The underwater soundscape consists of natural and man-made sounds

The underwater world is not silent. On the contrary, the underwater soundscape is complex. It is composed of natural sounds, as well as those produced by human activity. Man-made noise at sea has only recently been recognised as a problem.

In the Baltic Sea, thunderstorms and ice movements produce natural, short-lived sounds. Other sources of natural sound include surf, breaking waves, and rain. All of these sound sources produce noise in the 50Hz to 5kHz frequency range, in other words, within the range of human hearing.

Both fish and marine mammals are also capable of producing sounds in an even wider frequency range. Some sounds range from 1 Hz up to 100 kHz. Some of these sounds are inaudible to humans. The duration of the sounds generated by animals ranges from a millionth of a second to tens of seconds.

The propeller of a ship, photographed from below.
Marine traffic causes under-water noise.

Man produces underwater sound intentionally or unconsciously

Some human-made underwater sounds are intentional, while others are created as a by-product of other activities. Intentional sounds are generated during seismic soundings and other acoustic measurements.

Underwater noise is generated as an unwanted by-product by, among other things, maritime transport, civil engineering, as well as the construction and operation of wind farms.

The noise produced from construction is temporary, while that from shipping and wind turbines is practically continuous.

The noise from ship propellers is most pronounced near shipping lanes and along the coast. The noise from a single ship exceeds low-frequency background noises at distances of up to 5-10 km away. The momentary noise produced by a ship is most pronounced when it passes a short distance away.

Besides commercial maritime traffic, there is also busy recreational boating, particularly in the coastal areas of Finland. When small boats and jet skis travel close to the shoreline, they can disturb other people. Moreover, such recreational craft moving in proximity to fish spawning areas and bird nesting islets can cause major disturbance to nature.

However, this kind of noise disturbance to marine animals can be easily avoided by taking a better account of nature.

However, there are also sheltered places along the Finnish coast where the low frequencies are dominated by natural sounds.

An infographic about underwater noise.
Man-made noise at sea has only recently been recognised as a problem. Marine animals are sensitive to noises other than natural sounds, e.g. waves or wind. Man-made noises, e.g. ships, wind farms, and dredging, influence animal behaviour, habitat selection and communication, and causes stress. Underwater noise generated by humans is concentrated near shipping lanes. When at sea, take marine nature, as well as the activities of other sea goers into consideration. Do not drive motorboats or other watercraft near the shoreline. Do not interfere with seabird nesting areas or fish spawning grounds. Source: SYKE. Illustration: Kaskas Media Oy.

The underwater soundscape of the Baltic Sea differs from that of the oceans

The shallowness of the Baltic Sea acts to filter out low-frequency sounds. As a result, the soundscape of the Baltic Sea is very different from that of the oceans. Besides, the northern location and relatively small size of the Baltic Sea are also important.

The presence of vertical zones in the water column in which the salinity (halocline) or temperature (thermocline) changes rapidly with depth, create physical barriers in the water layers which dampen the vertical propagation of sounds. Conversely, the horizontal propagation of sound is amplified in the spaces between these clines.

In winter, the sounds of rain and waves are removed by ice cover. However, it effectively reflects sounds from beneath the sea ice and may even amplify the noise generated by ship traffic.

Along the Baltic Sea coastline, sound propagates differently compared to out on the open sea. The more labyrinthine the coast, the more it has an impact on the local soundscape.

The composition of the seabed also affects the underwater soundscape. Soft seafloor sediments reduce the reflection of sounds. Rocky seafloors, on the other hand, are highly effective in reflecting sounds. The variability of sea bottom quality is particularly high in the archipelagic coastal areas of Finland.

In general, the waters around Finland are small in area. As a result, a loud underwater sound could easily travel across the Gulf of Finland and the Gulf of Bothnia, while the same sound above water could not.