Sound Insulation : Definitions, Terms, Units and Measurements

Sound Insulation is the ability of building elements or structures to reduce sound transmission

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.

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 Test Equipment.

Airborne Sound Insulation Index : Ia” : former name for Weighted Apparent Sound Reduction Index : R’w

Apparent Sound Reduction Index : R’ : field measurements of Sound Reduction Index include Flanking and any other ‘on-site’ acoustic limitations.

R’ = D + 10 lg S/A (dB)

where
D = Level Difference
S = area of the test specimen (m2)
A = Equivalent Sound Absorption area of the receiving room

C and Ctr : Spectrum Adaption Terms

Coincidence Effect : Mass Law provides a good working rule to predict the airborne Sound Insulation of a partition up to the region of theCritical Frequency and the coincidence effect. The coincidence effect occurs when the Wavelength of the sound in air is the same as the bending waves in the partition.

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.

Critical Frequency : the lowest frequency at which the Coincidence Effect occurs in a partition is obtained when the incident sound waves graze the partition (parallel with it). This frequency is called the critical frequency in building acoustics.

Ctr : the traffic A-weighted spectrum can be added to the Weighted Standardized Level Difference : DnT,w to take account of low frequency traffic noise.

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Flanking Transmission : flanking is the transmission of sound from a source room to a receiving room by paths other than through the separating partition. For example, impact sound may be transmitted from one room to another through a common timber floor. Other common mechanisms for flanking transmission include suspended ceilings, pipework, ducting, etc.

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

Ia” : Airborne Sound Insulation Index

Impact Noise Rating : INR : a single number rating on the effectiveness of a floor construction.

Impact Sound : the sound produced by the collision of two solid objects. Typical sources are footsteps, dropped objects, etc., on an interior surface (wall, floor, or ceiling) of a building.

Impact Sound Insulation is expressed by a single value Ln,w or L’ n,w

Impact Sound Pressure Level : Li : the average Sound Pressure Level in a specific frequency band in the receiving room when the floor under test is excited by a Standardized Impact Sound Source.

Level Difference : D : airborne sound insulation – field measurements. The difference in the space and time averaged Sound Pressure Levels.

D = L1 – L2

L1 = average Sound Pressure Level in the source room
L2 = average Sound Pressure Level in the receiving room

L : Average Sound Pressure Level : in a room.

Li : Impact Sound Pressure Level : the average Sound Pressure Level in a specific frequency band in the receiving room when the floor under test is excited by a Standardized impact sound source.

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.

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Mass Law : a doubling in Mass or Frequency results in a 6 dB increase in the sound insulation of a single leaf partition over a defined frequency range.

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 theCoincidence Effect

Normalized

Normalized Impact Sound Pressure Level : Ln : laboratory measurement.

Ln = Li + 10 lg A/A0 dB

where:
A = measured Equivalent Sound Absorption area in the receiving room
A0 = Reference Absorption area.

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.

Normalized Impact Sound Pressure Level : L’n : field measurement.

The Normalisation formulae for Ln directly above also applies for L’n.

Normalized Level Difference : Dn : airborne sound transmission. The sound insulation index measured under field conditions, between ‘real’ rooms and therefore includes effects due to Flanking, different room sizes and other on-site considerations.

Dn = D – 10 lg A/A0

where
D = level difference in dB
A = Equivalent Sound Absorption area of the receiving room in square metres
A0 = Reference Absorption area in square metres (10 m²)

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.

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Sound Insulation : the ability of a building element or building structure to reduce the sound transmission through it. The sound insulation is measured at different frequencies, normally 100-3150 Hz.

Airborne sound insulation is expressed by a single value, Dn,t,w, Rw or R’w.

Impact sound insulation is expressed by a single value Ln,w or L’ n,w

Sound Reduction Index : R : the measured quantity which characterises the sound insulating properties of a material or building element in a stated frequency band – laboratory measurement.

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 (m2)
A: Equivalent Sound Absorption area of the receiving room

Sound Transmission Class : STC : American single number rating of a partition’s isolation value based on laboratory measurement. Results may not be compatible with Rw as a different range of frequencies is used.

Sound Transmission Loss : STL : a measure of sound insulation provided by a structural configuration. Expressed in decibels, it is 10 times the logarithm to the base 10 of the reciprocal of the sound transmission coefficient of the configuration.

Spatial Averaging : taking measurements at various positions and averaging the results. Mandatory in sound insulation measurements and recommended anywhere multiple reflections are present. See other types of Averaging.

Spectrum Adaptation Terms : C and Ctr : The single number rating method defined in BS EN ISO 717 uses a standard reference curve to determine the weighted value of airborne sound insulation.

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

C is an A-weighted Pink Noise spectrum.
Ctr is an A-weighted urban traffic noise spectrum.
Ctr can also be added to DnT,w or Rw to take into account low frequency noise

Standardized

Standardized Impact Sound Pressure Level : LnT : the impact Sound Pressure Level in a stated frequency band, corrected for the standardized reverberation time of 0.5 seconds. Laboratory measurement.

LnT = Li – 10 lg T/T0 dB

where:
T = measured Reverberation Time in seconds
T0 for dwellings = 0.5 seconds.

Standardized Impact Sound Pressure Level : L’nT : the impact Sound Pressure Level in a stated frequency band, corrected for the standardized reverberation time of 0.5 seconds. Field measurement, written L’nT to differentiate between LnT

Standardized Level Difference : DnT : airborne sound transmission. Similar to the Dn, 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.

DnT = D + 10 lg T/T0

where:
D = level difference
T = reverberation time in the receiving room
T0 = reference Reverberation Time, 0.5 seconds for dwellings.

Transmission Loss : TL : alternative name for sound reduction index, see R and R’

Weighted

Weighted Apparent Sound Reduction Index : R’w : a single number rating of airborne sound insulation between rooms, over a range of frequencies – field measurement.

Weighted Level Difference : Dw : single Integer number found by comparing the measured spectrum with the ‘standard’ curves for airborne and impact insulation.

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.

Weighted Normalized Impact Sound Pressure Level : Ln,w : Laboratory measurement of sound performance of a building element floor).

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

Weighted Sound Reduction Index : Rw : a single-number quantity which characterises the airborne sound insulation of a material or building element over a range of frequencies.

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

Weighted Standardized Impact Sound Pressure Level : LnT,w : single number quantity used to characterize the Impact Sound Insulation of floors, based on laboratory measurement of LnT.

Weighted Standardized Impact Sound Pressure Level : L’nT,w : single number quantity used to characterize the Impact Sound Insulation of floors, based on field measurement of L’nT.

Weighted Standardized Level Difference : DnT,w : field measurement of airborne sound transmission. Standardized to a standard 0.5 seconds reverberation time and weighted to provide a single figure value.

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

Weighted Standardized Level Difference with Spectrum Adaption Term : DnT,w + Ctr : a single quantity which characterises the airborne sound insulation between rooms using the Ctr Spectrum Adaption Term defined in BS EN ISO 717.

Definitions Specific to Sound Insulation Measurements

Normalized : The measured sound level difference and impact sound levels in similar dwellings may vary if the sound absorbing materials (soft furnishings) are different.

If the receiving room levels are normalized by adding 10 lg S/A – the Sabine Equation, then room to room variations in the ‘field’ will not influence the results.

Standardized : If the reverberation times differ between similar rooms in the field (actual dwellings), then there will be a related change in the sound levels measured in the receiving rooms.

Standardizing the impact and airborne sound pressure levels to a reverberation time of T = 0.5 s is equivalent to standardising the results to an equivalent absorption area of A0 = 0.32 V

where:
A0 is the equivalent absorption area in square metres
V is the volume of the receiving room in cubic metres.

Weighted : to establish a Single Figure Rating descriptor the Normalized or Standardized levels are compared to the Reference Curvespublished in BS EN ISO 717 for airborne or impact noise.

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