through water and interacts with the various elements present in the aquatic environment.

Decibels and Beyond

As the underwater realm is a cacophony of sounds, understanding the complexities of sound frequencies and noise levels beneath the waves is a source of various applications.

Frequency and Pitch:

Frequency refers to the number of oscillations or cycles of a sound wave per unit of time and is measured in Hertz (Hz). Higher frequencies result in higher-pitched sounds, while lower frequencies produce lower-pitched sounds.

Decibels and Noise Levels:

Decibels (dB) quantify the intensity or loudness of a sound wave and are measured on a logarithmic scale. A rise of 10 dB represents a tenfold increase in sound intensity, roughly corresponding with loudness, which is equivalent to increased loudness perception.

The Impact of Noise Pollution

Excessive noise pollution in the earth’s ocean harms marine organisms, disrupting vital behaviors such as communication, navigation, feeding, and reproduction. Whales and dolphins, which rely heavily on sound for social interactions and prey detection, are particularly susceptible to noise pollution. High-intensity anthropogenic noise can mask biologically important hearing, interfere with mating calls, and induce stress responses in marine mammals, which can affect their survival and well-being.

Mitigating Underwater Noise

Efforts to mitigate underwater noise pollution involve implementing regulations, technological innovations, and conservation measures to reduce the impact of human activities on marine ecosystems.

Strategies include implementing quieter vessel designs, modifying shipping routes to avoid sensitive habitats, and employing passive acoustic monitoring to assess noise levels and monitor marine mammal populations. Public awareness and stakeholder engagement are also essential for fostering responsible stewardship of the sea environment and minimizing the impacts of pollution on oceanic life.

Specific digital technologies developed by Sinay

Sinay leads the charge in revolutionizing maritime operations with cutting-edge digital technologies tailored for environmental sustainability and operational excellence. Among Sinay’s innovations are advanced sonar systems, boasting unparalleled precision in underwater monitoring and hazard detection. Designed to navigate through the complexities of ocean environments, these techniques utilize and edit cutting-edge technology to ensure accurate data collection while minimizing interference from background noise.

Complementing their sonar advancements, the maritime tech company has pioneered the development of underwater communication networks that redefine connectivity beneath the waves. (PAM buoys)

Seamlessly linking maritime vessels, sensors, and onshore facilities, these networks enable real-time collaboration and decision-making, fortified by robust encryption protocols for unparalleled security.

Sinay’s commitment to data-driven insights shines through its sophisticated analytics platforms, powered by artificial intelligence and machine learning. These platforms transform vast amounts of maritime data into actionable intelligence, empowering companies to optimize operations, mitigate environmental impact, and safeguard ocean biodiversity.

Passive acoustic monitoring buoy by Sinay

Acquiring knowledge of underwater acoustics is vital for protecting marine life by mitigating anthropogenic noise, conserving endangered species, assessing ecosystem health, informing oceanic spatial planning, and enhancing conservation efforts. By preserving the acoustic integrity of marine environments, safeguarding the rich biodiversity and ecological balance of the world’s oceans for future generations is ensured.

Frequently Asked Questions About Sound Underwater

Sound travels underwater through pressure waves. When an object creates a disturbance in the water, it creates waves that travel through the water, transmitting sound.

Several factors affect the speed of sound underwater, including temperature, salinity, and pressure. Generally, sound travels faster in warmer, denser water with higher salinity.

Sound travels faster in water than in air because water is denser and molecules are closer together, allowing sound waves to propagate more efficiently. This density facilitates faster transmission of sound waves.

The distance sound can travel underwater depends on various factors, including the frequency of the sound, the depth of the water, and the presence of obstacles. Low-frequency sounds can travel thousands of kilometers, while high-frequency sounds may only travel a few kilometers.

Sound is crucial underwater for communication, navigation, and sensing the environment. Marine animals use sound for various purposes, such as finding food, avoiding predators, and mating. Additionally, humans use sound underwater for activities like submarine communication, sonar mapping, and studying marine ecosystems.

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