German Federal Agency for Nature Conservation (BfN)


Impulse sound

Use of a driving hammer. Photo K. Betke
Use of a driving hammer. Photo K. Betke

Impulse sound is a single occurrence of sound or a series of sounds of very short duration which contain a high number of high frequency sound waves. Impulse sound largely occurs as a bang-like noise, such as that heard in an explosion, the use of a driving hammer or an air gun used in seismic surveys.

Impulse sound in wind farm construction

The energy switch is bringing about a considerable increase in offshore wind energy use. In the coming years, numerous wind farms will be built with dozens or even hundreds of wind turbines being installed and put into operation. The installation of large-scale wind farms involves using a ‘hammer’ to drive steel posts measuring several metres in diameter some 50 metres into the sea bed in order to provide a stable foundation on which to erect the wind turbine. This work can take several hours or even days, meaning thousands of hammer strikes for each turbine foundation (e.g. alpha ventus: depending on the type of foundation, the number of hammer strikes varies between 11,383 and 25,208 per post). The underwater noise generated during such construction phases is tremendous and can propagate over many kilometres. Without technical minimisation measures, the sound pressure level produced by an impulse hammer is extremely high (e.g. 177dB re 1 µPa²s at a distance of 750 m for a hammer used at alpha ventus).

Impact on harbour porpoises

The acoustic threshold at which hearing in harbour porpoises is damaged was measured during hearing tests. For a single impulse sound, the threshold was found to be a sound energy of approximately 164 dB re 1 µPa2s or a sound pressure level of 200 dB re 1 µPap-p. Studies conducted at alpha ventus, the first offshore wind farm, showed a significant drop in the presence of harbour porpoises in a radius of up to 20 kilometres during the construction phase.

See also: Impacts on marine species

Noise thresholds

Legislation is already in place to regulate the occurrence of impulse noise. According to provisions laid down by the German Federal Maritime and Hydrographic Agency (BSH), during wind farm construction sound energy may not exceed 160 dB re 1 mPa²s and a maximum sound pressure level of 190 dB re 1 mPa in a radius of 750 metres. Technical measures are thus indispensable in the construction of wind farms and oil rigs in order to enforce this provision and minimise the propagation of impulse sound.

Impulse sound from seismic surveying

Seismic surveys in which air guns are used pose a huge threat to marine life. The underwater sound waves produced and the reflections they generate are used to map the sea bed. This allows data to be gathered on rock formations and structures, and on the occurrence of oil and gas. The growing search for these valuable resources means a growing use of air guns. The type of sound waves they produce can have harmful effects, especially where marine mammals are concerned, due to the high sound pressure levels and high-frequency components involved.