Underwater noise disturbs the Baltic Sea - studies are just beginning

Even the depths of the Baltic Sea are no longer allowed to exist in solitude. Below the water surface, man-made noises echo everywhere. The issue of underwater noise is problematic because it does not disturb mankind itself. As a result, it is only in recent years that the problem has garnered any attention.

Noise can be extremely harmful to marine life. Their sense organs are sensitised to hearing underwater sound signals, which can often be subtle. Sudden, loud noises can shock aquatic animals. In the worst cases, it can even kill them.

View from onboard a passenger vessel over the sea and a wind farm.

Most antropogenic noise occurs in shipping lanes

Merchant shipping is a significant and, above all, continuous source of under-water noise. Most antropogenic noise occurs in and close to shipping lanes. The intensity of the noise by marine traffic depends on the volume of traffic. 

Distribution of continuous under-water noise in the Baltic Sea. The intensity of the noise increases from green to red. Source: HELCOM.

When considering all under-water noise, one has to remember that part of it originates from nature's own processes. Noise is generated by wind, waves and thunder. The importance of waves as the source of noise is more pronounced in the open sea.

 Punaisina erottuvat väylät. Vihreää löytyy lahtien pohjukoista.
Baltic Sea sound map. The intensity of the sound increases from green to red. Source: BIAS project.

Noise is created by engines, dredging and echo sounding

Water is very efficient at transmitting sound: sound waves travel almost five times faster in water than air. Even a small ship emits sounds that can carry five or even ten kilometres in calm weather. A large tanker may be detected using sound recording equipment up to a day before the ship itself appears!

Even louder noises than ship engines are produced from dredging activities. Worse still is underwater blasting, whose explosions can cause really strong noise peaks. Besides, underwater construction and warship sonar devices also produce a lot of powerful noise.

Kartta vedenalaisen melun esiintymisestä.
Spatial distribution of under-water noise in the Baltic Sea during 2003–2007 and its impact on cod, seals and harbour porpoise. In this case, noise level is based on noise generated by maritime traffic and wind farms. Impact level 1 means that animals can hear the noise. Al level 2 the noise disturbs animals' communication. At level 3 animals are avoiding noise, and at level 4 noise causes fysiologial changes. Source: HELCOM.

Noise is measured with a hydrophone

Underwater noise is measured with underwater microphones, also known as hydrophones. However, the way a marine animal experiences noise is affected by several factors other than the intensity of the noise itself. The frequency of the sound and whether it is continuous or random is also very important. Accordingly, different categories are used to assess underwater noise, depending on the impact that particular noise has on marine organisms.

Hydrofoni mittaa vedenalaista melua.
Under-water noise is measured with a hydrophone.

Within the European Union, criteria are being developed to determine whether or not a maritime area is in good condition in terms of underwater noise. Once these criteria have been established, the noise situation in the Finnish sea areas can also be assessed.