Acoustics glossary

Absorption
Also called the equivalent sound absorption area; it is the result of the area of the material multiplied by the material’s sound absorption coefficient.

A-weighting
An electronic filter built into a sound pressure meter which causes it to weight the measured noise in (almost) the same way as our ears. Usually, bass tones do not influence the measurement as much as treble tones.

Building Regulations (BR 08)
The Danish Building Regulations contain, among other things, requirements for the acoustic conditions in homes and in schools and day care institutions.

Danish Working Environment Authority guideline A.1.16
The guideline issued by the Danish Working Environment Authority (At-vejledning, A.1.16) in December 2008 relating to the acoustic conditions in workplaces.

Diffusion
Diffusion means (sound) spread. The phenomenon whereby sound, due to the shape of the room or obstacles in the room (e.g. furniture), is spread in a random manner around the room. Diffusion usually contributes to good acoustics in a room.

Echo
A single audible sound impulse which is reflected from a (large) surface when a sound impulse is sent (e.g. a handclap) towards the surface.

Equivalent, continuous sound pressure level, Leq
The average sound pressure which over the measurement period (e.g. a working day) results in the same sound energy as the current time-variety of the sound pressure.

Equivalent absorption area
See absorption

Flutter echo
A rapid pulse train which is reflected many times back and forth between two (large) parallel walls.  Flutter echoes can be produced by standing between two walls and clapping once with your hands.

Frequency
Number of oscillations per second. The unit of measurement is Hertz (Hz). Is used to quantifies pitch (bass, treble etc.). The human ear is capable of registering from about 20 Hz up to 18,000 -20,000 Hz.

Impulse train
See flutter echo.

Loudness
Measured in dB (decibels). The human ear is capable of registering very weak sounds measuring approx. 0 dB. There is no upper limit, but the human ear can be destroyed by even short exposure to sound levels of 120-130 dB. Is measured with a sound pressure meter, which is directly calibrated in dB.

Membrane absorber
An absorber consisting of a thin panel with a cavity behind it, for example a plasterboard wall or a wood floor on joists. The panel oscillates like a membrane, and in so doing absorbs sound, especially at low frequencies (bass).

Noise barometer
Popular graphical representation of the correlation between dB values and a number of everyday situations.

Peak value
The peak value (i.e. the absolute maximum value) of the sound pressure, for example from a shot.

Porous absorber
A type of absorber which consists of a porous material, such as mineral wool, fabric, textiles and carpets. Porous absorbers are usually medium tone/high tone absorbers, but they do not absorb bass tones.

Resonance absorber
An absorber consisting of a perforated panel, typically made of plaster or metal with a cavity behind. The resonance absorber absorbs medium tones, and is less effective at low and high frequencies.

Reverberation time
The time (in seconds) that it takes for the sound level to decay by 60 dB after the sound source (e.g. a speaker) has stopped producing the sound.

Sabine’s equation
Sabine’s reverberation equation states that the reverberation time (in seconds) for a room is equal to a proportionality factor of 0.16 multiplied by the room’s volume (in cubic metres) divided by the total absorption in the room (in m2 sabins or metric sabins).

Sound absorption coefficient
For a material, it expresses the relationship between the absorbed sound energy (the non-reflected sound energy) and the incident sound energy. It lies between 0 and 1, as an absorption coefficient of 0 indicates that the material does not absorb, while an absorption coefficient of 1 indicates that the material absorbs all sound.

Speech intelligibility
The ability of a room’s acoustics or a PA system to reproduce speech clearly so that the listeners are able to comprehend what is being said correctly.

Speech Transmission Index
A measure of speech intelligibility in a room or from a PA system. Uses a special test signal and measuring device, where the Speech Transmission Index (STI) can be read directly for the situation in question.