Oceans of noise - Whale and Dolphin Conservation Society

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2.1.2. Measuring sound intensity Ideally, acousticians would be able to measure intensity directly but practically it is easier to measure and detect changes in pressure and then convert these to intensities. However, the use of pressure as a measurement unit presents the acoustician with two problems, the first is related to the range of pressure differences that the human auditory system can detect (10_Pa – 100,000,000_Pa) and the second is related to the way in which the human auditory system processes differences in pressure, i.e. how it judges relative loudness. The former is a practical problem where the magnitude of pressure differences detectable by the human ear can make calculations clumsy, and the latter is a subjective problem whereby the human auditory system processes pressure differences logarithmically and judges these relatively. It is for these reasons that Decibel Scale and dimensionless unit the Decibel (dB) were introduced, and the terms Sound Pressure Level (SPL) and Sound Intensity Level (SIL) were defined. ⎛ ⎞ ⎜ p SPL( dB) = 20log ⎟ ⎜ ⎟ ⎝ p dB ⋅ re. 1µ Pa ref ⎠ Where p is the measured pressure and pref is the reference pressure. In underwater acoustics pref water = 1µPa, in air pref air = 20µPa ⎛ ⎞ ⎜ I SIL( dB) = 10log ⎟ ⎜ ⎟ ⎝ I dB ⋅ re. 1µ Pa ref ⎠ Where I is the measured intensity and Iref is the reference intensity. Historically, the reference intensity (Iref) in air was the sound intensity barely audible to humans at 1000Hz, i.e. 1x10 -12 Wm -2 (1 pico Wm - 2 ). Note that if both SPL and SIL are quoted in dB they are equivalent, i.e. ⎛ I ⎞ ⎛ p ⎞ SIL( dB) = 10log⎜ ⎟ = 20log⎜ ⎟ = SPL( dB) ⎜ I ⎟ ⎜ ref p ⎟ ⎝ ⎠ ⎝ ref ⎠ From the definitions of SPL and SIL above it should be clear that decibel scale is a log ratio scale of intensity that has dimensionless units – the decibel. It should also be clear that decibel scale overcomes the problems discussed above, (1) ratios are a convenient way of dealing with the large range of intensities (pressures) that the human ear can detect, (2) logarithms simplify computations since multiplication and division are reduced to addition and subtraction, and (3) the logarithmic scale approximates the mechanism by which the human auditory system judges relative loudness. It is important to note the dB scale is relative and that dB values are only meaningful if a reference level is included. The reference levels for SPL and SIL are equivalent but are reported in different units. In underwater acoustics a reference pressure of 1_Pa is commonly used, while the reference pressure in air is 20_Pa (this approximates the human hearing threshold at 1000Hz). The reference intensity in water can be calculated from reference pressure by: I ref ⎛ = ⎜ ⎝ ( ρ p c medium 2 ref medium ⎞ ⎟ ) ⎟ ⎠ 19
For water, pref = 1µPa rms, _ = 1x10 3 kgm -3 , c =1.5x10 3 ms -1 . I ref ⎛ ⎞ = ⎜ 3 ( 1 10 ) ( 1. 5 10 ) ⎟ Wm ⎝ × × × ⎠ 2 1 −19 −2 ⎟ = 6. 7 × 10 3 In the scientific literature you will often see the terms Source and Received Levels. In underwater acoustics, source level usually represents the sound level at a distance of one metre from the source, referenced to 1_Pa. On quoting a source level, the distance from the source at which the reference level was measured must also be cited; typically the units of SIL /SPL are dB relative to the reference intensity at 1 metre (e.g. 20 dB re 1µPa @ 1m). In practice, one can rarely measure source level at the standard 1m reference, so that source levels are usually estimated by measuring SPL at some known range from the source (assumed to be a single point), and then predicting and subtracting the attenuation effects from the measured value to estimate the level at the reference range. The received level is the sound level at the listener's actual position, which is usually considerably more distant that the reference source level. 2.1.3. Comparison of sound intensities measured in air and water For measurements made in air the Sound Intensity Level is defined as: SIL Air ⎛ ( dB) = 20log⎜ ⎜ ⎝ p p ref . Air ⎞ ⎟ ⎛ p ⎞ = 20log⎜ ⎟ ⎟ ⎠ ⎝ 20µ Pa ⎠ For measurements made in water the Sound Intensity Level is defined as SIL Water ⎛ ( dB) = 20log⎜ ⎜ ⎝ p p ref . Water 20 ⎞ ⎟ ⎛ p ⎞ = 20log⎜ ⎟ ⎟ ⎠ ⎝1µ Pa ⎠ It should be clear that direct comparisons of sound intensity levels measured in air and water cannot be made, unless levels are adjusted to take into account: (1) the differences in acoustic impedance between air and water (_cwater =1.5x10 6 and _cair =4.15x10 2 ) and (2) the differences in reference pressures used for air and water (pref water = 1µPa and pref air = 20µPa). However, although the physics behind these adjustments is correct it may not reflect the complexities of marine mammal hearing. Bearing this in mind and that direct comparisons of hearing in terrestrial mammals and marine mammals is both controversial and flawed, adjusting the levels to make such comparisons is a two stage process. Stage 1. Adjusting for differences in pressure reference levels ⎛ p ⎞ ⎛ 20 ⎞ 20 ⋅ log ⎜ = 20 ⋅ log⎜ ⎟ = + p ⎟ ⎝ water ⎠ ⎝1µ Pa ⎠ Air 26 dB
Page 1 and 2: Oceans of noise A WDCS Science repo
Page 3 and 4: Contents Author biographies 4 Prefa
Page 5 and 6: Author biographies Sarah Dolman Sar
Page 7 and 8: Preface Over the course of the last
Page 9 and 10: some thoughts on the mitigation and
Page 11 and 12: 1. Introduction Lindy Weilgart The
Page 13 and 14: 2. The physics of underwater sound
Page 15 and 16: Box 1. Characteristics of sound / l
Page 17 and 18: Box 2. The effects of temperature,
Page 19: Intensity The intensity of a sound
Page 23 and 24: 2.2. Types of sound source Sounds c
Page 25 and 26: Cylindrical spreading loss Cylindri
Page 27 and 28: Rain - - - - - - - - - 100-500 - Bi
Page 29 and 30: Energy source (Air gun array) 28 Se
Page 31 and 32: driving and pipe-laying (installati
Page 33 and 34: produced as the bubble collapses an
Page 35 and 36: and broadband noise sources decayed
Page 37 and 38: cetaceans to high levels of acousti
Page 39 and 40: Navigation (transponders) 38 7-60 1
Page 41 and 42: SONAR TYPE FREQUENCY RANGE AV. SOUR
Page 43 and 44: them from approaching fish farm cag
Page 45 and 46: 3.11. Research 3.11.1. Controlled E
Page 47 and 48: frequencies). These sound channels
Page 49 and 50: 4.3.6. Danger avoidance Many verteb
Page 51 and 52: Table 4.1. Summary of sound frequen
Page 53 and 54: Table 4.2. Summary of sound frequen
Page 55 and 56: 5. Noise as a problem for cetaceans
Page 57 and 58: Increased vulnerability to disease
Page 59 and 60: levels exceed the ear’s tolerance
Page 61 and 62: 5.6 A New Concern: Is noise causing
Page 63 and 64: stranding incident suggests that th
Page 65 and 66: The only cetaceans listed in Annex
Page 67 and 68: 6.4 The US Marine Mammal Protection
Page 69 and 70: 7. Solutions - mitigation and manag
Page 71 and 72:
2. More targeted studies into each
Page 73 and 74:
Noise pollution also needs to be co
Page 75 and 76:
ange. Therefore prey must be closer
Page 77 and 78:
identification of images collected.
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Pierson et al. (1998) indicated a m
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Baur, D.C., M.J. Bean, M.L. Gosline
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Cummings, W.C., Thompson, P.O. and
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Gedamke, J., Costa, D., Dunstan, A.
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Lagadere, J.P. 1982. Effects of noi
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National Research Council. 2000. Ma
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Sayigh, L.S., Tyack, P.L., Wells, R
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Todd, S., Lien, J. and Verhulst, A.
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December 1989, Pacific Grove, Calif
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consider just the definition of pol
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This evidence therefore suggests th
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jurisdiction (“the UNEP Conclusio
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Designation of Particularly Sensiti
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inter alia, internal waters, the te
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ief framework provision on pollutio
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definitions are located in the anne
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Questions arise as to the identity
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opportunity for, or a hurdle to, ad
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145 LOSC in respect of polymetallic
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Seabed Authority) has been establis
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One possibility is the adoption by
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Identification and Designation of P
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Provisions on Ships’ Routeing to
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4. Conclusion This paper has analys
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Instruments adopted under Bonn Conv
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Appendix B - Treaties underlying re
Page 131 and 132:
Appendix C - Selected provisions of
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abate, combat pollution of the Medi
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Annex 2. Guidelines for commercial
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1. Any commercial cetacean-watching
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Annex 3. Documented examples of cet
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Location: Sarasota, Florida, USA Pr
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Beluga whales, Delphinapterus leuca
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These results may indicate habituat
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Location: Northwest Atlantic Northe
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The dolphins started to display eva
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Boats that approached within 30m or
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Location: Hawaii On rare occasions
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Location: Beaufort Sea Aversion beh
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Location: Mexico Gray whale vocal b
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If boats changed their speed or cou
Page 161 and 162:
Corkeron, P.J. 1990. Aspects of the
Page 163 and 164:
Kibal’chich, A.A., Dzhamanov, G.A
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ates of fin whales, Balaenoptera ph
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Annex 4. A note of the Recommendati
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4. the inclusion of anthropogenic n
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