German Federal Agency for Nature Conservation (BfN)

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Continuous sound


Container ship. Photo: P. Huebner (BfN)
Container ship. Photo: P. Huebner (BfN)

Shipping is the main source of the continuous noise that causes permanent disturbance in the marine environment. While the associated sound emissions differ in frequency and intensity from the usual occurrences of loud or sudden impulse noise, they pose a significant risk due to their ongoing nature, especially where acoustics-reliant marine mammals are concerned.

Shipping noise

The underwater noise produced by ships is largely generated by their propellers and engines. Noise levels can vary greatly depending on the ship’s type and its operating mode (e.g. speed and load). Broadband levels can range from 160 dB re 1 µPa for quiet research ships and reach up to 190 dB re 1 µPa or more for commercial vessels. Noise levels are highest at deep frequencies under about 300 Hz, with rare instances of dominant continuous sounds at higher frequencies. An additional high-frequency sound source on larger ships is the sonar, which emits very high and strongly directional downward sound pressures that are therefore only noticeable in the immediate vicinity of a ship.

Shipping has steadily increased in recent decades. In heavily shipped areas, background noise has doubled or tripled since the 1960s. The North Sea is the most heavily shipped ocean in the world, but there are also parts of the Baltic Sea which are especially at risk. Although pre-determined routes and waterways regulate shipping traffic, deep-frequency shipping noise can be perceived across vast distances, so that large areas are affected by the permanent noise from ships.

Background noise research

Hydrophone and POD placement. Chart: ITAW / BfN

Chart description: Hyrophone and POD placement

Hydrophone and POD placement

In a research project on underwater noise (EEZ Research, Cluster 7), noise levels in the North and Baltic Seas were studied. Underwater acoustics recorders were tested and fishery-proof anchoring systems were developed to protect the highly expensive equipment.


Background noise measurements in the Baltic Sea. Chart: DW ShipConsult / BfN

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Chart description: Background noise measurements in the Baltic Sea

Arithmetic means of background noise measurements, Baltic Sea

The background noise measurements at monitoring stations in protected areas in the Baltic Sea showed a reduction in noise levels in the following areas: Fehmarn Belt – Western Rønne Bank – Kadet Trench – Pommernbank and Adler Ground – Odra Bank. Frequencies of between 20 and 1,000 Hz were measured in the Fehmarn Belt, with 90 to 116 dB re 1 µPa (median). These were largely caused by the heavy shipping traffic in the area (e.g. the ferry operating between Puttgarden and Rødby). The noise map allows detailed assessment of noise pressure in various parts of the North and Baltic Seas.

POD and hydrophone placement


Transcription of the video showing the placing of a POD and a hydrophone

Placing the equipment is difficult. Underwater microphones used to record harbour porpoise sounds (POD, beige pipe) are installed on the same anchoring system. The devices must be well-secured and be readily visible to shipping traffic by means of a colourful buoy. The sensitive equipment is protected by a casing (yellow).

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