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Federal Register / Vol. 86, No. 23 / Friday, February 5, 2021 / Notices lacking a single source or point Richardson et al., 1995. The sound level of a region is defined by the total acoustical energy being generated by known and unknown sources. These sources may include physical e.g., wind and waves, earthquakes, ice, atmospheric sound, biological e.g., sounds produced by marine mammals, fish, and invertebrates, and anthropogenic e.g., vessels, dredging, construction sound. A number of sources contribute to ambient sound, including wind and waves, which are a main source of naturally occurring ambient sound for frequencies between 200 Hz and 50 kHz ICES 1995. In general, ambient sound levels tend to increase with increasing wind speed and wave height. Precipitation can become an important component of total sound at frequencies above 500 Hz, and possibly down to 100 Hz during quiet times. Marine mammals can contribute significantly to ambient sound levels, as can some fish and snapping shrimp. The frequency band for biological contributions is from approximately 12
Hz to over 100 kHz. Sources of ambient sound related to human activity include transportation surface vessels, dredging and construction, oil and gas drilling and production, geophysical surveys, sonar, and explosions. Vessel noise typically dominates the total ambient sound for frequencies between 20 and 300 Hz. In general, the frequencies of anthropogenic sounds are below 1 kHz and, if higher frequency sound levels are created, they attenuate rapidly.
The sum of the various natural and anthropogenic sound sources that comprise ambient sound at any given location and time depends not only on the source levels as determined by current weather conditions and levels of biological and human activity but also on the ability of sound to propagate through the environment. In turn, sound propagation is dependent on the spatially and temporally varying properties of the water column and sea floor, and is frequency-dependent. As a result of the dependence on a large number of varying factors, ambient sound levels can be expected to vary widely over both coarse and fine spatial and temporal scales. Sound levels at a given frequency and location can vary by 1020 dB from day to day Richardson et al., 1995. The result is that, depending on the source type and its intensity, sound from the specified activity may be a negligible addition to the local environment or could form a distinctive signal that may affect marine mammals. Underwater ambient sound
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in the Atlantic Ocean southeast of Rhode Island is comprised of sounds produced by a number of natural and anthropogenic sources. Humangenerated sound is a significant contributor to the ambient acoustic environment in the project location.
Details of source types are described in the following text.
Sounds are often considered to fall into one of two general types: Impulsive and non-impulsive defined in the following. The distinction between these two sound types is important because they have differing potential to cause physical effects, particularly with regard to hearing e.g., Ward, 1997 in Southall et al., 2007. Please see Southall et al. 2007 for an in-depth discussion of these concepts. The distinction between these two sound types is not always obvious, as certain signals share properties of both impulsive and non-impulsive sounds. A
signal near a source could be categorized as impulsive, but due to propagation effects as it moves farther from the source, the signal duration becomes longer e.g., Greene and Richardson, 1988.
Impulsive sound sources e.g., airguns, explosions, gunshots, sonic booms, impact pile driving produce signals that are brief typically considered to be less than one second, broadband, atonal transients ANSI, 1986, 2005; Harris, 1998; NIOSH, 1998;
ISO, 2003 and occur either as isolated events or repeated in some succession.
Impulsive sounds are all characterized by a relatively rapid rise from ambient pressure to a maximal pressure value followed by a rapid decay period that may include a period of diminishing, oscillating maximal and minimal pressures, and generally have an increased capacity to induce physical injury as compared with sounds that lack these features.
Non-impulsive sounds can be tonal, narrowband, or broadband, brief or prolonged, and may be either continuous or intermittent ANSI, 1995;
NIOSH, 1998. Some of these nonimpulsive sounds can be transient signals of short duration but without the essential properties of pulses e.g., rapid rise time. Examples of non-impulsive sounds include those produced by vessels, aircraft, machinery operations such as drilling or dredging, vibratory pile driving, and active sonar systems.
The duration of such sounds, as received at a distance, can be greatly extended in a highly reverberant environment.
General background information on marine mammal hearing was provided previously see Description of Marine
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Mammals in the Area of the Specified Activities. Here, the potential effects of sound on marine mammals are discussed.
Potential Effects of Underwater SoundAnthropogenic sounds cover a broad range of frequencies and sound levels and can have a range of highly variable impacts on marine life, from none or minor to potentially severe responses, depending on received levels, duration of exposure, behavioral context, and various other factors. The potential effects of underwater sound from active acoustic sources can potentially result in one or more of the following: Temporary or permanent hearing impairment, non-auditory physical or physiological effects, behavioral disturbance, stress, and masking Richardson et al., 1995;
Gordon et al., 2003; Nowacek et al., 2007; Southall et al., 2007; Gotz et al., 2009. The degree of effect is intrinsically related to the signal characteristics, received level, distance from the source, and duration of the sound exposure. In general, sudden, high level sounds can cause hearing loss, as can longer exposures to lower level sounds. Temporary or permanent loss of hearing will occur almost exclusively for noise within an animals hearing range. We first describe specific manifestations of acoustic effects before providing discussion specific to pile driving.
Richardson et al. 1995 described zones of increasing intensity of effect that might be expected to occur, in relation to distance from a source and assuming that the signal is within an animals hearing range. First is the area within which the acoustic signal would be audible potentially perceived to the animal but not strong enough to elicit any overt behavioral or physiological response. The next zone corresponds with the area where the signal is audible to the animal and of sufficient intensity to elicit behavioral or physiological responsiveness. Third is a zone within which, for signals of high intensity, the received level is sufficient to potentially cause discomfort or tissue damage to auditory or other systems. Overlaying these zones to a certain extent is the area within which masking i.e., when a sound interferes with or masks the ability of an animal to detect a signal of interest that is above the absolute hearing threshold may occur; the masking zone may be highly variable in size.
We describe the more severe effects i.e., certain non-auditory physical or physiological effects only briefly as we do not expect that there is a reasonable likelihood that pile driving may result
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