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Impact Insulation Class (IIC)
A single-number rating derived from measured values of impact sound pressure levels which provides an estimate of the impact sound insulating performance of a floor-ceiling assembly.

Impact Noise
Noise caused by the collision (impact) of two objects.

Impedance
The ratio of force per unit area to the volume displacement of a given surface across which sound is transmitted.

Impulse Noise
A noise of short duration (typically less than a second), particularly of high intensity, such as that produced by an explosion, discharge of a firearm.

Initial Time Delay Gap (ITDG)
The time gap between the arrival of the direct sound and the first sound reflected from the surface of the room

Infrasonic
A sound that is below the human audible frequency, i.e., below 20 Hz. Also called infrasound.

Intensity
The rate of sound energy transmitted in a specified direction through a unit area.

Inter Aural Cross Corelation (IACC)
A binaural measure of the difference in the sounds at the two ears produced by a sound source on the stage of a performing space.

Inverse Square Law
At a distance from a source, under free-field conditions, sound intensity varies inversely with the square of the distance from the source, resulting in a decrease in sound pressure level of 6 dB for each doubling of distance.

Isolation
The ability of materials or constructions to resist that transmission of sound or vibration through them.