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Federal Register / Vol. 86, No. 106 / Friday, June 4, 2021 / Notices underwater, and exposure to anthropogenic sound can have deleterious effects. To appropriately assess the potential effects of exposure to sound, it is necessary to understand the frequency ranges marine mammals are able to hear. Current data indicate that not all marine mammal species have equal hearing capabilities e.g., Richardson et al., 1995; Wartzok and Ketten, 1999; Au and Hastings, 2008.
To reflect this, Southall et al. 2007
recommended that marine mammals be
divided into functional hearing groups based on directly measured or estimated hearing ranges on the basis of available behavioral response data, audiograms derived using auditory evoked potential techniques, anatomical modeling, and other data. Note that no direct measurements of hearing ability have been successfully completed for mysticetes i.e., low-frequency cetaceans. Subsequently, NMFS 2018
described generalized hearing ranges for these marine mammal hearing groups.

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Generalized hearing ranges were chosen based on the approximately 65 decibel dB threshold from the normalized composite audiograms, with the exception for lower limits for lowfrequency cetaceans where the lower bound was deemed to be biologically implausible and the lower bound from Southall et al. 2007 retained. Marine mammal hearing groups and their associated hearing ranges are provided in Table 2.

TABLE 2MARINE MAMMAL HEARING GROUPS
NMFS, 2018
Generalized hearing range

Hearing group Low-frequency LF cetaceans baleen whales
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

7 Hz to 35 kHz.
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.

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 detail concerning these groups and associated frequency ranges, please see NMFS 2018 for a review of available information. Twenty-one marine mammal species 16 cetacean and 5 pinniped 3 otariid and 2 phocid species are considered herein. Of the cetacean species that may be present, six are classified as low-frequency cetaceans i.e., all mysticete species, eight are classified as mid-frequency cetaceans i.e., all delphinid and ziphiid species and the sperm whale, and two are classified as high-frequency cetaceans i.e., porpoises.
Potential Effects of Specified Activities on Marine Mammals and Their Habitat This section includes a summary of the ways that LDEOs specified activity may impact marine mammals and their habitat. Detailed descriptions of the potential effects of similar specified activities have been provided in other recent Federal Register notices, including for survey activities using the same methodology and over a similar amount of time, and affecting similar species e.g., 83 FR 29212, June 22, 2018; 84 FR 14200, April 9, 2019; 85 FR

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19580, April 7, 2020. No significant new information is available, and we refer the reader to these documents for additional detail. The Estimated Take section includes a quantitative analysis of the number of individuals that are expected to be taken by LDEOs activity. The Negligible Impact Analysis and Determination section considers the potential effects of the specified activity, 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.
Background on Active Acoustic Sound Sources and Acoustic Terminology 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 to the specified activity and to the discussion of the effects of the specified activity on marine mammals 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
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unit of time and is measured in hertz 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 decibel. 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
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