Glossary - I
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I

ice keel
the underwater portion of an ice ridge.
IES
Inverted Echosounder; an instrument used to measure the temperature of the water column at a single point
immunological response
a bodily defense reaction that recognizes an invading substance (such as a virus, fungus, or bacteria, or a transplanted organ) and produces antibodies specific to that invading substance.
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.
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.
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.
inter-click interval
the time it takes an echolocation signal to be sent out and an echo to return, combined with the time the animal needs to receive and process that echo.
invertebrate
an animal that lacks a backbone (marine examples include lobsters, shrimp, squid, clams, crabs, and sea stars).
ion
an atom or a group of atoms that has an electric charge. Positive ions, or cations, are formed by the loss of electrons; negative ions, or anions, are formed by the gain of electrons.