Glossary for Discovery of Sound in the Sea

Glossary

absorption: the conversion of acoustic energy to heat energy
acoustic fish tag: a transmitter implanted or attached to a fish to monitor fish movement
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
acoustical shadowing: a condition that occurs when refraction or reflection prevents direct sound waves from reaching a region (called a shadow zone)
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 trauma: severe traumatic injury from sound
acoustic tomography: uses the travel time of sound in the ocean to measure the temperature of the ocean over large areas
active acoustics: sound is purposefully generated and received
ADCP: Acoustic Doppler Current Profiler; an instrument used to measure the current using acoustic sound and the knowledge of the Doppler effect
ambient noise: background sound in the ocean. Examples of sound sources contributing to ambient noise include waves, wind, rain, shrimp, earthquakes, and volcanoes.
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.
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
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.
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
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 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.

axial muscles

folded muscle segments that, when contracted, produce a wavelike motion that moves the fish through the water

backscatter: the deflection of sound in a scattering process through an angle greater than 90 degrees. Backscatter is the term commonly used to describe the return of sound from the seafloor to the receiver in an active sonar.
baleen: series of horny plates that hang from the gums of the upper jaw of some large whales (called Mysticetes). The baleen is made from the same materials as human hair and fingernails. It is used to filter small bits of food from the water.
baleen whales or Mysticetes: These large cetaceans are usually more than 9.1 m (30 ft) long and can be found in all of the world's oceans. Instead of teeth, mysticetes have a series of horny plates called baleen. The baleen is made from the same materials as human hair and fingernails. The baleen plates hang from the gums of the upper jaw and are used to filter small bits of food from the water. Baleen whales have symmetrical skulls and have two (or paired) blowholes.
The mysticetes are divided into four families: rorquals (blue, humpback, minke, sei, fin, and Bryde's whales), right whales, pygmy right whales, and gray whales. Rorquals have throat pleats, or ventral grooves, that expand when the whales gulp large amounts of water during feeding. Baleen whales can migrate up to hundreds of miles to feed in cooler areas with lots of food. On the feeding grounds baleen whales filter out small organisms from the water either by skimming the surface or gulping large quantities of water to filter. Baleen whales are not known to echolocate but produce a variety of sounds used for communication. Echoes from baleen whale vocalizations may help in navigating under ice or detecting the ocean floor.
basilar membrane: a membrane in the cochlea of the ear that vibrates in response to sound. As sound vibrations progress down the ear, a fluid wave that is created by the movement of the third ossicle, the stapes, moves the basilar membrane. The basilar membrane is the part of the cochlea that separates sounds according to their frequency.
bathymetry: charting of the sea floor using water depth measurements
beacon: an acoustic signaling device that continually sends out a repetitive signal. Acoustic beacons, sometimes called pingers, are used to mark the locations of underwater objects.
bearing: measurement of direction; the angle, with respect to magnetic north, to where the target is located
biologically significant: an action or activity that affects an animal's ability to grow, survive, or reproduce
biomass: measure of the amount of living material in an area, usually expressed in units of weight per unit volume
bowriding: a common activity of dolphins and other cetaceans in which marine mammals swim in front of a vessel, riding or surfing on the pressure wave created by the vessel
broadband: sounds that covers a wide range of frequencies
bubble feeding: a feeding process where whales trap a school of prey (fish or krill) by blowing a series of bubbles as the whales swim to the surface. The bubbles form a curtain that rises to the surface of the water and concentrates the prey in the center. The whales charge through with their mouths open to engulf the fish or krill.
buoyancy: the upward force on a free floating or submerged object, independent of the object's weight; gives submerged objects the "weightless" appearance
bycatch: the harvest of fish (or any marine organism) other than the species for which the fishing gear was set
byssal threads: string like substance that is secreted by mussels to allow the mussel to attach to hard substrates like rocks

caldera: depression formed at the summit of a volcano
callosites: thick, white patches of hardened skin, called chitin, that are covered with tiny crustaceans, called "whale lice." These patches are found on the heads, over the eyes, and around the mouths of whales, particularly right whales and bowhead whales. The patterns created by the patches are used by marine mammal researchers to identify individual whales.
canine teeth: sharp, fang-like teeth adapted for capturing and penetrating prey
capillaries: the smallest of the network of blood vessels throughout an organism
cavitation: formation of gas-filled cavities in liquids in motion when the pressure is reduced to a critical value. Low pressure regions are often created by rotating ship propellers. As the propellers rotate, bubbles form in the water. A loud acoustic sound is created when these bubbles collapse.
cephalopod: class of soft-bodied invertebrates that includes octopuses, squids, nautiluses and cuttlefishes. These animals have many arms and well-developed eyes.
cetacean: order of mammals that includes whales, dolphins and porpoises
cheliped: the first two claws of a crab. A male fiddler crab has an enlarged claw or cheliped.
chordotonal organs: for Crustaceans, they are located at the joint segments and they serve as mechanoreceptors (sensory organs).
cichlids: family (Cichlidae) of freshwater fishes found throughout tropical and subtropical waters of the Americas, Africa and parts of Asia
cilia: hair cells of the neuromast
click train: rapid sequence of clicks, produced by whales and dolphins, that are associated with echolocation. The clicks are emitted from the melon of the whale.
climatologist: a scientist who studies climate
clupeids: fish belonging to the herring family
coalesced: fused or grown together
cochlea: the spiral-shaped chamber within the inner ear that transforms sound waves into nerve impulses. The cochlea is a fluid-filled organ that houses many structures related to hearing, including the basilar membrane and the organ of Corti. It is considered "the organ of hearing."
coda: patterned set of clicks produced by sperm whales. Each sperm whale may have its own individually distinct coda pattern.
compression wave: a wave propagated by compressing the medium; longitudinal wave
computerized tomography (CT): method of constructing a three-dimensional image of an object from narrow x-ray beams that are passed through the object from several angles
Conductive hearing loss: transmission of sound to the inner ear is impaired
controlled experiments: tests or experiments used to demonstrate a cause and effect relationship. Results from tests performed on an experimental sample (which receives a particular treatment) are compared to those from a control sample (which does not receive the treatment) with all other aspects of the experiment remaining the same between the two groups.
convergence zone: a region of high intensity created by the refraction of sound waves in the SOFAR channel transmitted by a source near the sea surface. Convergence zones occur at about the same depth as the source approximately every 50-60 km away from it.
coronet: a star-shaped ossified crest mounted in a socket-like base
correlation: a relationship between two variables during a period of time, especially when there is a close match between the variables' movements. Correlation may indicate association between the variables; however, it does not mean that there is a cause and effect relationship.
corticosteroid: a type of hormone produced by the adrenal glands. Corticosteroids levels may be elevated as a response to stress.
courtship: behaviors in animals, that are used to initiate mating
crustaceans: a class of mainly aquatic, gill-breathing arthropods such as crabs, lobsters, shrimp, and barnacles. They usually have a hard exoskeleton and two pairs of antennae.
cupula: gel-like cover of cilia in the neuromast
cylindrical spreading: energy spreading out from a sound source in the shape of a cylinder; no energy radiates above the top or below the bottom of the cylinder

decibel: a relative unit used to describe sound intensities. Written as dB. See Advanced Topic: Introduction to Decibels.
decompression sickness: known as the bends, a condition that occurs in deep-sea divers caused by the formation of nitrogen bubbles in the blood and tissues following a sudden decrease in the surrounding pressure. This occurs when ascending rapidly from a deep dive and is characterized by severe pains in the joints and chest, skin irritation, cramps, and paralysis.
delphinid: cetaceans of the family Delphinidae, the most diverse of cetacean families. Includes oceanic whales and dolphins, such as, killer whales, pilot whales, common dolphins and bottlenose dolphins
detonation: to explode
dialect: vocalizations or calls of cetaceans that are characteristic of a particular group or pod
DIFAR: Directional Frequency and Ranging device; passive acoustic sonobuoy
digital data: information that is represented in a coded form, as a series of zeros and ones
directional: producing or receiving sound only from certain angles or directions
doppler effect: the raising or lowering of the frequency of a sound due to the source's or the listener's movement. The most common example is the rising frequency of a train whistle as the train approaches.
dorsal fin: the main fin found on the back of fishes and some marine mammals. Some whales, such as the killer whale, have tall dorsal fins, while other whales (i.e. belugas and bowheads) have no dorsal fin.
drumming: to vibrate a muscle in, on, or near, the swim bladder that produces a loud, low-pitched grunt sound
ducts: internal passage involved in the flow of fluids through an organism
duration: the length of a sound in seconds

echolocation: a process for locating distant or hard-to-see objects using the reflection of sound waves. The distance of objects or depth of the seafloor can be determined by measuring the time it takes for reflected sound waves (echoes) to return to the sound source. Some whales and dolphins use echolocation to identify underwater objects and to help find food.
echo signature: a unique sonar return (reflection) that can be used to identify individual species of marine organisms, such as fish, or other submerged objects
echosounding: the technique of measuring the depth of a body of water by means of an echo sounder, an electrical depth sounder that uses sound echoes. The instrument emits sound waves that travel to the bottom of the ocean and are reflected back. Depth is determined by timing how long it takes the sound pulse to leave the instrument, travel to the seafloor, and return to the receiver on the ship.
ecological risk assessment: the process of calculating the probability of adverse ecological effects
endangered: any species that is in danger of extinction
elasmobranchs: a group of fishes, including sharks, rays and skates, that has a skeleton made of cartilage rather than bone; they also do not have a swim bladder.
equal loudness curve: a graph of the perceived intensity (loudness) of sounds. The loudness of a sound is different for different frequencies.
equilibrium: the rest position of the particles in a medium
extrapolation: using evidence and inferences from a similar situation to project information about a related event or process. Extrapolation can also be used to apply or transfer experimental observations from a model to the real world.

fecund, prolific: fruitful in offspring or vegetation
fishes: two or more fish species. For example, if you have three fish species in an aquarium (such as a clownfish, damselfish, and goby), you would say "three fishes." If you have three clownfish (all of the same species), you would say "three fish."
focal animal observations: observations concentrated on individual animals that record everything they do
foraging: to search for food
foraging behavior: the way in which an animal searches for food; the process, or series of actions, that an animal goes through to find food
frequency: the rate of repetition of a regular event. The number of cycles of a wave per second. Expressed in units of Hertz (Hz)
frequency spectrum: a graph of a sound that plots the intensity of each frequency in the sound. Plural is spectra.
fright response: responding out of fear
fundamental frequency: the lowest frequency of a harmonic series. The fundamental frequency is also called the first harmonic frequency (f) of a harmonic series.

ganglion cells: nerve cells that have their cell bodies outside of the central nervous system (the brain and spinal cord). Auditory ganglion cells carry sound information from the inner hair cells to the auditory nerve.s
gas gland: a modification of the inner lining of the bladder, which works with the rete mirabile to force gases into the bladder

habituate: to become accustomed to something through repeated or prolonged exposure
harmonic frequency: the part of a signal whose frequency is an integer multiple of the fundamental frequency. Harmonic frequencies are related to each other by simple whole number ratios, for example if the fundamental frequency is f, the harmonics have frequencies of 2f, 3f, 4f, etc.
Heard Island Feasibility Test (HIFT): an expedition to Heard Island in the southern Indian Ocean during which acoustic sources suspended below a ship transmitted acoustic signals to receivers around the globe. Heard Island was selected because signals transmitted from that location can reach both coasts of North America. HIFT showed that underwater acoustic signals could be received worldwide and serve as a method for measuring global ocean warming.
hearing generalist: a fish species in which the swim bladder aids very little or not at all in hearing sensitivity
hearing specialist: a fish species in which the swim bladder is directly connected to the inner ear and provides increased hearing sensitivity
hearing threshold: the minimum intensity where a person with normal hearing can hear a sound. The intensity level varies with frequency. Lower frequency sounds generally have a much higher Threshold of Hearing. It ranges from 0 to 75 dB depending on the frequency.
hemorrhages: flow of blood from a ruptured blood vessels; excessive bleeding
hermaphroditic: an animal or plant having both male and female reproductive organs
Hertz: the unit of frequency; the number of cycles, or wavelengths, in a second (cycles/second)
hydroacoustic: acoustics in water
hydrodynamic: of, relating to, or operated by the force of liquid in motion
hydrophone: an underwater microphone that will listen to, or pick up, acoustic signals. A hydrophone converts acoustic energy into electrical energy and is used in passive underwater systems to listen only.
hydrophone array: several hydrophones attached to each other at known fixed distances so the location of sound sources can be calculated
hydrothermal vent: a hot spring on the seafloor
hypothesis: a tentative statement proposed by a scientist that is intended to explain certain facts or observations

IES

Inverted Echo Sounder; an instrument used to measure the temperature of the water column at a single point

impulsive sound: a broadband signal generated by sound sources such as explosions and airguns in which the sound pressure is very large at the instant of the explosion and then decays rapidly away; the duration of the peak pressure pulse is usually only a few milliseconds.
incident wave: the wave moving towards the reflector
infrasonic: sound waves that have a frequency that is lower than what humans can hear (i.e. below about 20 hertz). Baleen whales, such as blue and fin whales, produce these low frequency sounds. The sounds may be used to communicate over long distances and to detect large-scale topography of the seafloor. On land, elephants also use these powerful infrasonic sounds to communicate over long distances.
intensity: the average amount of sound power (sound energy per unit time) transmitted through a unit area in a specified direction. The unit of intensity is watts per square meter. For simplicity, the magnitude of the intensity is often referred to as the intensity, without specifying the direction in which the sound is traveling.
inner ear: the innermost part of the ear that is surrounded by the skull bone. It contains the organs of balance and hearing. The inner ear contains the vestibular system that helps maintain our balance. It also contains the cochlea that transforms sound waves into nerve impulses that are carried to the brain.

kerplunks: technique used by dolphins to drive fish away from protected areas such as sea grass beds. A dolphin will lift its tail and lower body out of the water and crash it down on the water surface. This causes a loud splash and creates a trail of bubbles under the water. The bubbles startle the fish hiding in the seagrass and flush them from their hiding places, making it easier for the dolphin to detect them.
krill, euphausiids: small, shrimp-like invertebrates that swarm in dense patches within the water column or at the sea surface. They have hard mouths and tail parts that reflect sound. Krill is an important food source for many marine organisms including whales and seals.

labyrinth: a complex system of interconnecting bony or membranous cavities, particularly those concerned with hearing and balance
lanugo: the soft, downy hair that covers some newborn mammals; in Arctic seals, it is a white fur that is highly prized by seal hunters, most famously associated with the hunting of harp seal pups
lateral line: sensory organ, found in fishes, that runs long the length of their body. The lateral line allows fish to sense movement.
larynx: the upper part of the trachea (air passage) that contains the vocal folds
ligaments: strong, flexible bands of tissue that hold two or more bones together at moveable joints. They help restrain the movement of bones at the joint.
localize: determine the direction the sound is coming from.
LOFAR: Low Frequency and Ranging sonobuoy; a type of passive acoustic sonobuoy
Lombard Vocal Response: the unconscious tendency of a person or animal to raise and/or lower their voice when going from a relatively quiet to a noisy environment and vice versa
longitudinal wave: a disturbance in which the particles and the energy move in the same direction
loudness: how loud a person perceives a sound to be. Not the same as the intensity of the sound. The perceived loudness varies with frequency.

magnetic resonance imaging (MRI): method of producing a three-dimensional image of object by recording the signals the object emits when placed in a magnetic field
masking: when a louder sound drowns out a quieter sound of the same frequency
mechanoreceptors: sensory organs that cause response to displacement, pressure and vibrations
medium, media (plural): substance or material that carries or transports the wave from its source to other locations. In the open ocean, the medium through which the wave travels is the ocean water.
melon: lipid-filled sac in the forehead of whales that helps to focus sound
mesopelagic boundary community: small (less than 4 inches long) fishes, shrimps, and squids that live in the middle of the water column and near islands
middle ear: the air-filled cavity that lies between the outer ear and the inner ear. The middle ear contains the ossicles which conduct sound vibration from the eardrum to the inner ear.
migration: movement of a group of animals from one location to another
molt: to shed the outer covering, or shell, which is then replaced by a new shell that is produced by the organism
mooring: equipment (often consisting of anchors and chains) which holds an item (such as a boat or underwater instrument) stable and secure in one place
morphology: the form and structure of an organism or any of its parts
multi-channel seismics: using multiple hydrophone arrays or streamers to record the reflected and refracted sounds from an air gun array

narrowband: sounds made up of only a small range of frequencies
necropsies: the examination and dissection of a body to determine the cause of death; autopsy
NeMO Net: New Millennium Observatory Network; a project, which records and transmits daily temperature and pressure readings from Axial Volcano
Neural impulses: An electrical and chemical signal sent along nerve fibers
neuromasts: small, sensory receptors of arranged hair cells located along the lateral line, which respond to motion between the fish and the surrounding water
noise: any undesired sound or any unwanted disturbance within a useful frequency band
non-vocal sounds: sounds made without the use of vocal folds or other body parts whose primary function is moving air for sound production. These sounds are typically made by slapping a body part on the water or land surface, or by forcefully clapping body parts together. Non-vocal sounds may be used to communicate acoustically.
normal line: a line that is perpendicular (makes a 90 degree angle) to a surface

oceanographic variable: a characteristic of the ocean that changes
octave band noise: noise over a range of frequencies where the frequency in Hertz of the upper end of the range is twice the frequency of the lower end
octavo-lateralis system: comprised of the lateral line and inner ear of fish; provides fish with balance, hearing, and the ability to feel vibrations from a distance
odontocete: group of mammals that includes the toothed cetaceans. This includes some whales (such as sperm whales, orca or killer whales, and beaked whales) as well as all dolphins and porpoises.
offspring: next generation or baby of a certain species
olfactory organ: an organ for smelling
omnidirectional: having no directional component; producing or receiving sounds from all directions
omnivorous: organisms that eat both animals and plants
organ of Corti: the sensory part of the inner ear that converts sound signals into nerve impulses. Located on the basilar membrane, it contains sensory cells that transform vibrations into neural code for sound processing by the brain. It contains many important cells including the inner and outer hair cells.
ossicles: three tiny bones - the incus (anvil), malleus (hammer), and stapes (stirrup) - that lie in the middle ear. The ossicles conduct sound across the middle ear to the inner ear by forming a bridge between the eardrum and the oval window.
otariids or eared seals: fur seals and sea lions that have visible ear flaps. They have an elongated neck and long front flippers. Their hind flippers can be turned forward for walking on land.
otolith: small bones in the inner ear which provide balance, and, in fish, aid in hearing
outer ear: the outermost part of the ear that is external to the ear drum or tympanic membrane. It directs sound waves down the air-filled ear canal onto the eardrum.
oval window: a membrane-covered opening between the middle ear and the inner ear. The oval window is connected to the third ear ossicle (stapes) and passes sound vibrations to the inner ear.

pack ice: a large expanse of floating ice
pan bone: thin bone in the back of the lower jaw that helps transmit sound to the middle ear
parallel: being an equal distance apart everywhere, extending in the same direction, never meeting or intersecting
passive acoustics: listening to sound sources; sound is only received
pectoral fins: the uppermost of the paired fins on a fish
peak pressure/0-to-peak pressure: the range in pressure between zero and the greatest pressure of the signal
peak-to-peak pressure: the range in pressure between the most negative and the most positive pressure of the signal
pectoral flipper: forelimbs of whales and dolphins that are used for stability and steering. These appendanges are generally flattened and paddle-like. Humpback whales have very large pectoral flippers, reaching 5 m (15 ft) in length.
pectoral free rays: rays that are part of the fishes pectoral fins but are free from the actual fin skin. These rays aid in the movement of the fish.
pectoral girdle: the bony or cartilaginous arch that supports the forelimbs of a vertebrate
"peer reviewed": scientific papers that have been subjected to evaluation by highly qualified experts in the field, the reviewers are usually anonymous and check the papers for inconsistencies, invalid conclusions and editorial errors
pelagic species: organisms that swim or drift in the water; these organisms are distinct from those living on or in the bottom sediments.
perpendicular: being at right angles
permanent threshold shift (PTS): a permanent increase in the threshold of hearing (minimum intensity needed to hear a sound) at a specific frequency above a previously established reference level
pharyngeal: teeth located in the gill or throat region
phocids or true seals: seals, such as harbor seals, that have no visible ear flap. They have a streamlined body with short front flippers. Their hind flippers are directed backward and they are not used for walking on land.
piezoelectric material: a material that produces electrical charges when subjected to pressure changes
pinna, auricle: the ear flap or outer part of the ear that extends from the head. The pinna funnels sound down the outer ear canal to the eardrum.
pinniped: group of mammals that includes seals, sea lions and walruses
plankton: relatively small organisms that drift or float passively in the water and are carried wherever currents and tides take them. Plankton are often microscopic and are an important food source for other aquatic organisms. There are two types of plankton- phytoplankton (plants and autotrophs) and zooplankton (animals).
plectrum: part of the spiny lobster that is found underneath the files. It is a soft piece of tissue that is found at the base of the antennae. The plecta is what the lobster pulls over the files to produce sound
pod: a social group of whales that are clustered together. Some toothed whales, such as orcas travel in large, sometimes stable pods. They may group together to hunt their prey and/or migrate.
polyps: small marine invertebrates that have hard, cup-like, limestone skeletons. Polyps live in large colonies where they take-in the calcium from the ocean and to build a hard shell around themselves. The skeletons grow one on top of one another and as a result a coral reef is able to grow.
pressure: the amount of force per unit area measured in units of atmospheres (atm)
profile: the change of an oceanographic variable with water depth
projector: an instrument that sends out sound waves; consists of a transmitter and a transducer
projector arrays: a collection of individual projectors used together to generate a directional sound beam
pure tone: a sound that consists of one single frequency

RAFOS floats: floating instruments designed to move with a current and track the current's movements
ramp-up: gradually increasing the sound source level
receiver: something that listens for or receives sound; it may or may not record the sound
recess: hole, corner or niche. For example, a rock recess, a place surrounded by rocks where a fish can hide
rectified diffusion: when acoustic energy causes supersaturated gas to be pumped into an existing small bubble, making the bubble increase in size
reflected wave: the wave moving away from the reflector
reflection: the deflection of the path of a sound wave by an object or by the boundary between two media
reflector: any boundary between two media that causes the reflection of a wave
refraction: the bending of a sound wave towards a region of slower sound speed
resonance: when sounds of specific frequencies cause air- or fluid-filled organs to vibrate with amplitudes that are large compared to the amplitude of incoming soundwaves
resonator: something that fills with sound and acts as a natural amplifier
rete mirabile: a tightly packed bundle of capillaries which works with the gas gland to force gases into the bladder
reverberation: echo
root-mean-square (rms) pressure: the square root of the average of the square of the pressure of the sound signal over a given duration
rorquals: whales of the family Balaenopteridae which includes humpbacks, sei, minke, fin, blue and Bryde's whales. They have throat pleats or ventral grooves that expand when the whales gulp large amounts of water while feeding.
rostrum: upper jaw of whales that is elongated and looks like a beak

salinity: the total amount of salt dissolved in seawater; the units most often used are parts per thousand (ppt) but practical salinity unit (psu) is now the accepted standard in oceanography. An average salinity value for seawater is 35 ppt (psu) or 35 parts of salt in 1000 parts of water.
Salt domes: a layer of salt in a domed structure that was formed beneath the Earth's surface by the movement of salt over long periods of time. Salt domes are often associated with oil and gas deposits.
sardines: small, silvery, slender fish of the herring family that have one short dorsal fin and no scales on their heads
Sargassum or Gulfweed: a type of brown algae (of the genus Sargassum) that have a branching thallus (body) with lateral outgrowths differentiated as leafy segments, air bladders, or spore-bearing structures
scattered/scattering: the diffusion of the sonar signal in many directions through refraction, diffraction and reflection, primarily due to the material properties of the seafloor. Scattering is one of the causes of attenuation in sonar, resulting in signal weakening.
scatterer: a particle in seawater or the roughness on the sea surface or seafloor that causes sound energy to be scattered
scattering: when the path of a sound wave is broken up by objects (volume scattering) or the sea floor or sea surface (boundary scattering)
school: a large group of marine animals, for example fish, that swim together. The appearance of a large number of individuals may ward off potential predators.
Sciaenidae: family of fishes that contains approximately 270 species distributed in the Atlantic, Pacific, and Indian Oceans. This family is commonly known as the drums, which are renowned for their sound producing ability.
scientific method: the orderly process by which scientists ask questions about the natural world and test their observations
seismic: relates to an earthquake, earth vibration or volcano
seismic reflection: a technique for examining the layers of the seafloor that uses the sound energy that is reflected by different layers in the seafloor.
seismic refraction: a technique for examining the layers of the seafloor that uses the sound energy that is refracted by different layers in the seafloor.
seismometer: an instrument that records ground movement; used to detect and measure earthquakes
Sensorineural hearing loss: hearing loss due to damage to the nerves or inner ear structures
sensory hair cells: bundles of hair like projections (cilia) located on the surface of the inner ear that become stimulated by movement of the otolith against them. Stimulation of the hair cells results in sending a signal to the brain which is interpreted as sound.
shadow zone: a region of low sound intensity that sound waves traveling away from a source in the ocean do not reach, usually because the sound waves are refracted away from that region
shelf-edged habitats: ocean habitat on the edge of the continental shelf
shoal: a large school of fish
side scan sonar: sonar is an acronym for sound navigation and ranging equipment. Sonar systems use sound waves to detect underwater objects by listening to the returning echoes. Side scan sonar uses the strength of the returning echo, not the travel time, to map the seafloor and identify objects on the seafloor.
signal: sound that is used for a specific task, such as to convey information.
signal-to-noise (SNR) ratio: the ratio that compares the received level of a sound signal and the background noise level. For example, it is easy to hear conversations in a quiet room, where the signal-to-noise ratio is high, but it is difficult to hear conversations at a noisy party, where the signal-to-noise ratio is low.
signal processing: analyzing sounds from a receiver to detect and classify sound sources
signature sound: a unique sound that is associated with a specific sound source
signature whistle: tonal sounds produced by whales and dolphins that are unique to a particular individual and distinct from any other member of the group. Signature whistles provide a way to recognize individuals and help maintain group cohesion.
sirenians: group of mammals that includes manatees and dugongs
SOFAR channel: SOFAR stands for SOund Fixing And Ranging. The sound speed minimum at 800-1000 meters of water depth is called the deep sound channel or, more historically, the SOFAR channel.
sonar: an acronym for sound navigation and ranging equipment. Sonar systems use sound waves to detect underwater objects by listening to the returning echoes. The distance to the object or the seafloor can be calculated by measuring the time between when the signal is sent out and when the reflected sound, or echo, is received.
sonic muscle - swim bladder: a combined mechanism used by fishes to produce sound. The swim bladder is a gas filled organ primarily used for buoyancy control and is also important for hearing in some fishes. The sonic muscle is attached to the swim bladder. Rapid flexure of the sonic muscle against the swim bladder produces drum-like sounds commonly associated with courtship and spawning behavior.
soniferous: sound producing
sonobuoy: instrument that is dropped into the ocean (from either an aircraft or ship) to record underwater sounds. It includes a hydrophone and a radio transmitter to send sound signals back to the aircraft or ship. The U.S. Navy uses this instrument to listen for enemy submarines. Sonobuoys may also be used to record marine mammal calls and listen for earthquake activity.
sound channel: an area of slow sound speed that causes sound waves to become focused at this water depth
sound channel axis: depth of the minimum sound speed within the sound channel. Sound waves cycle above and below the axis as they move through the sound channel.
sound field: the level of sound at different distances and depths from the source
sound receptor: something that receives sound; sound receiver
sound source: something that creates sound
sound spreading loss: the decrease in intensity that occurs when a sound wave expands as it moves away from a source
Sound Surveillance System (SOSUS): a network of hydrophones mounted on the seafloors of the North Atlantic and North Pacific oceans maintained by the US Navy
source level: the amount of sound radiated by a sound source. It is defined as the intensity of the radiated sound at a distance of 1 meter from the source, where intensity is the amount of sound power transmitted through a unit area in a specified direction. Source level is commonly given as a relative intensity in units named decibels (dB).
spawning: to produce, release, or deposit eggs for reproduction
spectrogram: a graphic presentation of a sound. A spectrogram plots the frequency vs. time and represents the different intensity of the frequencies with different colors. It is similar to a contour map or bathymetric map where the different colors represent different water depths. Also called a sonogram
spermaceti organ: an elongated connective tissue sac in the forehead of the sperm whale that contains a waxy fluid called spermaceti
spherical spreading: energy spreading out from a sound source in the shape of a sphere; the power is radiated equally in all directions from the sound source
spreading loss: a decrease in the intensity of a wave as it spreads out from a source
spy-hopping: a common whale activity in which they lift their heads above the surface of the water and observe what is happening on the surface. Whales will often spin around in order to observe in all directions.
squid: mollusks of the family Cephalopoda that are a favorite food of the sperm whale
static diffusion: when particles move from a region of higher concentration to a region of lower concentration
statistically significant: findings of an experiment or study that have a low probability of being due to chance alone
statistics: mathematical analysis that is used to explain and compare numerical data. This analysis helps make broader generalizations about a population from a smaller number of specific observations.
stereo: short for stereophonic: a sound-reproduction system that uses two or more separate channels to give a more natural distribution of sound.
stereocilia: long, flexible hair-like structures that occur as a brush border on the surface of some membranes
streamers: a long (2000-6000 m) string of hydrophones typically used with air-gun arrays
stridulate, stridulation: to produce a sound by rubbing two body parts together. Some fish make a shrill creaking noise by rubbing together bodily structures, especially skeletal parts.
subduction zones: places where two tectonic plates move toward each other, and one plate plunges beneath the other plate. Often ocean crust is subducting beneath continental crust
subsonic: a frequency that is below the audible range
substratum (substrate): the surface that an organism grows on or is attached to
supersaturated: more highly concentrated than is normally possible under given conditions of temperature and pressure
Surveillance Towed Array System (SURTASS): a passive sonar system used to listen for noises produced by submarines. The system consists of a long string of underwater hydrophones that are towed behind a ship and pick up sounds in the ocean.
swim bladder (also called an air bladder): an expandable, gas-filled sac that helps fish maintain buoyancy in the water. This organ is also important for hearing in some species of fish.
synapse: a junction between a nerve cell and another nerve cell or a nerve fiber or a sensory receptor

tectonic: relating to the deformation of the earth's crust
tectorial membrane: a membrane that covers the surface of the organ of Corti in the cochlea of the inner ear
telemetry: measuring and transmitting data from a remote location
temperature: a measure of the atomic and molecular vibration in a substance, in degrees. The response of a solid, liquid, or gas to the input or removal of heat energy.
temporary threshold shift (TTS): a temporary increase in the threshold of hearing (minimum intensity needed to hear a sound) at a specific frequency that returns to its pre-exposure level over time
terrestrial: something that lives on land as opposed to in the water. Some animals, such as sea lions spend time both on land and in the water, they are considered to be both terrestrial and marine.
territoriality/territorial: behavior in which an organism, for example a fish, defends its home
test: the outer skeleton of a sea urchin. It is made up of plates that encircle the sea urchin. Spines of the sea urchin grow from the test.
Thalassia: a broad-bladed seagrass occurring in shallow tropical and subtropical estuaries and nearshore marine waters
theory: a hypothesis or group of hypotheses that have been consistently validated through scientific observations or experiments. Geology's theory of plate tectonics is an example of a well-documented and widely accepted theory.
thermocline: a layer of water in an ocean or certain lakes, that separates warmer surface water from colder deep water. Temperature rapidly changes with depth in this region of the water column.
thermometry: the measurement of temperature
threshold of hearing: the minimum intensity where a person with normal hearing can hear a sound. The intensity level varies with frequency. Lower frequency sounds generally have a much higher Threshold of Hearing. It ranges from 0 to 75 dB depending on the frequency.
threshold of pain: the intensity level where sound is physically painful. Usually at 115-130 dB.
threshold shift: an increase (worsening) in the threshold of hearing for an ear at a specified frequency
towfish: an instrument, such as a side scan sonar, that is towed behind a ship
transducer: the part of the sonar system that functions like an antenna, sending out sonar signals (sound waves) and receiving return echoes. The transducer converts electrical energy into sound waves and vice versa.
transmitter: an instrument that sends out slectrical signals
transponder: an acoustic signaling device that automatically transmits a sonar signal upon reception of a designated incoming signal. Transponders are used to mark or track underwater objects.
transverse wave: a disturbance in which the particles vibrate up-and-down and the energy moves left-and-right
trill: a fluttering sound that alternates rapidly with another note
tympanic membrane or eardrum: a membrane that separates the outer ear from the middle ear. The eardrum vibrates in response to sound waves and passes the vibrations on to the bones of the middle ear.

ultrasonic: sound waves that have a frequency that is higher than what humans can hear (i.e. greater than 20,000 Hz). Bats and dolphins use these high frequency sounds for communication and navigation.
ultrasound signal: sound vibrations that have frequencies above the range of human hearing
underwater dB: the unit used to measure the intensity of an underwater sound. Underwater dB is a phrase that refers to "decibels referenced to 1 microPascal," which is abbreviated as "dB re 1 µPa." To be able to compare relative intensities given in dB to one another, a standard reference intensity or reference pressure must always be used. Scientists have agreed to use 1 microPascal (µPa) as the reference pressure for underwater sound. In air, however, scientists have agreed to use a higher reference pressure of 20 microPascals. It is important to remember that sound intensity given in underwater dB is not directly comparable to sound intensity given in air dB.

vacuum: a space from which all gas and other matter has been removed
velocity: the speed of sound. In air the speed of sound is about 344 meters per second. In water the speed of sound is about 1500 meters per second.
vertebrate: having vertebrae or having a backbone or spinal column. Fish and humans are examples of vertebrates.
vestibular system: a fluid-filled maze of canals and chambers inside the inner ear that helps maintain orientation and balance.
viscosity: a molecule�s resistance to motion
vocal fold ligament: connective tissue that strengthens the vocal folds via stiffness and support
vocalizations: sounds intentionally produced by animals that may be used for communication, navigation, and feeding. In humans, air is moved from the lungs and across the vocal folds (also known as vocal cords). The vibration of the vocal folds produce sounds that are formed into words and other vocal communication signals. We do not know how sound is produced by many species of marine mammals. The term "vocalization" is commonly used to refer to sounds that are produced by marine mammals; however, the use of the word vocalization does not imply that marine animals are using vocal folds to produce the sounds.

wave: disturbance caused by the movement of energy through a medium
waveform: A waveform presents the sound in a graph as positive and negative pressure on a relative scale (often from -1 to 1) through time. The relative pressure is related to the intensity of the sound.
wavelength: the length of one cycle of a wave (one crest and one trough)
Weberian ossicles: a series of bones which connect the swim bladder to the inner ear and carry vibrations between to the two, aiding in hearing

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