The ocean is naturally noisy, filled with the sounds of waves, weather, and marine life. Modern human activity has introduced loud and persistent sounds that fundamentally alter the underwater acoustic landscape, creating ocean noise pollution. This anthropogenic noise travels with remarkable efficiency in water. The high density and incompressibility of water allow sounds to propagate over vast distances, meaning a single, powerful source can impact a massive geographical area.
Noise from Commercial Shipping
Commercial shipping is widely recognized as the largest and most constant contributor to the background noise levels in the world’s oceans. The sheer number of vessels traversing global shipping lanes creates a continuous, low-frequency acoustic ‘haze.’ This pervasive sound is generated primarily through two distinct mechanisms: machinery noise and propeller noise.
The ship’s engine and auxiliary machinery produce vibrations that radiate through the hull and into the water column. This mechanical noise contributes to the overall sound signature, but it is often overshadowed by the propeller. The most intense shipping noise comes from propeller cavitation, which occurs as a ship moves through the water at speed.
Cavitation is the formation of vapor-filled bubbles on the low-pressure side of the propeller blades. As these bubbles move into a higher-pressure region, they rapidly collapse, generating a powerful, impulsive broadband noise. This constant creation and collapse creates a distinct low-frequency ‘hump’ in the noise spectrum, typically peaking between 30 and 100 Hertz. Since low-frequency sound travels the farthest, this continuous rumble can effectively mask the natural communication frequencies used by large marine animals, such as baleen whales, across entire ocean basins.
Seismic Surveying and Oil Exploration
A different source of noise pollution comes from seismic surveying, primarily conducted for oil and gas exploration. This process maps the geological structure of the seafloor and sub-seafloor layers to identify potential hydrocarbon or mineral deposits. The sound source employed is an array of airguns towed behind a vessel.
These airgun arrays rapidly release highly compressed air, creating an intense pressure pulse that travels through the water and penetrates the seabed. The sound waves reflect off various geological boundaries and return to hydrophones towed behind the same vessel. Each pulse is extremely loud and impulsive, representing one of the most powerful man-made sounds introduced into the ocean.
The dominant energy of the airgun impulse is concentrated in the low-frequency range, typically below 200 Hertz. While the arrays are designed to focus energy downward, acoustic energy also radiates horizontally into the water column. Source levels can reach over 250 decibels (re 1 µPa at 1 meter), and its sheer intensity means it can propagate for hundreds of kilometers, temporarily disrupting vast swathes of the marine environment with each blast.
Active Sonar and Military Operations
Military operations introduce noise into the ocean primarily through the use of active sonar systems for detection, navigation, and mapping. Active sonar works by transmitting a sound pulse, or ‘ping,’ into the water and then listening for the echo that reflects off submerged objects like submarines. This deliberate transmission of sound distinguishes it from passive sonar, which only listens to ambient noise.
Sonar systems are categorized by the frequency band they use, which dictates their range and resolution. Low-Frequency Active Sonar (LFAS), operating below 1 kilohertz, can travel hundreds of miles and is used for long-range surveillance. Mid-Frequency Sonar (MFAS), operating between 1 and 10 kilohertz, is commonly used by naval ships for submarine detection and has a shorter, but still significant, range.
These systems emit directed, high-intensity sound energy in focused beams. The sound is characterized by its high peak pressure and short duration, described as a repetitive series of powerful pings. Unlike the broadband noise of shipping or seismic surveys, military sonar is often a narrowband signal, but its concentrated energy output makes it a significant source of acoustic disturbance in the areas where it is deployed.
Noise from Marine Construction
The construction of coastal and offshore infrastructure also contributes intense, localized noise pollution to the marine environment. This includes the building of ports, bridges, offshore wind farms, and oil and gas platforms. The primary source of this noise is pile driving, where large steel or concrete piles are hammered into the seabed to form a stable foundation.
Impact pile driving uses a high-energy hammer to strike the pile repeatedly, producing an extremely loud, impulsive sound. Each strike creates a shock wave that radiates through the water and the substrate. This noise is highly localized but can be intense enough to cause significant disturbance in the immediate area, with thousands of strikes often required for a single foundation.
Alternatively, vibratory pile driving uses a lower-frequency method to shake the pile into the sediment. While less impulsive than impact driving, it produces a continuous, low-frequency sound, typically between 20 and 40 Hertz, that can last for minutes at a time. Other activities like dredging and rock-breaking also generate construction noise that affects specific coastal and nearshore habitats.