Glossary - A
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A

A-weighting
the most commonly used frequency weighting function for humans that accounts for the fact that human hearing is less sensitive to low frequencies; units dB(A) or dBA.
absorption
the conversion of acoustic energy to heat energy
acceleration
the rate of change of velocity with respect to magnitude or direction.
accelerometer
a device that measures the vibration, or change in motion (acceleration) of a structure or organism. The force caused by vibration or a change in motion causes piezoelectric material within the device to be squeezed, which produces an electrical charge that is proportional to the force exerted upon it. Since the charge is proportional to the force, and mass remains constant, the charge is also proportional to the acceleration.
accuracy
the degree of closeness of measurements of a quantity to its actual value.
acoustic fish tag
a transmitter implanted or attached to a fish to monitor fish movement
acoustic impedance
the amount of sound pressure generated by a given vibration at a specific frequency.
acoustic modem
a wireless communication device used to transmit data and information through the ocean
acoustic propagation models
conceptual and numerical models that compute how sound travels through the water, taking into account many variables such as water temperature, salinity, bottom topography, etc.
acoustic release
a device which holds onto the anchor of a buoyant instrument until it is commanded to release it
acoustic telemetry
to transmit acoustic signals automatically and at a distance, as between a ground station and an artificial satellite, space probe, or the like, especially in order to record information, operate guidance apparatus, etc
acoustic threshold
the received level at which an effect from acoustic exposure may begin to occur.
acoustic tomography
uses the travel time of sound in the ocean to measure the temperature of the ocean over large areas
acoustic trauma
severe traumatic injury from sound
acoustical shadowing
a condition that occurs when refraction or reflection prevents direct sound waves from reaching a region (called a shadow zone)
active acoustics
sound is purposefully generated and received
adaptation
an alteration in the structure or function of an organism or any of its parts that results in the organism becoming better fitted to survive and multiply in its environment.
ADCP
Acoustic Doppler Current Profiler; an instrument used to measure the current using acoustic sound and the knowledge of the Doppler effect
agonistic behavior
aggressive or defensive social interaction (such as fighting, fleeing, or submitting) between individuals usually of the same species
airgun
An airgun is designed to release compressed air, which forms bubbles. The formation of bubbles produces a loud sound that is used to explore the geologic structure of the ocean floor. Airguns primarily produce sound at low frequencies (between 10-500 Hz); however, high frequency noise is also created. A small airgun that releases 0.16 Liters of air can create source amplitudes up to 216 underwater dB at 1 meter. A large airgun that releases 32.8 Liters of air can have a source level of up to 232 underwater dB at 1 meter.
algorithm
a step-by-step procedure for calculations/solving a problem.
alongshore flow
A surface current that flows parallel to the shore.
Amazon river dolphin
Inia geoffrensis
ambient noise
background sound in the ocean. Examples of sound sources contributing to ambient noise include waves, wind, rain, shrimp, earthquakes, volcanoes, and distant sources, such as shipping and airguns.
American lobster
Homarus americanus
American shad
Alosa sapidissima
amphibious
living or able to live on land and in the water
amplitude
the maximum distance that a vibrating particle moves from its equilibrium; how much the medium is disturbed
anadromous
migrating from salt water into fresh water to breed/spawn (i.e. salmon are anadromous).
anemometer
a weather instrument that measures wind speed.
angle of incidence
the angle that the incident wave makes with a line perpendicular or normal to the reflecting surface
angle of reflection
the angle that the reflected wave makes with a line perpendicular or normal to the reflecting surface
animated frequency spectrum
An animated frequency spectrum is a series of frequency spectra that show just the frequencies present at each moment in time. You can see what frequencies are associated with each part of a sound.
anthropogenic
caused by humans
Aristotle's lantern
claw-like mouth on a sea urchin that contains five calcium carbonate teeth that are used for feeding
array elements
a single hydrophone in a receiving array or a single projector (sound source) in a projector array
arthropods
invertebrates of the phylum Arthropoda that have jointed appendages and a chitinous, segmented exoskeleton. Arthropods include insects, spiders, crabs, and lobsters.
Atlantic croaker
Micropogonias undulatus
Atlantic mackerel
Scomber scombrus
Atlantic salmon
Salmo salar
Atlantic spotted dolphin
Stenella frontalis
Atlantic Undersea Test and Evaluation Center (AUTEC)
a deep-water laboratory located in the Bahamas (in the Tongue of the Ocean (TOTO)) that is instrumented with a variety of acoustic beacons and sensors to provide testing, evaluation, and certification for U.S. Navy submarine captains and their crews, as well as the accuracy of their undersea weapons. A Marine Mammal Monitoring (M3R) system has been established at AUTEC to monitor vocalizing animals via the 91 range hydrophones.
attenuation
the decrease in the intensity of a wave due to the loss of acoustic energy to heat energy
audiogram
a graph expressing hearing loss (hearing sensitivity) as a function of frequency
audiometric curve
a graph displaying the range of sounds that humans can hear.
auditory brainstem response (ABR)
Whenever a sound wave is detected by the ear, it triggers a number of neuro-physiological responses along the auditory pathway. An auditory brainstem response test is an objective test that measures the electrical potential produced in response to sound stimuli by the synchronous discharge of the first through sixth order neurons in the auditory nerve and brainstem. Also sometimes known as brainstem auditory evoked potential (BAEP) or brainstem auditory evoked response (BAER).
auditory bulla
a hollow, bony structure that encloses parts of the middle and inner ear.
auditory fatigue
when the intensity level or duration of sound overwhelms the hair cells so they cannot respond to sounds appropriately
auditory meatus or ear canal
an air-filled canal that leads from the ear flap to the ear drum. It helps direct sound waves to the ear drum.
auditory system
the sensory system for hearing, consisting of the ear and the central nervous system.
auditory weighting function
a mathematical equation that compensates for the fact that animals do not hear equally well at all frequencies.
Australian freshwater crayfish
Cherax destructor
Automatic Identification System (AIS)
an automated tracking system used to electronically identify and locate ships. AIS uses GPS-linked, very high frequency radio signals that allow for ship-to-ship and ship-to-shore information transfer. Information transmitted includes a ship's name, position, speed, heading, and other information. These details are transmitted multiple times each minute.
autonomous
existing and/or functioning independently; with regards to underwater vehicles, one which travels underwater without requiring input from an operator.
axial muscles
folded muscle segments that, when contracted, produce a wavelike motion that moves the fish through the water