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The Silent Threat: Underwater Radiated Noise (URN) and its Impact on Oceans and Warfare

Beneath the shimmering surface of our oceans lies a hidden battleground, not of weapons and armies, but of sound. Underwater radiated noise (URN), the unintended sound emitted by human activities, is silently reshaping marine ecosystems and posing challenges for naval stealth. Let’s dive into the depths of this sonic issue and explore its multifaceted consequences.

Underwater radiated noise (URN)

Marine anthropogenic noise has increased significantly over the past few decades, and a growing body of research is highlighting the negative impacts this is having on marine eco-systems.

Underwater radiated noise (URN) refers to the sound emitted by ships, boats, and other underwater vehicles and structures. URN is caused by the movement of water and the vibration of machinery and equipment, and can travel long distances through the water. It can have a significant impact on marine life, as many species rely on sound for communication, navigation, and hunting.


URN Impact

URN can have a significant impact on marine life, as many species rely on sound for communication, navigation, and finding food. It can also interfere with underwater communication systems and military sonar operations.

For cetaceans like whales and dolphins, URN disrupts vital communication, navigation, and foraging. Blinding their echolocation abilities, it hinders their ability to hunt, avoid predators, and find mates. For fish, it masks crucial environmental cues, leading to disorientation and increased vulnerability to predation. URN can even cause physiological stress, impacting reproduction and survival rates.

Economic Echoes:

Fisheries Decline: URN disrupts fish communication and predator-prey interactions, leading to decreased catches and economic losses for fishing communities. Studies estimate billions of dollars lost annually due to URN-induced fish population declines.

Tourism Toll: Whale-watching tourism thrives on healthy cetacean populations. URN disrupts whale communication and drives them away from popular viewing areas, impacting tourism revenue and coastal economies.

Infrastructure Damage: URN from construction and seismic surveys can weaken underwater infrastructure, like pipelines and cables, leading to costly repairs and potential environmental disasters.

Military Impacts

Detection Dilemma: Sonar effectiveness diminishes in an URN-polluted ocean, making it harder to detect potential threats, raising security concerns for coastal nations.

Communication Chaos: Naval communication relies on underwater sound waves. URN disrupts these channels, hindering coordination and potentially compromising operational effectiveness.

Stealth Under Siege:

Naval vessels rely on silent operation for strategic advantage. Unfortunately, URN betrays their presence, alerting adversaries and compromising their missions. Submarine detection through passive sonar becomes easier, putting submarines at greater risk. These sounds can travel long distances through water and can be detected by underwater microphones, also known as hydrophones.

Additionally, URN from friendly vessels can mask enemy movements, creating tactical confusion and hampering operational effectiveness. URN can also interfere with sonar and other underwater communication systems used for military, scientific, and commercial purposes. In the past, URN has primarily been a concern for naval vessels, particularly those engaged in anti-submarine warfare or mine counter-measure vessels, and also some research vessels.


URN Mitigation

Mitigating URN requires a multi-pronged approach. Technological advancements in quieter engines, propellers, and sonar technologies are crucial. Operational changes, such as designated quiet zones and speed restrictions in sensitive areas, can further reduce noise pollution. International collaboration and policy frameworks are essential to address this transboundary issue effectively.


URN reduction technologies

With increasing pressure to reduce the noise generated by commercial and Naval ships, there is a need to develop new technologies and look at how existing technology can be applied to reducing vessel noise. To mitigate the effects of URN, regulations have been established to limit the amount of noise that can be emitted by ships, and mitigation measures such as underwater noise reduction technology are being developed. Industries are developing noise reduction technologies and regulations to limit the amount of noise generated by underwater vehicles and structures.

Biomimicry: Nature knows best. Quieter propellers inspired by fish fins and shark skin reduce cavitation and noise generation. Wartsila’s X-Flow propeller and Becker Marine Rudders mimic natural hydrodynamics, slashing URN and fuel consumption.

Electric Propulsion: A silent revolution. Replacing diesel engines with electric motors, especially in smaller vessels, significantly reduces noise pollution.

Marine propulsion is the mechanism or system used to generate thrust to move a naval vessel across the water. At present, 90% of sea-going naval ship are diesel-powered. Navies are gradually adopting electric propulsion for their ships. There are numerous benefits to electric motor propulsion including that it’s quieter, more efficient at lower speeds, and less smelly. It’s also expected to lower overall costs of ownership by reducing or eliminating the need for oil and transmission fluid changes, filter and impeller replacements and starter problems. There’s less to winterize too. Additionally, unlike diesel or gas engines, electric motors provide full torque instantly so boats get up on plane faster. Companies like ABB and Siemens Marine are pioneers in this transition, offering quieter, cleaner propulsion solutions.

Bubble Curtains: A sonic shield. Air bubbles released around noisy activities, like pile driving, create a physical barrier, absorbing and scattering URN before it reaches marine life. EcoGuard Barrier Systems offers inflatable bubble curtain solutions for various underwater operations.

Advanced Sonar Technologies: Seeing doesn’t have to scream. New sonar systems utilize optimized frequencies and targeted beams, minimizing energy output and reducing impact on marine mammals. Teledyne RESON and Kongsberg Maritime are developing quieter, smarter sonar solutions for quieter exploration and research.

Beyond the Technological Arsenal:

  • Operational Adjustments: Tweaking operational practices can make a big difference. Slowing down in sensitive areas, establishing quiet zones, and coordinating vessel movements can significantly reduce URN pollution.
  • Mapping and Monitoring: Understanding the noise landscape is crucial. By mapping URN sources and tracking noise propagation, targeted mitigation strategies can be developed. Marine Spatial Planning initiatives like those spearheaded by the European Union are paving the way for quieter seafloor usage.
  • International Collaboration: This global issue demands global solutions. International agreements like the IMO’s Underwater Noise Guidelines and regional initiatives like the Quiet Mediterranean project create frameworks for coordinated action and knowledge sharing.


Oscar Propulsion Unveils Silent Blade Tech: A Boon for Oceans and Marine Life

Ships have a new, quieter side, thanks to a revolutionary propeller technology from Oscar Propulsion and the University of Strathclyde. Their PressurePores™ system tackles a major environmental concern: underwater radiated noise (URN) from ship propellers. This silent threat disrupts marine fauna, affecting communication, behavior, and even survival.

The patented PressurePores™ system reduces propeller tip vortex cavitation by applying a small number of strategically placed holes in the propeller blades. The addition of these pressure-relieving holes allows ships to operate with a more silent propeller. “We have found the optimum number of holes required to reduce the noise. So long as the right number of holes are placed in the most effective positions, a cavitation sweet spot can be achieved,” said Eikeland.

Propeller cavitation noise can reach 188dB, audible to marine life 100 miles away. Anything above 160dB, as NOAA warns, can harm marine life, impacting reproduction, feeding, and even causing death. URN levels are expected to double by 2030, making PressurePores™ even more crucial.

“It’s not a case of simply drilling holes into the blades, as this will affect the propeller’s thrust capability. We know exactly where to place the holes for maximum efficiency and for optimum noise reduction.” Following four years of comprehensive computational fluid dynamics (CFD), modelling and cavitation tunnel tests during the solution’s development phase at Strathclyde, it was demonstrated that PressurePores can reduce cavitation volume by almost 14% and URN by up to 10dB.

Existing and Proposed International Regulations on URN Management

In response, international efforts are rising to curb this invisible threat, protecting marine life and safeguarding security interests. Let’s explore the existing and proposed regulations steering us towards a quieter future.

Existing Frameworks:

  • IMO Guidelines: The International Maritime Organization (IMO) is the primary actor, issuing voluntary “Guidelines for the Reduction of Underwater Noise from Shipping” in 2014. These encourage quieter ship design, operational adjustments like speed restrictions in sensitive areas, and research into URN reduction technologies.
  • Regional Initiatives: The European Union adopted a Marine Strategy Framework Directive that includes URN management, while the Mediterranean Sea is designated a “Quiet Area” under the Barcelona Convention, imposing stricter noise regulations.



The silent chorus of URN serves as a stark reminder of the interconnectedness of our planet. Protecting marine life and ensuring naval security necessitate tackling this invisible threat.

Tackling URN isn’t about silencing progress, but about finding harmony between human activities and the ocean’s delicate soundscape. By embracing these cutting-edge technologies, implementing best practices, and fostering international collaboration, we can orchestrate a future where the ocean whispers a lullaby of life, not a warning cry.

This isn’t just about protecting marine life; it’s about safeguarding our economies, our security, and the future of our planet.



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