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Ocean Noise and Marine Mammals (2003) / Chapter Skim
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2. Sources of Sound in the Ocean and Long-Term Trends in Ocean Noise
Pages 27-82

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From page 27...
... , source spectral density level (units of dB re 1 ,uPa2 per Hz at 1 m) , and time-integrated source pressure amplitude squared for use with transient signals (units of dB re 1 ,uPa2 at 1 m)
From page 28...
... These anthropogenic sound sources contribute to ocean noise over the complete 1-Hz to 200-kHz band of interest in this report. In the lowest bands, 1-10 Hz, the contributors are ship propellers, explosives, seismic sources, and aircraft sonic booms.
From page 29...
... Across most of the remainder of this band, the primary sources are bubbles that are oscillating, both individually and collectively in a cloud, in the water column. Several good references on natural physical sources of ocean noise and the properties of the ambient noise field are available (e.g., Urick, 1984; Zakarauskas, 1986; Ross, 1976; Kerman, 1988, 1993; Buckingham and Potter, 1995; Leighton, 1997; Deane, 1999~.
From page 30...
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From page 34...
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From page 37...
... 37 no ~ ~ ~ o no ho do o v o ° o V ~ s°~' o s°~' pa En pa Us Us o o o o U
From page 38...
... The parallel nature of the "curves" for various sea states signifies that the noise level increases with increasing sea state by the same amount at all frequencies. Although developed more than a half-century ago, the Knudsen curves continue to be widely used to predict natural ocean noise levels at frequencies from 1 to 100 kHz.
From page 39...
... Movement of sediment by current flow across the ocean bottom can be a significant source of ambient noise at frequencies from 1 kHz to greater than 200 kHz (Thorpe, 1986~. Effects of Ice An ice cover at the ocean surface radically alters the ocean noise field.
From page 40...
... The underwater sound pulses that are emitted typically are a few milliseconds in duration and so have broad spectral content from 100 Hz to 1 kHz. Though sound is created within moving ice packs from the relative motion of adjacent ice blocks, much
From page 41...
... Biological Sources of Underwater Sound Biological contributions to the underwater sound field are discussed in this section. This discussion is presented not only to help satisfy the committee's task of evaluating "the human and natural contributions to marine ambient noise" but also to provide an idea of how these sounds are similar to, or different from, natural sounds from physical sources and noise from anthropogenic sources.
From page 42...
... Several Type II species produce signals with peak energy at higher frequencies, for example, Tursiops truncates, when tested in a high-noise environment (Au, 1993~. Type I echolocators are inshore and riverine dolphins that operate in acoustically complex waters.
From page 43...
... Odontocete whistles have much lower source levels than echolocation clicks, ranging from less than 110 dB re 1 Spa at 1 m for spinner dolphins (Stenella longirostris) (Watkins and Schevill, 1974)
From page 44...
... Marine Mammal Contributions to Ocean Noise Along the U.S. West Coast, the Navy's sound surveillance system (SOSUS)
From page 45...
... Other marine mammals such as the eared seals, manatees, dugongs, and sea otters have relatively low-level underwater vocalizations and add little to the acoustic scene. Except for the vocalizations of baleen whales, which can be detected for hundreds of kilometers, the contributions of marine mammals to the ocean sound ambient are localized in space.
From page 46...
... Fish produce sounds by a variety of mechanisms. Many of these involve striking two bony structures against one another.
From page 47...
... and may be used by males to attract females to spawning sites (Wine, 1964; Holt, 2002~. While less recognized as sound producers than fish or marine mammals, a number of marine invertebrates produce sound.
From page 48...
... Recent work extending the measurements to frequencies above the sonic band (Cato and Bell, 1992; Cato, 1992; Au and Banks, 1998) showed that snapping shrimp sounds contain energy up to 200 kHz and that individual snaps can have peak-to-peak source levels as great as 189 dB re 1 Spa at 1 m.
From page 49...
... , propulsion machinery such as diesel engines, gears, and major auxiliaries such as diesel generators (Ross, 1976~. Particular vessels produce unique noise source levels with frequency, known as acoustic signatures.
From page 50...
... The contribution from recreational boating to the underwater noise field has not been quantified. Much of this boating activity occurs in shallow coastal waters, environments that are inhabited by many marine mammal species.
From page 51...
... However, high-quality shipping noise modeling probably requires only representative source spectra for the different classes of ships (Figure 2-2~. Source spectral densities for the five classes of surface ships are used in the ANDES (Ambient Noise Directionality Estimation System)
From page 53...
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From page 54...
... 1 '000 FIGURE 2-2 (a) Modeled surface ship source spectral densities for the five classes of ships used in the RANDI ambient noise model.
From page 55...
... of the acoustic source used in Wales and Heitmeyer to derive the source spectral densities from the measured received spectral densities is a vertical line of incoherent point sources, rather than a single-point source, in order to more accurately account for the character of the acoustic source region about the ship propeller. An interesting observation is that the decrease in ship spectral density levels with frequency above 400 Hz has the 5-6 dB/octave dependence as seen in the Knudsen curves for wind-generated noise (see Plate 1~.
From page 56...
... 56 Cal ._ C~ o Cal Cal Cal Cal Cal I o o Cal ._ ._ C~ ~4 ˘ sit ~4 ._ U
From page 57...
... Results from a recent study of source signatures from outboard, inboard-outboard, and inboard powerboats shows that source levels for the largest amplitude narrowband tones typically range between 150 and 165 dB re 1,uPa at 1 m and the broadband radiated energy, which is engine RPM dependent, has maximum source spectral density levels in the 350-1,200-Hz band of 145-150 dB 1,uPa2/Hz (Bartlett and Wilson, 2002~. Additional examples of individual ship signatures in these classes can be found in Richardson et al.
From page 58...
... Marine seismic reflection profiling currently relies on the use of arrays of air-guns. These arrays have replaced the explosive charges that previously were used as sources.2 Air-guns release a volume of air under high pressure, creating a sound pressure wave that is capable of penetrating the 1The numbers provided by one database group may differ from those provided by another group because of the use of different categories of activity (e.g., under contract versus actually working)
From page 59...
... (0.041 m3) array having the same number of guns.3 The acoustic pressure signal of air-gun arrays is focused vertically, being 12-15 dB stronger or more in the vertical direction for some arrays in use today.
From page 60...
... The peak amplitude of an air-gun array is also a function of the frequency (Figure 2-4~. The peak pressure levels emitted from commonly used seismic industry air-gun arrays are in the 5-300 Hz range.
From page 61...
... This plot shows pressure levels as a function of frequency for a signal generated by a 4,550-cu.-in. air-gun array.
From page 62...
... . The sound pressure levels created by the different drilling methods are not well known.
From page 63...
... Research is needed to make accurate measurements of the sound pressure levels generated by various drilling techniques. The compilation of drilling activity numbers over time with a conventional geographic breakdown as illustrated by Table 2-4 may not be particularly useful to describe the drilling ensonification of the oceans.
From page 64...
... Sound pressure level measurements are needed to conclude how these changes in oil industry techniques affect ocean noise. Offshore structure emplacement will create some localized unintentional noise for relatively brief periods of time.
From page 65...
... systems use acoustic energy to characterize physical properties and locate objects beneath the ocean surface. The wide range of applications requires systems that vary greatly in engineering specifications and deployment strategies.
From page 66...
... In addition, the U.S. Navy reports that the hullmounted AN/SQS-53C tactical sonars can generate pulses in the 1-5 kHz band and have been operated at source levels of 235 dB re 1,uPa at 1 m and that the AN/SQS-56 sonars generate pulses in the 5-10 kHz band and have operated at 223 dB re 1 Spa at 1 m source levels (Evans and England, 2001~.
From page 67...
... are designed to be sufficiently high level to exclude marine mammals, usually pinnipeds, from areas such as aquaculture sites, sections of river systems where migrating salmonids are vulnerable, and some fishing equipment. Whereas the lowpower "pingers" have maximum source levels typically between 130 and 150 dB re 1 mPa at 1 m, the high-power AHDs have source levels in the 190-200 dB re 1 Spa at 1 m range.
From page 68...
... No porpoises were found within 400 m of the device, and the sighting rates between 2.5 and 3.5 km were 10 percent of the control rates. 1 1 Underwater Sound Sources in Basic Ocean Acoustics Research This section summarizes the characteristics of underwater sound sources used in basic ocean acoustic and acoustical oceanography research programs in the United States.
From page 69...
... It is a moving coil-type device designed to transmit signals in the 30-900 Hz band with a maximum source level of approximately 170 dB re 1 Spa at 1 m (Ivey, 1991~. A wide variety of waveforms are transmitted by these sonars over a wide range of frequency bands, source levels, and duty cycles because of the large number of research questions addressed in these programs.
From page 70...
... All federal and relevant state environmental laws must be followed, including the Marine Mammal Protection Act, the Endangered Species Act, and the Magnuson-Stevens Fisheries Conservation and Management Act, as well as other laws, regulations, and executive orders. Future Trends in Sonar Use Commercial sonars will continue to proliferate in the oceans.
From page 71...
... Explosive Sources Explosive sources are broacibanci with very high zero-to-peak source levels. In fact, the highest zero-to-peak pressure levels from man-macle sources probably were created by the nuclear tests conclucteci in the ocean, in the atmosphere over the ocean, and on oceanic islands in the past half century.
From page 72...
... charge detonated at a depth of 20 fathoms (36.6 m) and are equal to the actual source level at each frequency for a signal of 1-see duration.
From page 73...
... This broad range of activities produces a range of source levels and acoustic patterns: · pile driving (impulsive, very high amplitudes) , · power plants (very strong 60-cycle and harmonics)
From page 74...
... What parameters, other than direct noise measurements, might be related to the overall sound levels produced by the myriad of sources described? What, if any, modeling capabilities exist to predict ocean noise levels and other noise characteristics in the decades to come?
From page 75...
... with the opportunity to determine the "usual lowest ocean noise" level from an extensive suite of measurements (Plate 5~. There are several striking features that appear in these data.
From page 76...
... Long-term trends in ambient noise levels can be observed at low frequencies (unspecified, but presumably below 200 Hz) in the East Pacific and East and West Atlantic Oceans (Figure 2-7~.
From page 77...
... and Wenz (1969) to obtain an overall 16-dB increase in low-frequency noise level from shipping during the period 1950-2000.
From page 78...
... Nevertheless, it is possible to make some general well-founded comments regarding ship-radiated noise and shipping traffic and their possible implications for long-term ocean noise levels. First, there is no doubt that ships generate noise, principally by propeller cavitation and machinery.
From page 79...
... Duringl990-1998 growth averaged3.2 percent per annum." The 50 percent trade increase is comparable to the 38 percent increase in gross tonnage during the same period. Applying Equation 2-2 gives a result, 20 ~log(1.38)
From page 80...
... Only further study can elucidate whether the similarity in these figures is purely coincidental or scientifically meaningful. Long-Term Trends in Other Sources of Ocean Noise No long-term systematic ocean acoustics data set exists to permit a scientific assessment of trends of noise in the ocean.
From page 81...
... However, given that exploration methods have been changing, for example, large explosive sources have been replaced by air-guns, which have evolved into air-gun arrays that focus the radiated acoustic energy in the vertical direction, and that undiscovered oil and gas reserves probably are deeper within the earth and/or are to be found in deeper waters, the overall impact of changes on the ocean sound field is difficult to evaluate without a combined ocean noise measurement and numerical modeling effort. In contrast to focusing of acoustic energy in the vertical by present-day geophysical exploration sources, the newly developing low-frequency navy sonars radiate acoustic energy preferentially in the horizontal direction.
From page 82...
... since the occurrence of war and resulting contributions to the ocean noise field are highly unpredictable and extremely episodic. Long-term trends in the use of smaller explosive devices also may be taking place.


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