N 7 2021 
A Advances in knowledge of 
anthropogenic underwater 
sound ın the Ocean 
Progress has been made in the past decade on the 
characterization of both impulsive and continuous sound 
sources such as airguns, pile-driving, shipping, and dredging; 
Some sources have still not been sufficiently 
characterized, which is essential for impact assessments; 
A substantial increase in activities producing impulsive 
sounds has been observed over the last decade in European 
waters due to offshore construction, e.g. pile-driving; 
in this chapter, we define some basic terms such as ‘sound’ and 
'noise’. We then provide a summary of knowledge built over the 
past decade on the different anthropogenic sound sources that 
contribute to the soundscape today. Further, we highlight the 
knowledge gained on spatial distribution of noise in Europe (e.g. 
Recent projects have provided new information on 
ambient noise levels in parts of Europe, but there are still 
arge gaps in some regions such as the Black Sea and the 
Mediterranean Sea; and 
The on-going growth in the number of commercial ships 
might, without countermeasures, substantially increase 
ambient noise levels. 
noise hotspots), followed by what we know about the latest trends 
in ambient sound. To make it more easily accessible we do not 
provide formulas and/or (in most cases) numbers. Detailed reviews 
on the acoustics of underwater sound already exist elsewhere (e.g. 
OSPAR Commission, 2009b; Ainslie, 2010). 
2.1 Background on sound 
Sound or noise? 
Boyd et al., (2008) did not define the terms ‘sound’ or ‘noise’ and consequently, the terms were used interchangeably, which is also 
often the case in other work and can lead to confusion. 
Within this Future Science Brief, we apply the definitions used in both US and EU regulatory contexts. Accordingly, the term ‘sound’ 
is used to refer to the acoustic energy radiated from a vibrating object, with no reference to its function or potential effect. ‘Noise’ is 
sound that is not a useful signal or cue, ie. it has no adaptive value or biological meaning for the receiver, and may either be neutral or 
may have adverse effects (Van der Graaf et al., 2012; Southhall, 2018; see also ISO, 2017). The term ‘soundscape’ is defined as ‘ambient 
sound in terms of its spatial, temporal, and frequency attributes, and the types of sources contributing to the sound field’ (ISO, 2017). 
What is sound? 
Sound is a variation in pressure which propagates through a compressible medium (e.g. water). Sound pressure levels (SPL) are referred 
to in decibels (dB) with 1 uWPa (one microPascal) as a reference unit. A decibel is a unit for measuring the relative loudness of sounds. 
SPLs are useful when analysing sounds of relatively long duration, such as the noise of a ship. The received levels from shorter duration 
sound sources such as pile-driving strikes, are often denoted in peak or peak-to peak SPLs, and Sound Exposure Level (SEL, 1 WPa2.s), 
which is a measure of the energy of the emitted sound (ISO, 2017). 
Besides decibels, another important metric is frequency, which is the number of sound wave cycles per second. It is given in hertz (Hz): 
1 hertz = one cycle per second, 1 kHz = 1000 cycles per second. Different marine animals use and are affected by different frequencies 
(see Chapter 3). 
With the scope of this Future Science Brief reaching beyond marine mammals, it is important to understand that, in addition to 
pressure, sound also manifests itself in a particle motion component. Particle motion includes the movements of the particles in the 
sound wave (as acceleration, velocity and displacement). Acoustic waves can propagate within the substrate (e.g. the seabed) or at 
che substrate water interface, generating high levels of particle motion. Fishes and invertebrates are principally sensitive to particle 
motion. Marine mammals are sensitive to sound pressure, and some fishes are sensitive to both sound pressure and particle motion 
(Fay & Wilber, 1989: Nummela, 2009}.No full text available for this image