Acoustic Glossary


 

Sound Insulation and related terms, definitions, units etc., ..


Sound Insulation

is the ability of building elements or structures to reduce sound transmission. Field results often fall short of laboratory measurements ...
 

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 Definition

IEC 801-31-39, of a partition, for a specified frequency band, difference in decibels between the average sound pressure levels in the reverberant source and receiving rooms, plus ten times the logarithm to the base ten of the ratio of the area of the common partition to the total sabine absorption in the receiving room

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 • normalizedstandardized and weighted


Related Terms - listed alphabetically

Acoustical Insulation Material Definition

IEC 801-31-45, material used in insulating against the transmission of sound

Airborne Sound Insulation

expressed by a single values, Dn,t,w, Rw, and R'w for example

Airborne Sound Insulation Index (Ia'') : former name for the weighted apparent sound reduction index (R'w)

Apparent Sound Reduction Index (R')

Average Sound Pressure Level in a Room Definition

IEC 801-31-36, in decibels, ten times the common logarithm of the ratio of the space and time average of the sound pressure squared to the square of the reference sound pressure, the space average being taken over the entire room with the exception of those parts where the direct radiation of any sound source or the near field of the boundaries is of significant influence
Note : depending on the size of the room it is 'normal' to measure in at least 6 locations, see spatial averaging.

C and Ctr : under 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 the critical 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 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 • normalizedstandardizedweighted and spectrum adaption terms


Field Normalized Impact Sound Pressure Level Definition

IEC 801-31-43, for a specified frequency band, impact sound pressure level in decibels minus ten times the logarithm to the base ten of the ratio of the reverberation time in the receiving room to the reference reverberation time of 0.5 second

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

Flanking Transmission Definition

IEC 801-31-40, transmission of sound from a source room to an adjacent receiving room but not via the common partition

Ia'' (airborne sound insulation index) : former name for the weighted apparent sound reduction index (R'w)

Impact Noise Rating

(INR) : 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 : L' n,w : etc.

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
 

Impact Sound Pressure Level

IEC 801-31-41, the average sound pressure level in a specified frequency band in the receiving room when the floor under test is excited by the standardized impact sound source

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 levels

Level Difference

(D) airborne sound insulation - field measurements. The difference in the averaged sound pressure levels measured, in several positions in the two rooms.

D = L1 - L2
L1 = average sound pressure level in the source room
L2 = average sound pressure level in the receiving room

Level Difference Definition

IEC 801-31-37, in decibels, difference in space and time average sound pressure levels produced in two rooms by one or more sound sources in one of them and is also known as the sound insolation between rooms

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 • normalizedstandardized and weighted


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 the coincidence effect


Normalized

: the measured sound level differences and impact sound levels in similar dwellings may vary if the sound absorbing materials (soft furnishings) are different. However if the receiving room levels are normalized by adding 10 lg (S/A), then any room to room variations in the 'field' will not influence the results.

See also • standardized and weighted


Normalized Impact Sound Pressure Level

(Ln) - laboratory measurement.

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.

Normalized Impact Sound Pressure Level Definition

IEC 801-31-42, for a specified frequency band, average sound pressure level in a receiving room in decibels due to the standardized impact sound source, plus ten times the logarithm to the base ten of the ratio of the sabine absorption in the receiving room to the reference absorption of ten decibel metre-squared

See also • other impact levels


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/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)
 

Normalized Level Difference Definition

IEC 801-31-38, in decibels, level difference between rooms plus ten times the common logarithm of the ratio of the reverberation time in the receiving room to the reference reverberation time
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 • normalizedstandardized and weighted


Single Figure Rating under weighted

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 : 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..


Sound Isolation between Rooms : under level difference

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 Reduction Index Definition

IEC 801-31-39, of a partition, for a specified frequency band, difference in decibels between the average sound pressure levels in the reverberant source and receiving rooms, plus ten times the logarithm to the base ten of the ratio of the area of the common partition to the total sabine absorption in 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.

Apparent Sound Reduction Index (R')

, field measurements of the sound reduction index, includes flanking and any other 'on-site' acoustic limitations.

See other • sound reduction indexes


Sound Transmission Class

(STC) American single number rating of a partition's isolation value based on laboratory measurement of the STL - sound transmission loss. Results may not be compatible with Rw as a different range of frequencies are used.

Sound Transmission Loss

(STL) a measure of sound insulation expressed in decibels in octave or 1/3 octave bands. Widely used in America, see the STC - sound transmission class entry above.

See also • the IEC Definition of Transmission Loss


Spatial Averaging

taking measurements at various positions and averaging the results. Mandatory in sound insulation measurements and recommended anywhere multiple reflections are present. See also the average sound pressure level in a room definition

Other types of average and averaging in acoustics.


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 : 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


Standardized

: if the reverberation times differ between similar rooms in the field (actual dwellings), 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:

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


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/To) dB
where :
T = measured reverberation time in seconds
To 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.

See also • other impact levels


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/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


Transmission Loss (TL).

a term widely used when discussing sound insulation of a partition, see the Sound Insulation Definition IEC 801-31-39. However it should not be confused with the Propagation Transmission Loss.

Weighted

: to establish a single figure rating descriptor the normalized or standardized levels are compared to the reference curves published 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

See also • normalized and standardized


Weighted Apparent Sound Reduction Index

(R'w) : a single number rating of airborne sound insulation between rooms, over a range of frequencies - field measurement.

See also • other sound reduction indexes


Weighted Level Difference

(Dw) : a single integer number found by comparing the measured spectrum with the 'standard' curves in BS EN ISO 717 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.

See also • other level differences


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.

See also • other impact levels


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.

See also • other sound reduction indexes


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.

See also • other impact levels


Weighted Standardized Level Difference

(DnT,w) : field measurement of airborne sound insulation, standardized to 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.

See also • other level differences

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