To compare sound insulation properties you need to take into account the area of the dividing partition/wall, as well as the volume and sound absorption properties of the receiving room. To do this, measurements are normalized to a reference absorption value or standardized reverberation time.

The absorption and reverberation time are mathematically related so if the reverberation time is measured in the receiving room then both procedures are catered for.

A single number to present the results and compare products would be useful, this is where the weighted term comes in.

*Sound Insulation* is also known as the *Sound Reduction Index* and *Transmission Loss*, but should not be confused with the IEC 801-23-39 Transmission Propagation Loss Definition.

*See also • normalized • standardized and weighted*

Related Terms - listed alphabetically

Apparent Sound Reduction Index (R')

*C and Ctr* : under spectrum adaption terms

At a certain frequency and angle of incidence, the bending oscillation of the partition will be amplified and the acoustic energy will be transmitted through the partition almost without attenuation.

**Ctr** the traffic spectrum can be added to the weighted standardized level difference (DnT,w), to take account of low frequency traffic noise.

**D : level difference
Dn : normalized level difference
DnT : standardized level difference
DnT,w : weighted standardized level difference
DnT,w + Ctr : weighted standardized level difference with spectrum adaption term Ctr
Dw** : weighted level difference

See also • normalized • standardized • weighted and spectrum adaption terms

**Flanking** is always present, except in the 'ideal' acoustics laboratory. In practice the sound insulation is often limited by the flanking transmission.

● Note: the standardized impact sound source is a tapping machine specified in ISO 140/6, which causes hammers whose effective mass each is 0,5 kg to fall 40 mm at the rate of 10 impacts per second.

See also • other impact levelsD = L1 - L2

L1 = average sound pressure level in the source room

L2 = average sound pressure level in the receiving room

See also • other level differences

**L** : average sound pressure level - in a room.

**Li** : impact sound pressure level - in a room

**Ln** : normalized impact sound pressure level - laboratory measurement.

**L'n** : normalized impact sound pressure level - field measurement.

**LnT** : standardized impact sound pressure level - laboratory measurement.

**L'nT** : standardized impact sound pressure level - field measurement.

**Ln,w** : weighted normalized impact sound pressure level - laboratory measurement.

**LnT,w** : weighted standardized impact sound pressure level - based on laboratory measurement of LnT.

**L'nT,w** : weighted standardized impact sound pressure level - based on field measurement of L'nT.

See also • normalized • standardized and weighted

Mass Law provides a good working rule to predict the airborne sound insulation of a partition up to the region of the critical frequency and the coincidence effect

*See also • standardized and weighted*

Ln = Li + 10 lg (A/Ao) dB

where :

A = measured equivalent sound absorption area in the receiving room

Ao = reference absorption area.

**Field Measurements** : in all cases where it is uncertain whether results are obtained without flanking transmission the *normalized impact sound pressure level should be denoted by L'n* - a field measurement. The Normalisation formulae for Ln above also applies for L'n.

See also • other impact levels

Dn = D - 10 lg (A/Ao)

where :D = level difference in dB

A = equivalent sound absorption area of the receiving room in square metres

Ao = reference absorption area in square metres (10 m²)

● Note: for dwellings, reference reverberation time is 0.5 seconds.

See also • other level difference

**R** : sound reduction index - laboratory measurement.

**R'** : apparent sound reduction index - field measurement.

**Rw** : weighted sound reduction index - laboratory measurement.

**R'w** : weighted apparent sound reduction index - field measurement.

See also • normalized • standardized and weighted

**Airborne Sound Insulation** : expressed by a single value, Dn,t,w, Rw or R'w etc..

**Impact Sound Insulation** : expressed by a single value, Ln,w or L' n,w etc..

R = L1 - L2 + 10 lg (S/A) dB

where:L1 : average sound pressure level in the source room

L2 : average sound pressure level in the receiving room

S : area of the test specimen (m

A : equivalent sound absorption area of the receiving room

*Sound Reduction Index* is also known as the *Sound Insulation* and the *Transmission Loss*, **but should not be confused with the IEC 801-23-39 Transmission Loss Definition.**

See other • sound reduction indexes

See also • the IEC Definition of Transmission Loss

Other types of average and averaging in acoustics.

**The spectrum adaptation terms C and Ctr** may be used to take into account different source spectra as indicated in the standard.

C : A-weighted
pink noise spectrum.

Ctr : A-weighted urban traffic noise
spectrum.

Ctr : can also be added to
DnT,w or
Rw to take into account low frequency noise

Ao is the equivalent sound absorption area in square metres

V is the volume of the receiving room in cubic metres.

*See also • normalized and weighted*

LnT = Li - 10 lg (T/To) dB

where :

T = measured reverberation time in seconds

To for dwellings = 0.5 seconds.

See also • other impact levels

DnT = D + 10 lg (T/To)

where:D = level difference

T = reverberation time in the receiving room

To = reference reverberation time, 0.5 seconds for dwellings.

See also • other level-differences

To evaluate the levels in one-third octave bands the reference curve is moved in 1 dB steps towards the measured curve, until the average unfavourable deviation is not more than 2.0 dB. **

An unfavourable deviation at any frequency occurs when the measurement value exceeds the reference value in the case if impact levels, or is less than the reference value in the case or airborne measurements. Only the unfavourable deviations are taken into account.

The **single rating number** in decibels, is now the reference curve value at 500 Hz.

** 32 dB is often quoted as the unfavourable deviation. This is the total deviation across the 16 one-third octave bands ... 32/16 = 2.0 dB average deviation

See also • normalized and standardized

See also • other sound reduction indexes

The Dw value is where the curve meets the 500 Hz curve and the unfavourable deviation is 32 dB. Dw will be identical to DnT,w when T = 0.5 seconds.

See also • other level differences

There is no flanking (indirect) transmission loss, so only the element under test needs to be considered.

See also • other impact levels

Laboratory measurements - so Rw may be used to compare building elements.

See also • other sound reduction indexes

See also • other impact levels

The results will include flanking transmission so the test is for the total transmission between the rooms, not just the partition.

See also • other level differences

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