N° 7 2021 
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1,000 10,000 100 900 
Frequency (Hz) 
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Figure 4. Comparing sound sources, where the order shows the relative sound pressure levels (dB) of the noise sources and the colour code allows 
comparison of the levels between categories (orange being highest dB level to dark blue being lowest). The areas with hatching indicate the frequency 
range with most energy. MFAS = Medium Frequency Active Sonar. LFAS = Low Frequency Active Sonar. ADD = Acoustic Deterrent Device 
impulsive sound emitters of high relevance are explosions, airgun 
arrays and navy sonar. Pile-driving could be regionally important, 
for example in Northern Europe. The main and most important 
amitter of continuous sound is commercial shipping. These sources 
of anthropogenic sound are described in more detail below. For easy 
reading, we avoid in most cases any reference to specific units such 
as decibel levels and frequency. For frequency, ‘low’ refers to 1 Hz — 
500 Hz, ‘mid’ means >500 Hz — 10 kHz and ‘high’ refers to >10 kHz 
200 kHz (and more, see Figure 4; see Tasker et al., 2010). Concerning 
decibel levels, ‘high’ and ‘low’ are always referred to as relative terms, 
ie. in comparison to other sources. Due to the different physical 
properties of impulsive vs continuous sounds, these comparisons 
have to be viewed with caution. 
Anthropogenic impulsive sounds 
Explosions are caused by the use of explosives e.g. for the removal of 
structures from the seabed, in military operations or when clearing 
unexploded ordnance such as those deposited in the North Sea after 
World War Il. Across all the impulsive sound sources, explosions 
produce the highest peak levels of noise. This is why there is serious 
risk of direct injury to marine organisms, which depends on the 
weight of the charge and the depth of the detonation (Hildebrand, 
2005; OSPAR Commission, 2009b). 
Seismic surveys are conducted to map geological structures beneath 
the seabed, both for the oil and gas industries and research purposes, 
using arrays of airguns towed from seismic vessels. An airgun is a 
compressed-air-filled cylinder and when the air is suddenly released, 
it causes a transient high-pressure peak that can create a sound with 
very high sound pressure levels (>230 dB re 1 WPa m). The sound is 
reflected by the seabed and is detected, providing information about 
the sub-sea properties. The main energy content is at low frequencies 
{see Figure 4), with some mid- and high frequency content, although 
the extent of the high frequency component is still uncertain (OSPAR 
Commission, 2009b; Genesis Oil and Gas Consultants, 2011). Sub- 
bottom profilers are also used to survey the seabed and are highly 
directional sound sources. Sound levels can be relatively high 
and there is a wide variety of profilers operating across a range of 
frequencies (from low to high). Given their prevalent use in some 
areas, more measurements are needed to fully characterize these 
sources and assess their potential impact. 
Much research has been done since 2008 on pile-driving, where piles 
are driven into the seabed to provide foundation support for offshore 
structures. This activity is undertaken in the construction of offshore 
platforms, including those for wind farms. Pile-driving emits short 
pulses of intense sound with a relatively high SPL in the low frequency 
range but extending to higher frequencies as well. Propagated 
sound levels depend on a number of factors including the maximum 
energy rating of the hammers and the fact that the sound is not only 
transferred into the water column but also to some extent into the 
substrate (Bellmann et al, 2020; Jimenez-Arranz et al, 2020}.