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Federal Register / Vol. 86, No. 23 / Friday, February 5, 2021 / Notices TABLE 4MARINE MAMMAL HEARING GROUPSContinued NMFS, 2018
Generalized hearing range

Hearing group Mid-frequency MF cetaceans dolphins, toothed whales, beaked whales, bottlenose whales
High-frequency HF cetaceans true porpoises, Kogia, river dolphins, cephalorhynchid, Lagenorhynchus cruciger & L.
australis.
Phocid pinnipeds PW underwater true seals
Otariid pinnipeds OW underwater sea lions and fur seals

150 Hz to 160 kHz.
275 Hz to 160 kHz.
50 Hz to 86 kHz.
60 Hz to 39 kHz.

Represents the generalized hearing range for the entire group as a composite i.e., all species within the group, where individual species hearing ranges are typically not as broad. Generalized hearing range chosen based on 65 dB threshold from normalized composite audiogram, with the exception for lower limits for LF cetaceans Southall et al. 2007 and PW pinniped approximation.

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The pinniped functional hearing group was modified from Southall et al.
2007 on the basis of data indicating that phocid species have consistently demonstrated an extended frequency range of hearing compared to otariids, especially in the higher frequency range Hemila et al., 2006; Kastelein et al., 2009; Reichmuth and Holt, 2013.
For more details concerning these groups and associated frequency ranges, please see NMFS 2018 for a review of available information. Sixteen marine mammal species 14 cetacean and 2
pinniped both phocid species have the reasonable potential to co-occur with the proposed activities Table 3.
Of the cetacean species that may be present, six are classified as lowfrequency cetaceans i.e., all mysticete species, seven are classified as midfrequency cetaceans i.e., all delphinid species and the sperm whale, and one is classified as a high-frequency cetacean i.e., harbor porpoise.
Potential Effects of Specified Activities on Marine Mammals and Their Habitat This section includes a summary and discussion of the ways that components of the specified activity may impact marine mammals and their habitat. The Estimated Take section later in this document includes a quantitative analysis of the number of individuals that are expected to be taken by this activity. The Negligible Impact Analysis and Determination section considers the content of this section, the Estimated Take section, and the Proposed Mitigation section, to draw conclusions regarding the likely impacts of these activities on the reproductive success or survivorship of individuals and how those impacts on individuals are likely to impact marine mammal species or stocks.
Description of Sound Sources This section contains a brief technical background on sound, on the characteristics of certain sound types, and on metrics used in this proposal inasmuch as the information is relevant
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to the specified activity and to a discussion of the potential effects of the specified activity on marine mammals found later in this document. For general information on sound and its interaction with the marine environment, please see, e.g., Au and Hastings 2008; Richardson et al.
1995; Urick 1983.
Sound travels in waves, the basic components of which are frequency, wavelength, velocity, and amplitude.
Frequency is the number of pressure waves that pass by a reference point per unit of time and is measured in Hz or cycles per second. Wavelength is the distance between two peaks or corresponding points of a sound wave length of one cycle. Higher frequency sounds have shorter wavelengths than lower frequency sounds, and typically attenuate decrease more rapidly, except in certain cases in shallower water. Amplitude is the height of the sound pressure wave or the loudness of a sound and is typically described using the relative unit of the dB. A
sound pressure level SPL in dB is described as the ratio between a measured pressure and a reference pressure for underwater sound, this is 1 microPascal mPa, and is a logarithmic unit that accounts for large variations in amplitude; therefore, a relatively small change in dB
corresponds to large changes in sound pressure. The source level SL
represents the SPL referenced at a distance of 1 m from the source referenced to 1 mPa, while the received level is the SPL at the listeners position referenced to 1 mPa.
Root mean square rms is the quadratic mean sound pressure over the duration of an impulse. Root mean square is calculated by squaring all of the sound amplitudes, averaging the squares, and then taking the square root of the average Urick, 1983. Root mean square accounts for both positive and negative values; squaring the pressures makes all values positive so that they may be accounted for in the summation
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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 micropascalsquared second 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
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