Acoustic Glossary

Airborne sound

Sound that arrives at the point of interest, such as one side of a partition, by propagation through air.

Ambient noise

The composite of airborne sound from many sources near and far associated with a given environment. No particular sound is singled out for interest.

Background noise

Noise from all sources unrelated to a particular sound that is the object of interest. Background noise may include airborne, structure borne, and instrument noise.

Indoor Ambient Noise Level

The A-weighted Sound Pressure level inside a given room resulting from the transmission of sound energy from noise sources located outside of the room.


Noise level exceeded for 10% of the measurement period. Represents the higher noise level (ie. average near-maximum).


Noise level exceeded for 90% of the measurement period. Represents the background noise level (ie. near-minimum).


Energy average noise level for the measurement period


Maximum noise level for the measurement period


Peak instantaneous sound pressure (OHS)

Sound Insulation

The capacity of a structure (e.g. a partition such as a wall or a floor) to prevent sound from reaching a receiving location. Sound energy is not necessarily absorbed; impedance mismatch, or reflection back toward the source, is often the principal mechanism.

Sound Transmission Loss (TL)

Of a partition, in a specified frequency band, ten times the common logarithm of the ratio of the airborne sound power incident on the partition to the sound power transmitted by the partition and radiated on the other side. The quantity so obtained is expressed in decibels. The reduction in sound level when sound passes through a partition or ceiling system.

Sound Transmission Class (STC)

A single-number rating (defined in ASTM E413), calculated using values of sound transmission loss. It provides an estimate of the performance of a partition in terms of airborne sound insulation in the 125Hz to 4kHz frequency range. In instances where a noise source has significant sound energy below 125Hz (e.g. aircraft, music, road traffic, machinery) STC is regarded as inadequate to fully quantify sound insulation and DW or RW may be used as an alternative.

Peak sound pressure level

In decibels, is 10 times the logarithm to the base 10, of the ratio of the square of the maximum instantaneous sound pressure to the square of the reference sound pressure (20 μPa). (Symbol: Lpeak)

Partial noise exposure

If a person is exposed to different noises at different workstations or tasks over the day, each exposure is referred to as a partial noise exposure. The sum of all partial noise exposures is the person’s total daily noise exposure.

Sound Level Conversion

SLC80 (Sound Level Conversion) is a single number rating commonly used in Australia and New Zealand to compare the acoustic performance of hearing protectors. The subscript ’80’ indicates that in well-managed hearing protector programs, the protection provided is expected to equal or exceed the SLC80 in 80% of protector-wearer noise spectrum combinations.