Intensity - The amount of sound energy radiated per unit area, measured in watts per square centimeter.

Initial time-delay gap (signal delay) - The time interval between the arrival of a direct sound and its first reflection from the surfaces of the room.

In phase
- Two periodic Wavesreaching peaks and going through zero at the same instant are said to be “in phase”.

Impulse - A very short, transient, acoustical (or electrical) signal.

Impaling Clip (also: Impaler) - A mounting fastener that is a metal plate with tines (prongs or spikes) sticking out of it. It is used along with adhesive to mount acoustical panels. The impaling clip is attached to the wall with the tines facing outward and the panel is pushed onto it (impaled).

Impact noise - The noise heard as a result of vibrations transferred through the structure of a room. Foot thumps are impact noise.

Impact Isolation Class (IIC) - A system for rating the ability of a structure to isolate impact noise (i.e. footsteps, and other vibrational disturbances). Normally used in reference to floor and ceiling constructions, the IIC method utilizes whole positive numbers for rating purposes.

Image shift
- Sound dislocated from its correct position, to be more left and / or right of center.

Hertz
- The measuring unit of frequency or the speed of vibration of a sound wave. Synonymous with “cycles per second” (CPS).

Helmholtz resonator - A reactive, tuned, sound absorber. A bottle is such a resonator. It can employ a perforated cover or slats over a cavity. An acoustic guitar or violin body is a Helmholtz resonator.