Increased underwater noise from human activities, especially vessel traffic, is harming marine life prompting international efforts like Canada’s Quiet Vessel Initiative.
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In the dark undersea world, sound is essential. For marine life, sounds often play a crucial role in communication, foraging, reproduction, and navigation. However, as human activity in the oceans increases, so does underwater noise. Various industries contribute to ocean noise—offshore energy, construction, military operations, and of course vessel traffic. The noise generated by vessels is referred to as underwater radiated noise. With growing activity from commercial shipping, offshore developments, fishing and recreational vessels, underwater radiated noise pollution is increasing.
This increase has serious consequences for marine life, particularly species that rely on sound to survive, such as marine mammals. Underwater radiated noise can mask natural sounds, which may make it difficult for individuals to communicate, find a mate or prey, or even avoid predators. Some may change their behaviours, such as communicating less or avoiding or abandoning important habitats. Underwater radiated noise may also increase stress and cause other health problems.
Addressing underwater radiated noise is gaining attention. On the international stage, the International Maritime Organization updated its non-mandatory guidelines on reducing underwater radiated noise in 2023. In Canada, the federal government is developing an Ocean Noise Strategy to enhance the science, knowledge gathering, innovation, assessment, and guide management of underwater radiated noise while raising awareness of the issue.
What is sound, and how does it travel through water?
Sound is a form of energy created by vibrations. When something vibrates, like strings on a guitar or a car engine, the surrounding molecules also vibrate and bump into their neighbours, which bump into their neighbours, creating sound waves. These waves travel outward from the source in all directions. If the sound waves reach a receiver – like your ear – before they lose too much energy (and if the waves are within your hearing range), they are interpreted as sound.
Sound can be described by its amplitude (how loud) and frequency (high or low pitch). To human ears, larger amplitude is perceived as a louder sound. Amplitude generally decreases with distance travelled, which is why sounds from a source far away are quieter than those close by. Frequency refers to the number of cycles of a sound wave in a given time period. The more sound-wave cycles that occur a time period, the higher the frequency. Low-frequency sounds, like a foghorn or blue whale call in deep water, travel farther than high-frequency sounds because they lose less energy over distance.
When in water, sound travels faster and farther than in the air because the different physical properties of water mean it is denser than air and can transfer energy faster. Ocean conditions influence how far and how fast sound travels. Sound travels faster at higher water temperatures, salinity levels (saltiness), and pressure. Temperatures drop and pressure increases with greater water depth, meaning sound can travel at different speeds depending on depth.
Water depth is also significant because of sound attenuation. When a sound wave reaches the sea surface or seafloor, sound energy can be absorbed or reflected and refracted (bounced around), reducing the intensity of acoustic energy. The type of seabed material influences the amount of energy loss. For example, sand tends to absorb more sound than rock. In the Arctic, ice cover both absorbs and reflects sound, depending on the smoothness of the ice. For these reasons, a sound wave could travel farther and faster in one location than another. Even in a single area, how sound behaves can change from one day to the next.
Underwater sound is considered to be noise when it is generated by human actions, transmitted below the surface of the water and has a range of impacts on marine animals. Underwater noise can be continuous and long-lasting (like a ship propeller) or impulsive with an instantaneous start and stop (like a seismic survey air gun or echosounder).
The need to mitigate underwater radiated noise to protect marine ecosystems is gaining attention and urgency. Canada’s approach to reducing underwater noise includes supporting voluntary and mandatory operational measures (e.g., vessel seasonal slowdowns), and since 2017, promoting quieter ship designs through the International Maritime Organization. Canada’s efforts led to the adoption of updated guidelines on reducing underwater radiated noise in 2023. To support these revised guidelines, the International Maritime Organization also approved an underwater radiated noise Action Plan with an experience-building phase for different countries and international organizations to share best practices to address underwater noise. Further, the Canadian federal government is developing an Ocean Noise Strategy to enhance the science, knowledge gathering, innovation, assessment, and management of underwater noise while raising awareness of the issue.
In response to these concerns, Transport Canada’s Quiet Vessel Initiative (QVI) is stepping up to help reduce underwater noise, aiming to reduce noise at the source. QVI provides federal funding for research and development projects to develop and test promising technologies, vessel designs, retrofits and operational practices to make vessels quieter and more efficient, contributing to the protection and recovery of the marine environment and endangered marine mammals. This series of articles explores the results and shares the challenges identified by some of the QVI-funded projects.
This article is part of a five articles series on technology for detecting and analysing underwater vessel noise.
Continue learning about the new discoveries and challenges in making vessels quieter with the other topics in this series here
The Quiet Vessel Initiative is a federally funded program through Transport Canada. Industry partners and researchers interested in potential research and development collaborations to advance innovative solutions in marine technology are invited to contact the Quiet Vessel Initiative team at Marine-RDD-maritime@tc.gc.ca.
Featured image credit: Erik Mclean via Unsplash
Marine life: mammals, fish, and invertebrates living in the ocean environment.
Underwater noise: sound generated below water by human activity in the ocean environment. Various industries contribute to underwater noise—offshore energy, construction, military operations, and of course vessel traffic. The noise generated by vessels is referred to as underwater radiated noise.
Commercial shipping: vessels engaged in the transport of goods or people for the purpose of economic trade, including (but not limited to) container ships, bulk cargo carriers, vehicle carriers, tankers, cruise ships and ferries.
Mask, masking: a phenomenon where one or more sounds influence how another sound is perceived. This interference makes it difficult for the listener to accurately grasp and identify the sound of interest, causing it to become harder to understand. Masking can occur underwater when background noise, such as waves, wind, rain, or human activities, interferes with the detection or communication of sounds of interest and importance to marine life.
Amplitude: refers to the height of the sound pressure wave or the “loudness” of a sound. It is often measured using the decibel (dB) scale. Small variations in amplitude (“short” pressure waves) produce weak or quiet sounds, while large variations (“tall” pressure waves) produce strong or loud sounds. For example, imagine a wave on the surface of a pond. The amplitude would be the maximum height of the water above or below its calm level.
Frequency: sound moves through a medium like water as a wave, thus the term sound wave. Frequency, also known as pitch, indicates how often a sound wave repeats within a single second. Measured in Hertz (Hz), also known as cycles per second, higher numbers signify higher-pitched sounds and lower numbers mean lower-pitched sounds.
Salinity: the amount of salt dissolved in a body of water. The speed of sound traveling through water tends to increase as salinity increases.
Attenuation: the reduction in the intensity of acoustic waves over time and distance, also referred to as transmission loss.
Intensity: the amount of energy contained in a sound wave, measured in a given area at a given time; this also translates to the subjective perception of sound pressure and loudness.
Acoustic energy: the energy that travels through a substance in the form of sound waves is referred to as acoustic energy. When sound passes through any medium, it creates waves of vibrations, which vary in their energy in proportion to the amplitude of the sound waves.
Continuous sound: a type of sound that is long lasting and does not have impulsive characteristics.
Impulsive sound: an acoustic signal with an instantaneous start and stop. Underwater impulsive sounds are generated by certain human activities such as geophysical surveys, impact pile driving, acoustic deterrent devices, multi-beam echosounders, and detonation of explosives.
Seismic survey: a geophysical operation that uses a seismic air source to generate acoustic waves that propagate through the water and sediment, are reflected from or refracted along subsurface layers of the sediment, and are subsequently recorded by hydrophones at the surface.
Echosounder: a device for determining the depth of the seabed or detecting objects in water by measuring the time taken for sound echoes to return.
