Acoustic and Soundproofing Definitions

Acoustic Terminology

TL-Transmission Loss

Transmission Loss (TL) is a figure which rates the ability of a material to block sound. It is usually measured in 1/3 octave band intervals. Mathematically it is defined as the ratio of the sound energy transmitted through a material to the sound energy incident on the material.

The Transmission Loss (TL) of a material is measured by mounting a sample of the material in an opening of a wall separating two reverberant test rooms. Broadband noise is played in one room (source).

The difference between the sound levels in the source room and the other (receiving) room is defined as the Noise Reduction (NR). As the frequency and/or density increases the Transmission Loss also increases. The density of the material is directly related to Transmission Loss.

SRI

The ASTM (American) 1/3rd octave Sound Transmission Losses measured above are referred to by the European ISO standards as Sound Reduction Indices (R).

BS EN ISO 717 defines a standard contour and a procedure for fitting the contour to the measured sound reduction indices to determine a single-number rating of a sound transmission loss spectrum. This rating is called the Weighted Sound Reduction Index (R_w).

Unlike the STC contour, the R_w contour is defined over a slightly lower frequency range of 100 Hz to 3.15 kHz.
The contour fitting procedure requires that:

• the sound reduction values be determined to one decimal place
• the contour be raised in 1 dB increments to a point where the average deficiency over the contour frequency range is as close to, but not exceeding, 2.0 dB.
• if an 8dB or larger deficiency exists in the sound reduction index data, then the deficiency amount in dB and the frequencies at which they occur must be reported.

The average deficiency is the sum of all deficiencies in all frequency bands divided by 16, the number of 1/3rd octave frequency bands spanned by the contour.

An actual R_w value is equal to the fitted contour value at 500 Hz.

It should be noted that the SRI value has been developed to approximate the performance of a material in reducing the transmission of speech.

The SRI value obtained from the TL data is useful for a quick comparison of materials but does not give a true idea with respect to non-speech sounds such as music, traffic, trains, aircraft etc.

STC

STC is the American ASTM, standard E413, equivalent of SRI and is based on the averaged sound insulation achieved between 125Hz and 4kHz.

As before, the standard defines a procedure for determining the STC rating for a TLoss spectrum by fitting a contour to the 1/3rd octave data. This procedure involves raising or lowering the contour following these rules:

• the contour may not be raised above the point at which the Tloss in any 1/3rd octave band falls more than 8dB below the contour.
• the contour may not be raised above the point at which the total number of deficiencies is greater than 32.

A deficiency occurs when the TL data in any 1/3rd octave band falls below the contour by 1dB.

The STC rating resulting from the contour fitting procedure is the TL value of the contour at 500Hz.

It should be noted that the STC value has been developed to approximate the performance of a material in reducing the transmission of speech.

The STC value obtained from the TL data is useful for a quick comparison of materials but does not give a true idea with respect to non-speech sounds such as music, traffic, trains, aircraft etc.

NRC

The Noise Reduction Coefficient, defines how much sound specific materials absorb. It is the average sound absorption between 250Hz – 2kHz rounded to the nearest 0.05.
This is analogous to a room’s finishes.

A material with a low NRC rating (wood) absorbs little sound and a material with a higher NRC rating (carpet) absorbs more sound.

dB(A)

The human ear does not hear all frequencies with the same intensity. It is most sensitive to sounds in the 500Hz-8kHz range.

Above and below this range the ear becomes progressively less sensitive. To compensate for this, sound level meters incorporate electronic filtering to correspond with the varying sensitivity of the ear.

This filtering is called A-weighting and Sound Pressure Levels obtained with this weighting are referred to as A-weighted and signified as dB(A).

R_w

This is a laboratory value as defined in BS EN ISO 717-1. It is the weighted sound reduction index, a single figure used to give an easy to read average decibel value.

R_w does not take into account flanking noise loss but describes the airborne sound insulating performance of a building element such as a wall, floor, ceiling, door, window.

Because R_w is a laboratory derived value it should be noted that onsite sound tested value will generally be 5dB to 10dB lower than the published R_w value.

Absorption coefficient

Expressed as a value between 0 and 1 where 0 represents no absorption (perfect reflection) and 1 represents total absorption. It can also be represented as a percentage.

C_tr

The correction to a sound quantity such as the weighted standardized sound level difference D_nTw to take account of the specific sound spectrum.

Decibel

A unit of level derived from the logarithm of the ratio between the value of a quantity and a reference value.

It is used to describe the level of many different quantities. For sound pressure level the reference quantity is 20 μPa, the threshold of normal hearing is in the region of 0dB and 140dB is the threshold of pain.

A change of 1dB is only perceptible under controlled conditions.

D_n

Normalized Level Difference: for airborne sound transmission.

A sound insulation index measured under field conditions, between ‘real’ rooms and deliberately includes effects due to flanking routes and differences in the relative size of the rooms.

It attempts however to normalize the measured difference level to the level which would be present when the rooms are furnished. This is achieved by measuring the acoustic absorption in the receiving room. The level difference is then correcting to what would be expected if there was 10m² sabine absorption in the receiving room.

Accurate knowledge of the dimensions of the receiving room are required. The higher the figure the better.

D_nT

Standardized Level Difference: for airborne sound transmission. Similar to the D_n, but this index corrects the measured difference to a standardized reverberation time of 0.5 seconds.

This RT value is often cited as approximately average for a medium sized, carpeted and furnished living room. It does not require detailed and accurate knowledge of the dimensions of the test rooms. The higher the figure the better.

D_nTw

Weighted Standardized Level Difference: for airborne sound transmission.

A single number quantity which characterises the airborne sound insulation between rooms, calculated from the D_nT.

Building Regulations : Approved Document E – preferred parameter. The higher the figure the better.

D_nTw + C_tr

A single quantity which characterises the airborne sound insulation between rooms using spectrum no 2 as defined in BS EN ISO 717-1   – see also C – Spectrum Adaption Term and C_tr.

Building Regulations: Approved Document E – preferred parameter. The higher the figure the better.

Flanking Sound

In sound transmission measurement, flanking is the transmission of sound from the source room to the receiving room by paths other than through the partition under test.

For example, impact sound may be transmitted from one room to another through a timber floor. It may also be through the supporting wall.

Other common mechanisms for flanking transmission include internal leaves of external cavity walls which adjoin separating walls. Suspended ceilings and partition walls. Pipework and ducting.

L_A90T

The A-weighted level of noise exceeded for 90% of the specified measurement period, T. In BS 4142:1997 it is used to define background noise level.

L_Aeq

The equivalent continuous A-weighted sound pressure level – the sound level of a notionally steady sound having the same energy as a fluctuating sound over a specified measurement period, T.

L_nw

Is a laboratory value of impact sound insulation that does not take into account the reverberation time of the receiving room. The lower the figure the better.

L_nTw

A site measured value of impact sound insulation taking into account the reverberation time of the receiving room. The lower the figure the better.

Reverberation time (T)

Reverberation time is defined as the time required for a steady state sound pressure level in an enclosed space to decay by 60 dB, measured from the moment the sound source is switched off.

By averaging multiple decays within the space it can be accurately measured. Consequently it and is one of the most important criteria for rooms involving speech and music.

With field measurement it is often too noisy to be able to record a decay of 60dB as there is not a wide enough dynamic range available. In these situations, T can be evaluated based on a smaller dynamic range (T₂₀) and extrapolated to a decay time of 60dB.

Therefore, if T is derived from the time at which the decay curve first reaches 5dB and 25dB below the initial level, it is labelled T₂₀. If decay values of 5dB to 35dB below the initial level are used, it is labelled T₃₀.

Sound level meter

SLM: an instrument, usually hand-held, designed to measure a frequency-weighted value of the sound pressure level. This should be in accordance with an accepted national or international standard.

The instrument is normally equipped with F, S and possibly time-weightings as an aid to measuring fluctuating sounds.

With suitable circuitry it can also perform frequency analysis, typically either with octave or one-third octave bands.

Sound pressure level

Or sound level L_p is a logarithmic measure of the rms sound pressure of a sound relative to a reference value. It is measured in decibel (dB).
L_p = 20 log (p/po) dB re 20 µPa for air.