The depths of the sea form a delicate universe, far from surface noise, where marine life communicates through sound. Whale songs, dolphin clicks, fish crackles… Yet in recent years, this natural symphony has been overshadowed by a heavy background sound: underwater radiated noise (URN) from ships.
The bulletin published by ClassNK in July 2025 (Technical Journal No.11) addresses this issue on an international scale. The study, aligned with IMO guidelines, includes measurement criteria and proposed solutions, bringing to the forefront a concept the maritime sector will increasingly encounter in the coming years: quiet ships.
The following graphic from the ClassNK Technical Journal vividly illustrates the impacts of anthropogenic underwater noise on marine life. Low-frequency sounds from ships, in particular, can travel for kilometers with little attenuation in water, creating serious stress and threats for marine species.
High-frequency sounds (above 1000 Hz) attenuate quickly in water and cannot travel long distances. However, low-frequency sounds generated by large ships, air guns, wind turbines, and mining activities can spread across vast areas. Especially within the so-called SOFAR channel (Sound Fixing and Ranging), formed depending on water temperature and depth, these sounds can travel thousands of kilometers.
Such noise may:
- Negatively affect essential behaviors of whales, dolphins, seals, sharks, and fish, such as navigation, hunting, migration, and communication.
- Cause reduced hearing sensitivity, and in some species, permanent damage to auditory organs.
- Lead to habitat abandonment and shifts in species distribution.
The graph below clearly shows the overlap between the hearing ranges of marine species and anthropogenic noise sources. For example, the continuous noise from large ship engines falls within the hearing range of many species.
These findings highlight that underwater noise is now a critical environmental stressor that can no longer be overlooked.
Where Does the Noise Come From?
- Propeller cavitation → The strongest source; low-frequency sounds can travel kilometers.
- Main and auxiliary engines → Motor vibrations are transmitted to the hull.
- Hull-propeller mismatch → Flow irregularities increase vibration.
Why Does It Matter? Echoes in the Ecosystem
- Communication disruption: Mating, hunting, and navigation behaviors are hindered.
- Physiological impacts: Long-term exposure may cause hearing loss in marine mammals.
- Migration route changes: Marine life avoids noise-intensive regions.
The Role of IMO and Classification Societies
In 2014, the IMO published its first voluntary guidelines for reducing underwater noise. Updated in 2023, the guidance includes measurement methods and noise management plans.
In line with this, ClassNK issued its own Guidelines for Underwater Noise from Ships in 2023. These cover:
- Measurement conditions (depth, loading status, sea state),
- Design requirements (hull-propeller alignment, vibration-reducing components),
- Classification categories (SUN-C, SUN-A).
How Is It Measured?
- Deep water condition: ≥150 m or 1.5 times ship length.
- Full load measurement: Engine power, wave, and wind within set limits.
- Hydrophone recording: Sound pressure measured as the ship passes.
- Data analysis: One-third octave band evaluation, background noise corrections applied.
Challenges in Practice
- Cost: New propeller designs and vibration-reducing systems require investment.
- Shallow seas: Measurement becomes complicated in regions such as the Marmara and Baltic Seas.
- Voluntary nature: Not yet mandatory, limiting implementation.
What Does It Mean for Turkey?
Turkey’s seas host heavy maritime traffic: the Istanbul and Çanakkale Straits, Mediterranean ports, and tourism areas.
- Fisheries and tourism could be directly affected by noise.
- Sensitive species such as dolphins and seals may lose habitat.
- For shipyards, SUN-C and SUN-A classifications can offer advantages in both prestige and exports.
Looking Ahead: Quiet Ships
- IMO guidelines will continue to evolve.
- Noise management plans and databases will become widespread.
- New research will focus on measurement methods in shallow waters.
The key for the sector is this: noise cannot be completely eliminated, but it can be reduced. This benefits both the environment and ship efficiency.
Different Species, Different Reactions
Research has shown that the effects of ship noise often begin quietly. At first, marine animals struggle to communicate, then their behaviors start to change, and in the long run even their survival and overall fitness can be affected.
Not every species or region responds the same way. For example, dolphins living in coastal waters are more likely to show stress and avoidance when ships pass by, while large whales are especially vulnerable because the noise interferes with the low-frequency sounds they use to communicate.
Scientists also point out that the impact of noise depends a lot on the conditions: the type of ship, its speed, how heavily it is loaded, and even the physical environment around it. Ignoring these details can lead to misleading results.
Another key message from the research is that we still lack enough long-term studies. Many investigations have been short snapshots in time, but to really understand how underwater noise shapes the future of marine populations, scientists need to follow animals over much longer periods.
Final Note
Ships will always generate noise; the challenge is managing its echoes in the ecosystem. ClassNK’s guidelines and IMO’s vision provide concrete steps in this regard. For maritime nations like Turkey, the issue is not only about external pressure but also about ensuring the sustainability of their own seas.
Protecting the silence of the oceans may be one of the strongest legacies left for future generations.
References: IMO, ClassNK, EMSA, NRDC.