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The scientific study of sound, especially of its generation, transmission, and reception.

Ambient noise

The typical or persistent environmental background noise regularly present in the ocean.

Anthropogenic noise

Noise related to or produced by human activities.

Antisubmarine warfare (ASW)

Naval operations conducted against submarines, their supporting forces, and operating bases.


The filtering plates that hang from the upper jaw of baleen whales that capture zooplankton and small fishes.

Baleen whales

Whales that have baleen plates instead of teeth and filter their food from the ocean; also known as mysticetes.

Biologically important activities/behaviors

Those activities or behaviors essential to the continued existence of a species, such as migration, breeding/calving, or feeding.

Biologically important areas (offshore)

Offshore biologically important areas or OBIAs are defined as those areas of the world’s oceans that lie beyond 12 nautical miles (22 kilometers) from any emergent land, including islands, where marine mammals carry out biologically important activities. Biologically important areas include:

  • Migration corridors;
  • Breeding and calving grounds; and
  • Feeding grounds.

A member of the Cetacea taxonomic order, which includes aquatic mammals with anterior flippers, no posterior limbs, and a dorsal fin; whales, dolphins, and porpoises are all cetaceans.

Convergence zone (CZ)

The region in the deep ocean where sound rays, refracted from the depths, arrive at the surface in successive intervals of 55 to 64 kilometers (30 to 35 nautical miles). The repeated occurrence of these zones to several hundred miles from the sound source depends on the refraction of sound at depth and the reflection of these rays at the surface.

Critical habitat

The area where the species of concern resides that contains physical or biological characteristics essential to the survival of the species, or the area surrounding such habitats, which are essential to the survival of the species. However, it does not include all habitats that could be used by the species.

Decibel (dB)

A unit used to express the relative difference in power, usually between acoustic or electrical signals, equal to ten times the common logarithm of the ratio of the two levels.


A layer in the ocean where refraction and probably reflection result in the trapping of sound waves.

Duty cycle

The ratio of the time the sound is being transmitted over the total time period, measured in percent.

Endangered species

Defined in 16 U.S.C. 1532 as any species that is in danger of extinction throughout all or a significant portion of its range (other than a species of Class Insecta designated as a pest). Federally endangered species are listed in 50 CFR 17.11 and 17.12.


Description of the rate of disturbance, or vibration, measured in cycles per second. Cycles per second are usually referred to as the unit of measure of Hertz (Hz). In acoustics, frequency is characterized in general terms as low, mid, or high. The U.S. Navy categorizes these as follows:

  • Low frequency (LF) sound is below 1,000 Hz;
  • Mid frequency (MF) sound is between 1 and 10 kHz; and
  • High frequency (HF) sound is above 10 kHz.

Place where an animal or plant normally lives, often characterized by a dominant plant form or physical characteristic.


Under the Marine Mammal Protection Act, any act of pursuit, torment, or annoyance that has the  potential to:

  • Injure a marine mammal or marine mammal stock in the wild; or
  • Disturb a marine mammal or marine mammal stock in the wild by causing disruption of behavioral patterns, including, but not limited to, migration, breathing, nursing, breeding, feeding, or sheltering.
Hertz (Hz)

The unit of measure of frequency in cycles per second. 1,000 Hz is usually referred to as 1 kiloHertz (kHz).

Impedance (acoustic)

The product of density and sound speed.

LFA mitigation zone

The LFA mitigation zone covers a volume insonified to a level > 180 dB by the SURTASS LFA sonar transmit array. Under normal operating conditions, this zone will vary between the nominal ranges of 0.75 to 1.0 km (0.40 to 0.54 nm) from the source array over a depth of approximately 87 to 157 m (285 to 515 ft).


The obscuring of sounds of interest by interfering sounds, generally at similar frequencies.


Any of several whales having symmetrical skulls, paired blow holes, and plates of whale bone (baleen plates) instead of teeth of the suborder Mysticeti. Filter-feeding whales, also referred to as baleen whales.


Any of the toothed whales (without baleen plates) having a single blow hole and asymmetric skull of the suborder Odontoceti, such as orcas, dolphins, and porpoises.


One of three families of Pinnipedia having small but well formed ears (known as “eared” seals) including eared seals, sea lions, and fur seals.


Living in the water column. Plants and animals that are free-floating and drift passively, or animals that are strong swimmers.


The time required for a wave crest to traverse a distance equal to one wavelength.

Permanent threshold shift (PTS)

The deterioration of hearing due to prolonged or repeated exposure sounds which accelerate the normal process of gradual hearing loss, and the permanent hearing damage due to brief exposure to  extremely high sound levels.


Of or belonging to the Pinnipedia, an order of aquatic mammals that include seals, sea lions, walruses and similar animals having fin-like flippers for locomotion. They are carnivorous and “haul out” on shore to have their pups.

Received level (RL)

The level of sound that arrives at the receiver, or listening device (hydrophone). It is measured in decibels referenced to 1 microPascal root-mean-squared (rms). Put simply, the received level is the source level  minus the transmission losses from the sound traveling through the water.

Record of Decision (ROD)

In regard to an Environmental Impact Statement (EIS), the notice published in the Federal Register that contains the lead agency’s decision, and identifies both the alternatives and the mitigation measures to be used.


Process by which a traveling wave is deflected by a boundary between two media. Angle of reflection equals angle of incidence.


Bending of a sound wave passing through a boundary between two media; may also occur when physical properties of a single medium change along the propagation path.

Root mean squared

The square root of the arithmetic mean of the squares of a set of numbers.


A measure of the quantity of dissolved salts in seawater. It is formally defined as the total amount of dissolved solids in seawater in parts per thousand (‰) by weight when all the carbonate has been converted to oxide, the bromide and iodide to chloride, and all organic matter is completely oxidized.


Early consultation with federal and state agencies, and interested public to identify possible alternatives and the significant issues to be addressed in the EIS.

Single Ping Equivalent (SPE)

The single ping equivalent (SPE) is the methodology used during the acoustic modeling of potential impacts to marine animals from exposure to low frequency sound. This method estimates the total exposure of each individually modeled animal, which was exposed to multiple pings over an extended period of time. This was accomplished by the summation of the intensities for all received pings into an equivalent exposure from one ping, which is always at a higher level than the highest individual ping received.


A herbivorous aquatic mammal of the order Sirenia, which include the manatee and dugong.


An acronym for SOund NAvigation and Ranging. It includes any system that uses underwater sound, or acoustics, for observations and communications. There are two broad types of sonar:

  • Passive sonar detects the sound created by an object (source) in the water. This is a one-way transmission of sound waves traveling through the water from the source to the receiver; and
  • Active sonar detects objects by creating a sound pulse, or ping, that transmits through the water and reflects off the target, returning in the form of an echo. This is a two-way transmission (source to reflector to receiver) and is a form of echolocation.
Sound channel axis

The depth at which minimum sound velocity occurs in the ocean. Sound pressure level (SPL): Twenty times the logarithm to the base 10 of the ratio of the pressure to the reference pressure, in decibels at a specific point. The reference pressure shall be explicitly stated. SPL is usually measured in decibels referenced to 1 microPascal (rms).

Sound speed

Sound speed is the velocity that sound waves travel through a medium. Sound speed through seawater is approximately 1,500 meters per second (4,920 feet per second). It varies with water temperature, salinity, and depth (pressure). Sound speed increases with increases in temperature and pressure (depth), and to a lesser extent with increase in salinity. This change in speed as sound travels through water causes the travel path to bend in the direction of lower velocity.

Sound speed profile (SSP)

The sound speed profile (SSP) is a graphic representation of the sound speed versus depth of the ocean. These profiles vary with latitude, season, and time of day.

Source Level (SL)

The sound transmitted into the water by a sound source, such as an active sonar ping. SL is usually measured in decibels referenced to 1 microPascal at 1 m (3.28 ft).


Long-range, low frequency (between 100 and 500 Hz) sonar system composed of both active and passive components. SURTASS (Surveillance Towed Array Sensor System) is the passive component. LFA (Low Frequency Active) is the active component.


To harass, harm, pursue, hunt, shoot, wound, kill, trap, capture, or collect, or to attempt any of these activities.

Temporary threshold shift (TTS)

Temporary increases in threshold occurring after exposure to high noise levels, which can last from minutes to hours.

Transmission loss (TL)

Energy losses as the pressure wave, or sound, travels through the water, the associated wavefront diminishes due to the spreading of the sound over an increasingly larger volume and the absorption of  some of the energy by seawater.

Threatened species

Any species that is likely to become an endangered species within the foreseeable future throughout all or a significant portion of its range. Threatened species are listed in 50 CFR 17.12.


A member of the subphylum Vertebrata, a primary division of the phylum Chordata that includes fishes, amphibians, reptile, birds, and mammals, all which are characterized by a segmented bony or cartilaginous spinal column (i.e. backbone).


The distance between corresponding points of two successive waves.