The underwater soundscape is complex. It is composed of natural sounds, as well as those produced by human activity. Noise from maritime traffic is predicted to more than double by 2030 compared to 2010 levels. This means that the proportion of human-caused underwater noise will become increasingly dominant among all underwater sounds.
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.
Some human-made underwater sounds are intentional, while others are created as a by-product of other activities. Intentional sounds are generated, for example, by sonar used for ship navigation, fishing, and underwater research such as seismic soundings. These surveys generate powerful sounds ranging from a few hertz to hundreds of kilohertz.
Unwanted by-products of underwater noise arise from maritime transport, offshore wind farms, underwater construction, and dredging. The noise produced from construction is temporary, while that from shipping is practically continuous. Noise from wind turbines – specifically the rotation of the rotor – is entirely continuous, thus raising the background level of underwater sound pressure.
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.
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, there are also sheltered places along the Finnish coast where the low frequencies are dominated by natural sounds.
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The shallowness of the Baltic Sea, natural obstacles (such as islands), variations in the seabed material, and physical-chemical conditions significantly affect the propagation of sounds. The surface area and water volume of the Baltic Sea are small compared to the oceans, and maritime traffic has continuously increased. For these reasons, the soundscape of the Baltic Sea differs greatly from that of the oceans
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.
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.
Sound travels faster in water than in air. 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.
