cMB FUTURE SCIENCE BRIEF 
Sound emitted from active military sonars (Low- and Medium 
Frequency Active Sonar, LFAS and MFAS) depends on the 
operational purpose, which determines the frequency range and 
t*he source strength. Most active military sonar used for submarine 
detection operate in the low to medium frequency range, however 
possible higher frequency sound content of the sonar is often not 
specified and may still be significant (OSPAR Commission, 2009b). 
High-frequency sonars, used for civilian purposes such as fisheries, 
surveying and research, generally produce signals directed towards 
the seabed. This category includes Single-beam and Multibeam 
Echosounders and Side Scan Sonars. 
Acoustic Deterrent Devices (ADD) or pingers are used to deter marine 
mammals away from human activities such as fishing vessels and 
aquaculture farms. They operate in the medium-high frequency 
range with source levels reaching from relatively low to high levels. 
Acoustically, there is significant variation in pingers. Concerning 
their acoustical characteristics, ADDs are not easily categorized 
as impulsive or continuous. They are considered impulsive if they 
operate with a low duty cycle, Le. where the duration for which 
the sound is active is short compared to the duration for which 
:he sound is inactive. If they operate continuousliy, or with a high 
duty cycle (i.e. the duration for which the sound is active is long 
zompared to the duration for which the sound is inactive), they are 
not considered impulsive (see Dekeling et al., 2014). 
Anthropogenic continuous sounds 
Sounds from shipping have been researched intensively in the last 
decade. The sounds from ships cover a wide range of frequencies 
From low to high. There is also significant variation in emitted sound 
levels (OSPAR Commission, 2009a; Erbe et al, 2019). The exact 
characteristics of the sound emissions depend on variables such as 
vessel type, size and operational mode. In general, the larger the ship 
is, the more intense the sound levels and the lower the frequency. A 
notable exception are modern military vessels which use technology 
to suppress the radiated noise. 
Large commercial vessels produce relatively intense and 
„redominately low frequency sounds, with the most energy 
concentrated below 100 Hz (OSPAR Commission, 2009a). Large 
vessels dominate low-frequency background noise on a global scale 
and, due to the steady increase in shipping over the past decades 
(estimated to continue at 4% per year globally between 2018-2023), 
oressure on the marine environment will potentially also increase 
(Erbe et al., 2019). Sound from recreational craft, while relatively less 
powerful than commercial vessels, can vary significantly between 
vessels (Erbe et al., 2016) and is concentrated in coastal areas 
Offshore drilling and especially marine dredging produce sound 
levels in the range of small — medium sized vessels, which are below 
the emissions of large commercial vessels. They can contribute 
'ocally to the soundscape (CEDA, 2011; WODA, 2013; Prideaux, 
2017). Due to more dedicated measurements in the past decade, 
the sound output of operating offshore wind turbines is better 
understood. Source levels are low compared to other continuous 
sound sources discussed here. However, in low ambient noise 
conditions, noise from individual turbines can overlap and lead 
to higher noise levels at least within the wind farm area and its 
immediate vicinity. It should thus not be overlooked in impact 
assessments. This is especially important when considering the 
development of larger and potentially more noisy turbines in the 
future (Tougaard et al., 2020; Stöber & Thomsen, 2021). Wave and 
tidal turbines have only been investigated in the past decade and 
studies show moderate sound levels with maximum energy below 
400 Hz (Thomsen et al., 2015). 
Dredaer off Ostend beach in Belgium