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Federal Register / Vol. 86, No. 122 / Tuesday, June 29, 2021 / Notices
omnidirectional sources, as is the case for sound produced by the pile driving activity considered here. The compressions and decompressions associated with sound waves are detected as changes in pressure by aquatic life and man-made sound receptors such as hydrophones.
Even in the absence of sound from the specified activity, the underwater environment is typically loud due to ambient sound, which is defined as environmental background sound levels 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 kilohertz kHz Mitson, 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
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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 decibels 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.
Sounds are often considered to fall into one of two general types: Pulsed and non-pulsed 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 pulsed and non-pulsed sounds. A signal near a source could be categorized as a pulse, but due to propagation effects as it moves farther from the source, the signal duration becomes longer e.g., Greene and Richardson, 1988.
Pulsed 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. Pulsed 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-pulsed sounds can be tonal, narrowband, or broadband, brief or prolonged, and may be either continuous or intermittent ANSI, 1995;
NIOSH, 1998. Some of these nonpulsed sounds can be transient signals of short duration but without the essential properties of pulses e.g., rapid rise time. Examples of non-pulsed 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.

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The impulsive sound generated by impact hammers is characterized by rapid rise times and high peak levels.
Vibratory hammers produce nonimpulsive, continuous noise at levels significantly lower than those produced by impact hammers. Rise time is slower, reducing the probability and severity of injury, and sound energy is distributed over a greater amount of time e.g., Nedwell and Edwards, 2002; Carlson et al., 2005. DTH is believed to produce sound with both impulsive and continuous characteristics e.g., Denes et al., 2016.
Acoustic Effects on Marine Mammals We previously provided general background information on marine mammal hearing see Description of Marine Mammals in the Area of Specified Activities.
Here, we discuss the potential effects of sound on marine mammals.
Anthropogenic 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., 2004; 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 and removal activities.
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
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