Acoustic Glossary

Sound Insulation

Sound Insulation is the ability of building elements or structures to reduce sound transmission and is measured over a range of frequencies, normally 100 ~ 3150 Hz. On-site results often fall short of laboratory measurements due to the flanking transmission.

To compare sound insulation properties, the area of the dividing partition, the volume and the sound absorption properties of the receiving room, need to taken into account. Fortunately the sound absorption and reverberation time are mathematically related, so if the reverberation time is measured in the receiving room then both properties are established and the sound insulation measurements are standardised to the reference reverberation time and the reference sound absorption value.

To compare products, a single 'insulation value' would be useful - see the weighted sound insulation procedure and the various D • L and R - single value insulation terms.

Sound Insulation should not be confused with propagation loss.

See also • level difference • normalised sound insulation • sound reduction index • standardised sound insulation • transmission loss • weighted sound insulation procedure

Acoustical Insulation Material

Acoustical Insulation Material Definition (IEC 801-31-45) material used in insulating against the transmission of sound

Airborne Sound Insulation

Airborne Sound Insulation is ranked by a single value, for example D and R single number insulation levels.

Average Sound Pressure Level in a Room

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

Bending Oscillation

Bending Oscillation at a certain frequency and a certain angle of incidence of the incident sound waves, the sound energy will be transmitted through the partition, almost without attenuation

Coincidence Effect

Coincidence Effect occurs when the wavelength of the sound in air is the same as the bending waves in the partition.

Critical Frequency

Critical Frequency is the lowest frequency at which the coincidence effect occurs in a partition and the incident sound wave travels parallel to the surface of the partition.

Field Measurements refer to non-laboratory sound insulation testing

Field normalised impact sound pressure level (L'n), the impact sound pressure level is normalised for a standard absorption area in the receiving room.
L'n = Li + 10 lg (A/Ao) dB
where :
A = measured equivalent sound absorption area in the receiving room
Ao = reference absorption area.

Flanking Sound

Flanking is the transmission of sound 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, pipe work, ducting, electricity cables etc., passing through the partition. The latter may not meet the flanking definition, but they are common and have the same effect.

Flanking sound is always present, except in the 'ideal' acoustics laboratory, so in practice the sound insulation is often limited by the flanking sound and associated sources.

Impact Noise Rating

Impact Noise Rating (INR) a single number rating on the effectiveness of a floor construction to reduce the impact sound.

Impact Sound

Impact Sound, the structure-borne noise produced by the collision of two solid objects within a building. Typically footsteps or objects dropped onto an interior surface or floor. Tested using a tapping machine.

Impact Sound Insulation Level

Impact Sound Level, expressed by single values, see the Ln • L'n • L'nT • Ln,w • LnT,w • L'nT,w levels.

Impact Sound Pressure Level

Impact Sound Pressure Level (Li), the average sound pressure levels in the receiving room during impact noise testing

Impact Testing - sound insulation

Impact Testing, the method of measuring the impact sound transmission response of a building using a tapping machine (also known as a footstep machine).

See also the impact sound pressure level definition.

Sound Insulation Level Difference

Level Difference (D) also known as the sound level difference, is the difference between the sound pressure level in the source room and the sound pressure level in the receiving room, measured and averaged in several positions in both rooms.
D = L1 - L2 where;
L1 = average sound pressure level in the source room
L2 = average sound pressure level in the receiving room

See also the other D single value insulation levels

Sound Insulation Mass Law

Mass Law provides guidance for predicting the airborne sound insulation of a partition up to the region of the critical frequency and the coincidence effect

Mass Law in the mass controlled region, the sound insulation of a single leaf wall or partition, increases by up to 6 dB, for each doubling of the frequency, or doubling the mass (thickness of the wall) at a given frequency

Normalised Impact Sound Pressure Level

Normalised Impact Sound Pressure Level (Ln) - laboratory measurements, the impact sound pressure level is normalised for a standard absorption area in the receiving room.

Ln = Li + 10 lg (A/Ao) dB
where :
A = measured equivalent sound absorption area in the receiving room
Ao = reference absorption area.

Normalised Level Difference

Normalised Level Difference (Dn), the level differences measured on-site vary due to the different room sizes encountered. Normalising the results to a reference absorption minimises these differences. Dn is also known as normalised noise reduction.
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)

See also the other D single value insulation terms

Normalised Sound Insulation

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

See also standardised sound insulation • weighted sound insulation

Sound Isolation Between Rooms

Sound Isolation Between Rooms 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. Also known as the level-difference

Sound Reduction Index - sound insulation

Sound Reduction Index (R), the laboratory measurement of the sound insulating properties of a material or building element in a stated frequency band is given by the formula;
R = 10 lg (W1/W2 in decibels, where
W1 is the sound power incident on the element under test and
W2 is the sound power on the other side.

For laboratory measurements using sound pressure, the sound reduction index is calculated using: 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 is also known as the sound insulation and the transmission loss, but should not be confused with the transmission loss.

Sound Transmission Class

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

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

Spatial Averaging, taking measurements at various positions and averaging the results. Required 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 averaging in acoustics.

Spectrum Adaption Terms

Spectrum Adaption 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 adaption terms C and Ctr may be used to take into account different source spectra as indicated in the standard.

Standardised Impact Sound Pressure Level

Standardised Impact Sound Pressure Level (LnT) is the impact sound pressure level (Li), standardised for the reference reverberation time of 0.5 seconds - laboratory measurements:
LnT = Li - 10 lg (T/To) dB
where :
T = measured reverberation time in seconds
To for dwellings = 0.5 seconds.

Normalised Impact Sound Pressure Level (L'n) - field measurements, are normalised as above.

See also other L - single number insulation levels

Standardised Level Difference

Standardised Level Difference (DnT), the difference in sound level between a pair of rooms, in a stated frequency band and standardised to the reference reverberation time of 0.5 seconds.
DnT = D + 10 lg (T/To)
where:
D = the level difference
T = the reverberation time in the receiving room
To = the reference reverberation time, 0.5 seconds for dwellings.

Standardised level difference takes account of all sound transmission paths between the rooms and provides a straightforward correlation to the subjective impression of the airborne sound insulation.

See also other D - single number insulation levels

Standardised Sound Insulation

Standardised Sound Insulation, 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. Standardising the impact and airborne sound pressure levels to a reference 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, normalised sound insulation • weighted sound insulation

Tapping Machine

Tapping Machine, a standardised impact sound source, used to carry out impact sound level measurements in accordance with the sound insulation regulations and standards - both on-site and laboratory measurements, see the impact sound pressure level definition

Transmission Loss

Transmission Loss (TL) a term widely used when discussing sound insulation of a partition. Also known as sound insulation and sound reduction index, but should not be confused with the propagation loss or the transmission loss..

Weighted Sound Insulation Procedure

Weighted Sound Insulation Procedure, to establish a single figure rating descriptor, the normalised or standardised sound insulation 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.

See also, normalised sound insulation • standardised sound insulation

Weighted Apparent Sound Reduction Index

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 R - single number insulation levels

Weighted Level Difference

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.

According to BS 8233, weighted level difference is used to characterize the insulation between rooms in building as they are. Values cannot normally be compared with measurements made under other conditions.

See also other D - single number insulation levels

Weighted Normalised Impact Sound Pressure Level

Weighted Normalised 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 L - single number insulation levels

Weighted Sound Reduction Index

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, based on laboratory measurements.

Weighted Sound Reduction Index (R'w), based on the field measurements of Rw and originally known as the apparent sound reduction index (la'').

See also other R - single number insulation levels

Weighted Standardised Impact Sound Pressure Level

Weighted Standardised Impact Sound Pressure Level (LnT,w), a single number quantity used to characterise the impact sound insulation of floors, based on laboratory measurement of LnT.

See also other L - single number insulation levels

Weighted Standardised Level Difference

Weighted Standardised Level Difference (DnT,w) single number value of the airborne sound insulation between room, used to characterize the sound insulation between rooms in buildings.

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

See also other D - single number insulation levels