Glossary of Acoustic Terms

A-weighting

Frequency weighting applied to the level in each stated octave band by a specified amount, in order to better represent the response of the human ear. The letter ‘A’ will follow a descriptor, indicating the value has been ‘A’ weighted. An ‘A’ weighted noise level may also be written as dB(A).
Acoustic Privacy
See Privacy.

Absorption Class

In order to categorise the absorptive effects of different elements (such as ceiling tiles), classes from A to E were derived, as per BS EN ISO 11654:1997. A class ‘A’ absorber would be very acoustically absorptive, a Class ‘E’ absorber would be less absorptive and more reflective. A product that is highly reflective may not be classified.

Absorption Coefficient, α or alpha

A value usually between 0 and 1 assigned to a material to indicate how acoustically absorptive it is. 0 indicates a material is entirely reflective (and therefore not absorptive), and 1 indicates a material is entirely absorptive (and therefore not reflective). Absorption coefficients are usually given for each octave band between 125 Hz and 4 kHz, or as an overall ‘practical’ coefficient.

Airborne sound

Sound transmitted through air.

CRTN Shortened Method

Department of Transport document ‘Calculation of Road Traffic Noise’ (CRTN), published in 1988. The memorandum was prepared to enable entitlement under the Noise Insulation Regulations 1975 to be determined, but it is stated in the document that the guidance is equally appropriate to the calculation of traffic noise for land use planning purposes.

The CRTN shortened measurement method involves taking traffic noise measurements (LA10) over representative time periods within any three consecutive hours between 10:00 and 17:00. By using the LA10,3hour, as the arithmetic mean of the measured LA10 values, the LA10,18hour value can then be calculated; CRTN states that ‘LA10,18hour = LA10,3hour, – 1dB(A)’. The LA10,18hour values can then been converted into the equivalent LAeq,16hour values by subtracting 2.2 dB from the LA10,18hour for use in the subsequent assessment.

Impact sound pressure level values

Different metrics are used to describe the level of impact noise measured in a space.

L’nTw

A single figure impact sound pressure level value, normalised to the reverberation time measured or expected in a receive room below a source room.

Infra-sound

Very low level frequencies below the accepted range of human hearing, located between 1 Hz and 20 Hz.

Insertion Loss

The amount of sound reduction offered by an attenuator or louvre once placed in the path of a noise level.

Noise

Unwanted sound.

Noise Criterion, NC

Noise Criterion (NC) level, an octave band dependent system of rating noise levels similar to the Noise Rating system.

Noise Rating, NR

Noise Rating (NR) level. A frequency dependent system of noise level curves developed by the International Organisation for Standardisation (ISO). NR is used to categorise and determine the acceptable indoor environment in terms of hearing preservation, speech communication and annoyance in any given application as a single figure level. The US predominantly uses the Noise Criterion (NC) system

Octave

The interval between a frequency in Hz (f) and either half or double that frequency (0.5f or 2f).

Pressure, Pascals in Pa

Pascals, the Systeme International (SI) I unit to describe pressure, after physicist Blaise Pascal.

Privacy or acoustic privacy

Privacy is the addition of the level of sound insulation between two rooms and the background noise within a receiving room. It can be used to assess the level of privacy afforded in the ‘receiving room’ for speech from the ‘source room’.

The ‘privacy factor’ is a value that is the combination of the average ‘A’ weighted background noise level in the receiving room dB and the weighted sound level difference (Dw) of the separating structure in dB.

The ‘speech privacy potential‘ is another measure of privacy, and this is the combination of the D‘w of the separating structure in dB and the Noise Rating (NR) level in the receiving room.

Reverberation Time

The time taken in seconds for a sound to diminish within a room by 1,000 times its original level, corresponding to a drop in sound pressure of 60 dB. When taking field measurements and where background noise levels are high, the units RT20 or RT30 are used (measuring drops of 20 or 30 dB respectively). Sometimes given as a mid-frequency reverberation time, Tmf which is the average of reverberation time values at 500 Hz, 1 kHz and 2 kHz.

Sound level difference, D

The sound level difference between two internal spaces, or between internal and external spaces. The ‘D’ value is used to denote the differences at each third octave or octave band, with a single figure ‘weighted’ value to describe an overall performance. Note that the ‘D’ value will always describe an in-situ or on-site acoustic performance. All values are described using the decibel.

Dw
Single figure weighted sound level difference, simply the measured source noise level minus receiver noise level, not adjusted to reference conditions
DnT,w
Weighted normalised sound level difference – a single, weighted sound insulation value, normalised to a reference reverberation time using the measured reverberation time in the receive room
DnT,w + Ctr
As above, with a spectral adaptation term applied to account for the effects of low frequency noise, and based on urban traffic noise
Dnf,w
Overall flanking normalised level difference – A parameter that defines the flanking transmission of sound from room to room where a dividing partition or floor construction abuts a flanking building element common to both rooms, such as the building façade or ceiling

Sound power level, SWL or LWA

A sound level obtained by calculation from measurement data, given at the face of an item of plant or machinery. Referenced to 10-12 Watts or 1pW.

 

Sound pressure level, SPL or LPA

A sound level measured or given at a distance from a source or a number of sources. Referenced to 2×10-5 Pa. Can be represented by a number of different metrics, as set out below.

LAeq,T
The ‘A’ weighted ‘equivalent’ noise level, or the average noise level over the time period T, described or measured.
LA90,T
The ‘A’ weighted noise level exceeded for 90% of the time period T, described or measured. The ‘90’ can be substituted for any value between 1 and 99 to indicate the noise level exceeded for the corresponding percentage of time described or measured.
LAmax
The ‘A’ weighted maximum measured noise level. Can be measured with a ‘slow’ (1 sec) or ‘fast’ (0.125 sec) time weighting.
LAmin
The ‘A’ weighted minimum measured noise level.

Sound reduction Index, R

Rw
Weighted single figure sound reduction index
Rw + Ctr
As above, with a spectral adaptation term applied to account for the effects of low frequency noise, and based on urban traffic noise
R’w
The ‘apparent sound reduction index’, a field measurement to obtain the sound reduction index of a material or element, with all effects of site installation accepted.

Speech Privacy Potential, SPP

The SPP is used to define the expected acoustic privacy between two spaces. The table below details the SPP outcomes associated with calculated rating value.

Tones, Tonality

Tones are prominent frequencies (or octaves, or third-octaves) that are ‘proud’ of the immediately adjacent frequency bands such that they are distinguishable, for example, as a ‘hum’, ‘whine’ or ‘whistle’.

Ultra-sound

Very high level frequencies above the accepted range of human hearing, located above 20 kHz.

Watts, W

Watts, the Systeme International (SI) unit to describe power, after engineer James Watt.