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Federal Register / Vol. 86, No. 106 / Friday, June 4, 2021 / Notices
may be accounted for in the summation of pressure levels Hastings and Popper, 2005. This measurement is often used in the context of discussing behavioral effects, in part because behavioral effects, which often result from auditory cues, may be better expressed through averaged units than by peak pressures.
Sound exposure level SEL;
represented as dB re 1 mPa2-s represents the total energy in a stated frequency band over a stated time interval or event and considers both intensity and duration of exposure. The per-pulse SEL is calculated over the time window containing the entire pulse i.e., 100 percent of the acoustic energy. SEL is a cumulative metric; it can be accumulated over a single pulse, or calculated over periods containing multiple pulses. Cumulative SEL
represents the total energy accumulated by a receiver over a defined time window or during an event. Peak sound pressure also referred to as zero-to-peak sound pressure or 0-pk is the maximum instantaneous sound pressure measurable in the water at a specified distance from the source and is represented in the same units as the rms sound pressure.
When underwater objects vibrate or activity occurs, sound-pressure waves are created. These waves alternately compress and decompress the water as the sound wave travels. Underwater sound waves radiate in a manner similar to ripples on the surface of a pond and may be either directed in a beam or beams or may radiate in all directions 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
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200 hertz 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 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. Details of source types are described in the following text.
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,
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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.
Airgun arrays produce pulsed signals with energy in a frequency range from about 102,000 Hz, with most energy radiated at frequencies below 200 Hz.
The amplitude of the acoustic wave emitted from the source is equal in all directions i.e., omnidirectional, but airgun arrays do possess some directionality due to different phase delays between guns in different directions. Airgun arrays are typically tuned to maximize functionality for data acquisition purposes, meaning that sound transmitted in horizontal directions and at higher frequencies is minimized to the extent possible.
Summary on Specific Potential Effects of Acoustic Sound Sources Underwater sound from active acoustic sources can include one or more of the following: Temporary or permanent hearing impairment, nonauditory physical or physiological effects, behavioral disturbance, stress, and masking. The degree of effect is intrinsically related to the signal characteristics, received level, distance from the source, and duration of the
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