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Technical Support Information to the CMS Family Guidelines on Environmental Impact Assessments for Marine Noise-Generating Activities

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Technical Support Information to the CMS Family Guidelines on Environmental Impact Assessments for Marine Noise-Generating Activities Technical Support Information to the CMS Family Guidelines on Environmental Impact Assessments for Marine Noise-generating Activities Module A. Sound in Water is Complex The full CMS Family Guidelines on Environmental Impact Assessments for Marine Noise-generating Activities and the stand-alone modules are online at: cms.int/guidelines/cms-family-guidelines-EIAs-marine-noise A. Sound in Water is Complex Geoff Prideaux Wild Migration The ocean environment is filled with activity in the region and under the conditions natural sound from animals and physical they plan to operate. processes. Species living in this environment Understanding the basic concepts that are adapted to these sounds. Over the past should be presented is important to assess if century many anthropogenic marine activities the Environmental Impact Assessment is have increased levels of noise. (André et al defensible and sufficient. 2010, Hildebrand 2009) These modern anthropogenic noises have the potential for physical, physiological and behavioural A.1. Basic concepts impacts on marine fauna–mammals, reptiles, fish and invertebrates. (Southall et al 2007) The study of acoustics is a specialized The propagation of sound in water is and technical field. Professional acousticians complex and requires many variables to be will consider many more complexities beyond carefully considered before it can be known if the scope of this paper. a noise-generating activity is appropriate or The basic concepts that decision-makers not. It is inappropriate to generalize sound may need to understand are outlined in a very transmission without fully investigating simplified form, specifically to be accessible to propagation. a lay-audience. Often, statements are made in Environmental Impact Assessments that a A.1.1. Elasticity noise-generating activity is ‘X’ distance from The speed of sound is not a fixed ‘Y’ species or habitat and therefore, will have numerical value. Sound wave speed varies no impact. In these cases distance is used as a widely and depends on the medium, or basic proxy for impact but is rarely backed material, it is transmitted through, such as with scientifically modelled information. solids, gas or liquids. Sound waves move (Wright et al 2013, Prideaux and Prideaux through a medium by transferring kinetic 2015) energy from one molecule to the next. (Lurton The behaviour of sound in the marine 2010) Each medium has its own elasticity (or environment is different from sound in air. The resistance to molecular deformity). This extent and way that sound travels elasticity factor affects the sound wave’s (propagation) is affected by many factors, movement significantly. Solid mediums, such including the frequency of the sound, water as metal, transmit sound waves extremely fast depth and density differences within the water because the solid molecules are tightly packed column that vary with temperature, salinity and together, providing only tiny spaces for pressure. (Clay and Medwin 1997, Etter 2013, vibration. Through this high-elasticity Lurton 2010, Wagstaff 1981) Seawater is medium, solid molecules act like small springs roughly 800–1,500 times denser than air and aiding the wave’s movement. The speed of sound travels around five times faster in this sound through aluminium, for example, is medium. (Lurton 2010) Consequently, a sound around 6,319ms-1. Gas, such as air, vibrates at arriving at an animal is subject to propagation a slower speed because of larger spaces conditions that are complex. (Calambokidis et between each molecule. This allows greater al 2002, Hildebrand 2009, Lurton 2010, deformation and results in lower elasticity. McCauley et al 2000) Sound waves moving through air at a To present a defensible Environmental temperature of 20°C will only travel around Impact Assessment for any noise-generating 342ms-1. Liquid molecules, such as seawater, activity proposal, proponents need to have bond together in a tighter formation compared expertly modelled the noise of the proposed with gas molecules. This results in less CMS Family Guidelines on EIA for Marine Noise Module A-7 deformation, creating a higher elasticity than the interactive effect of temperature and water gas. Sound waves moving through water at pressure (and, to a smaller extent, salinity). 22°C travel at around 1,484ms-1. This occurs because pressure in the ocean (Brekhovskikh and Lysanov 2006, Au and increases with depth, but temperature is more Hastings 2009, Ross 2013) Temperature also variable, generally falling rapidly in the main has an effect on molecules. Molecules move thermocline from the surface to around a faster under higher temperatures, transmitting thousand metres deep and then remaining sound waves more rapidly across the medium. almost unchanged from there to the ocean Conversely, decreasing temperatures cause the floor. Near the surface, the rapidly falling molecules to vibrate at a slower pace, temperature causes a decrease in sound speed hindering the sound wave’s movement. (or a negative sound speed gradient). With (Brekhovskikh and Lysanov 2006, Au and increasing depth, the increasing pressure Hastings 2009, Ross 2013) The temperature of causes an increase in sound speed (or a seawater at different depths is therefore of positive sound speed gradient). The depth importance to modelling. where the sound speed is at a minimum is called the sound channel axis. The speed A.1.2. Spherical Spreading, gradient above and below the sound channel axis acts like a lens, bending sound towards the Cylindrical Spreading and depth of minimum speed. The portion of sound Transmission Loss that remains within the sound channel The way sound propagates is also encounters no acoustic loss from reflection of important. Spherical spreading is simply sound the sea surface and sea floor. Because of this leaving a point source in an expanding low transmission loss, very long distances can spherical shape. As sound waves reach the sea be obtained from moderate acoustic power. surface and sea floor, they can no longer (Urick 1983, Brekhovskikh and Lysanov 2006, maintain their spherical shape and they begin Lurton 2010, Jensen et al 2011) to resemble the shape of an expanding cheese wheel. This is called cylindrical spreading. A.1.4. Decibels dB The transmission loss, or the decrease in the sound intensity levels, happens uniformly The decibel (dB), 1/10th of a Bel, is in all directions during spherical transmission. used to measure sound level. It is the unit that However, when sound is in a state of will be presented in documentation. cylindrical transmission, it cannot propagate The dB is a logarithmic unit used to uniformly. The sound is effectively contained describe a ratio. The ratio may be power, between the sea surface and the sea floor, sound pressure or intensity. while the radius still expands uniformly (the The logarithm of a number is the sides of the cheese wheel). The height is now exponent to which another fixed value, the fixed and so the sound intensity level decreases base, must be raised to produce that number. more slowly. (Urick 1983, Au and Hastings For example, the logarithm of 1,000 to base 10 2009, Lurton 2010, Jensen et al 2011) is 3, because 1,000 is 10 to the power 3: 1,000 = 10 × 10 × 10 = 103. In actuality, the seabed is rarely, if ever, y flat and parallel to the sea surface. These More generally, if x = b , then y is the natural variations add extra complexities to logarithm of x to base b, and is written y = modelling cylindrical spreading. However, logb(x), so log10 (1,000) = 3. (Au and these characteristics must be known to model Hastings 2009, Jensen et al 2011, Ross, 2013) spreading accurately, as should the water depth A common mistake is to assume that and the rise and fall of the seabed surrounding 10dB is half as loud as 20dB and a third of it. (Lurton 2010, Jensen et al 2011) 30dB. To disprove this false assumption, suppose there are two loudspeakers, the first A.1.3. Sound Fixing and Ranging playing a sound with power P1, and another Channels (SOFAR) playing a louder version of the same sound As well as spherical and cylindrical with power P2, but everything else (distance spreading, another variable can impact how far and frequency) remains the same. sound will be transmitted. This is usually The difference in decibels between the called a Sound Fixing and Ranging Channel two is defined as: (SOFAR) and is a horizontal layer of water in 10 log (P2/P1) dB where the log the ocean at which depth, the speed of sound is is to base 10. at its minimum. If the second produces twice as much The SOFAR channel is created through power as the first, the difference in dB is: 10 log (P2/P1) = 10 log 2 = 3 dB. Module A-8 CMS Family Guidelines on EIA for Marine Noise To continue the example, if the second According to NOAA's paper, Guidance has 10 times the power of the first, the for Assessing the Effects of Anthropogenic difference in dB is: Sound on Marine Mammal Hearing, (NOAA, 10 log (P2/P1) = 10 log 10 = 2016) sound exposure level works well for 10 dB. marine mammals but not well for other marine If the second has a million times the species (crustaceans, bivalves, cephalopods, power of the first, the difference in dB is: finned fish, etc) because non-mammal marine 10 log (P2/P1) = 10 log 1,000,000 species detect sound through particle motion = 60 dB. (the organism resonating in sympathy with the This example shows one feature of surrounding sound waves) rather than through decibel scales that is useful in discussing a tympanic mechanism as with marine sound: they can describe very big ratios using mammals. A more informed measurement manageable numbers. introduced to modelling is sound exposure level cumulative (SELcum) by which a time A.1.5.
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