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Figure 1. Overview of the risk-based approach (World Organization of Dredging Associations, WODA 2013; after Boyd et al., 2008) 
Building on Boyd et al., (2008), the World Organization of Dredging 
Associations (WODA (2013)) emphasized the use of the risk-based 
approach in impact assessments of underwater noise. There is 
now a broad appreciation among regulators and scientists that 
the fundamental way to investigate the potential effects of noise 
is the risk-based approach developed in EMB Position Paper N° 13 
(McQueen et al., 2020; Popper et al., 2020). 
A second important framework is based on the “zones of noise 
influence” described in Richardson et al., (1995), which has been 
Further revised since 2008 by Hawkins & Popper (2016). The revised 
Framework suggests that underwater noise can have a variety of 
effects on marine organisms (Figure 2), which can be conceptualised 
as overlapping zones of influence relative to a sound source. This 
simplified model assumes that effects are related to the received 
sound level. The received sound level in turn is dependent on 
*he distance between a sound source and the marine organism 
potentially affected. Thus, different effects may extend to varying 
distances from the source. This basic model has been used in many 
studies and impact assessments (see Hawkins & Popper, 2016), but 
it also has limitations which are outlined in Chapter 3 in more detail. 
The key features of the model include: 
The zone of masking: the area where noise interferes 
with the detection of biologically relevant signals or cues 
used for communication and navigation, meaning that 
these sounds cannot be heard, or are less clear; 
The zone of behavioural response: the area within which 
a marine animal changes its behaviour in response to 
noise, e.g. by swimming away or diving deeper; 
The zone of impaired hearing: delineates the areas in 
which noise can lead to changes in hearing sensitivity. 
These changes can be temporary (temporary threshold 
shift, TTS) or permanent (permanent threshold shift, 
PTS). In most cases, TTS and PTS relate to changed 
sensitivity to certain frequencies. For an animal to detect 
a certain frequency, it will need to be louder. Generally, 
it does not mean that there has been a complete loss of 
hearing ability; 
The zone of physical! and/or physiological? effects: 
the zone where tissue damage and physiological effects 
other than those associated with hearing can occur. In 
extreme cases, the damage can lead to the death of the 
marine organism. It should be noted that death can also 
result, albeit indirectly, from any of the other effects 
listed above. 
°hysical effects of noise can include damage to internal tissue and/or to the auditory system 
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