Absorption
Conversion of sound energy into heat thereby reducing the amount of sound energy that is reflected.

Absorption Coefficient
Fraction of the incident sound energy absorbed at any surface. It has a value between 0 and 1.

Acoustics
The art and science of sound.

Acoustical Treatment
The use of acoustical absorbing, diffusing or reflecting materials or noise-isolating structures to modify the acoustical environment.

Active Noise Control
Introduction of a soundwave that is a mirror image of the original sound wave resulting in the cancelling out of the original sound.

Airborne Sound
Sound propagated through air as a medium.

Ambient Noise
Ambient noise encompasses all sound present in a given environment, being usually a composite of sounds from many sources near and far.

Anechoic
No echo. The sound field is a free field, implying freedom from echoes.

Antinode
Point of maximum vibration.

Architectural Acoustics
The control of noise in a building space to adequately support the communications function within the space and its effect on the occupants.

Articulation
The percentage of speech units correctly perceived by a listener.

Articulation Class (AC)
A classification method that rates building systems and subsystems for speech privacy purposes

Articulation Index (AI)
A numerically calculated index whose value lies between 0 and 1. A value of 0 implies zero speech intelligibility and a value of 1, perfect intelligibility.

Attenuation
Reducing the magnitude of a sound signal by separation of a sound source from a receptor.

Audio frequency
An acoustical or electrical signal whose frequency is within the audible range of the human ear - 20 Hz to 20 kHz.

Aural
Pertaining to the human ear or hearing.

A-weighting (dBA)
Means of electronically simulating the unequal sensitivity of the human ear at various frequencies by filtering the sound in the sound level meter.

Axial mode
Room resonances associated with each pair of parallel walls.