Acoustical Material: A material used to alter a sound field. The material may be used to absorb, damp or block acoustical energy.

Airborne Noise: A condition when sound waves are being carried by the atmosphere.

Ambient Noise: All the sounds from many sources associated with a given environment.

Anechoic Room: A test chamber which has a lining of absorbent acoustical material to eliminate all sound reflections. It is most often used to determine the sound radiation characteristics of equipment.

Damping: The process of dissipating mechanical vibratory energy into heat. In noise control, a damping material is usually applied to a vibrating surface to reduce the noise radiating from that surface.

Dissipative Silencer: A device inserted into an air duct or opening to reduce noise transmitted through the duct or opening. Noise reduction is accomplished through the use of internal sound absorbing materials.

Flanking Transmission: Noise that reaches an observer by paths around or over an acoustical barrier.

Frequency Spectrum: A graph or plot of the sound pressure level in each band from a set of octave or 1/3 octave bands.

Insertion Loss: The reduction of sound power level attained by inserting a silencer or muffler in an acoustic transmission system (see ASTM E-477).

Loudness: Loudness is the subjective human definition of the intensity of a sound. Human reaction to sound is highly dependent on the sound pressure and frequency.

Noise: Any undesired sound.

Noise Reduction (NR): The reduction in sound pressure level caused by making some alteration to a sound field.

Noise Reduction Coefficient (NRC): A single number rating which is the average of the sound absorption coefficients in the octave bands centered at 250, 500, 1000 and 2000 Hz expressed to the nearest integral multiple of 0.05 (see ASTM C-423).

Radiation: The process whereby structure-borne vibration is converted into airborne sound.

Reverberation: Reverberation is the echoing of previously generated sound caused by reflection of acoustic waves from the surface of enclosed spaces.

Reverberation Room: A test chamber so designed that the reverberant sound field within the room has an intensity that is approximately the same in all directions and at every point. It is commonly used to measure sound absorption, ASTM C-423 and transmission loss, ASTM E-90.

Sabin: The unit of measure of sound absorption. The number of square feet of sound absorbing material multiplied by the material absorption coefficient.

Sound: Pressure waves that are traveling in the air or other elastic materials.

Sound Absorption: The acoustical process whereby sound energy is dissipated as heat rather than reflected back to the environment.

Sound Level Meter: An instrument used to measure sound pressure level. Sound level meters are commonly either Type 1, precision instruments, or Type 2, general purpose instruments. Both types can have weighting and filter networks to provide dB readings by octave band in the A, B, or C scales.

Sound Power Level (Lw): A measure of the total airborne acoustic power generated by a noise source, expressed on a decibel scale referenced to some standard (usually 10-12 watts).

Sound Pressure Level (Lp): A measure of the air pressure change caused by a sound wave, expressed on a decibel scale referenced to 20µPa.

Sound Transmission Class (STC): A single number rating derived from measured values of transmission loss in accordance with ASTM 413. The rating provides an estimate of the performance of a barrier in certain common noise attenuation applications.

Structure-borne Noise: Mechanical vibration in a structure which can ultimately become audible sound. Until such time as radiation occurs, these vibrations are inaudible and of little concern.

Transmission Loss (TL): The reduction in sound power that is caused by placing a wall or barrier between the source and receiver. Transmission loss is expressed in decibels.