Acoustic Glossary


 

Sound Insulation Terms, Definitions, Units, Measurements ...


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 flanking sound.

To compare noise 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 standardized to the reference reverberation time and the reference sound absorption value.

A single number, to present the results and compare products would be useful, this is where the weighted sound insulation terms 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 propagation loss.

See also • normalized sound insulationstandardized sound insulation and weighted sound insulation


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, DnT,w and Rw for example

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

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. Also known as the energy-average sound pressure level in a room.

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

C and Ctr under spectrum adaption terms


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

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.


Dlevel difference - field measurements.
Dnnormalized level difference.
DnTstandardized level difference.
DnT,wweighted standardized level difference.
DnT,w + Ctrweighted standardized level difference + spectrum adaption term.
Dwweighted level difference

See also • normalized sound insulationstandardized sound insulationweighted sound insulation and spectrum adaption terms


Energy-average sound pressure level in a room
Flanking Sound 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, electricity cables ...

Flanking Sound is always present, except in the 'ideal' acoustics laboratory, so 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 apparent sound reduction index (R'w)

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

Impact Sound is the structure-borne noise produced by the collision of two solid objects, typically footsteps, dropped objects, etc., on an interior surface (floor or ceiling), within a building.

Impact Sound Insulation Levels expressed by single values, see Ln ..... L'nT,w.

Impact Sound Pressure Level (Li), the average sound pressure levels in the receiving room during impact noise testing
 
Impact Sound Pressure Level Definition 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 BS EN ISO 140-7, which causes hammers whose effective mass each is 0,5 kg to fall 40 mm at the rate of 10 impacts per second.

Impact Testing, a method of measuring the frequency response function of a structure by hitting it with a calibrated impact hammer and measuring the system's response.

The impact hammer has a transducer to measure the input force pulse while the response is typically measured using an accelerometer. The impact imparts a force pulse that excites the structure over a broad frequency range.

See also • other impact levels


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
 
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

Laverage sound pressure level - in a room.
Liimpact sound pressure level - in a room.


Lnnormalized impact sound pressure level - laboratory measurement.
L'nnormalized impact sound pressure level - field measurement.
L'nTstandardized impact sound pressure level - field measurement.
Ln,wweighted normalized impact sound pressure level - laboratory measurement.
LnT,wweighted standardized impact sound pressure level - based on laboratory measurement of LnT.
L'nT,wweighted standardized impact sound pressure level - based on field measurement of L'nT.

See also • normalized sound insulationstandardized sound insulation and weighted sound insulation


Mass Law provides guidance 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


Normalized Sound Insulation :

Normalized 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 normalized by adding 10 lg (S/A), then any room to room variations in the 'field' will not influence the results.

See also • standardized sound insulation and weighted sound insulation


Normalized Impact Sound Pressure Level (Ln), impact sound pressure level normalized for a standard absorption area in the receiving room - laboratory measurements.
Ln = Li + 10 lg (A/Ao) dB
where :
A = measured equivalent sound absorption area in the receiving room
Ao = reference absorption area.

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.

Normalized Impact Sound Pressure Level (L'n) - field measurements, are normalized as above but signified as L'n.

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

See also • other impact levels


Normalized Level Difference (Dn), the level differences (D) measured on-site, vary due to the different room sizes encountered. Normalizing the results to a reference absorption minimises these differences. Dn is also known as normalized 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)
 
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 the reference reverberation time is 0.5 seconds.

Rsound reduction index - laboratory measurement.
R'apparent sound reduction index - field measurement.
Rwweighted sound reduction index - laboratory measurement.
R'wweighted-apparent sound reduction index - field measurement.

See also • normalized sound insulationstandardized sound insulation and weighted sound insulation


Reverberation Time

Single Figure Ratings under weighted sound insulation levels

Sound Level Difference under level difference

Sound Isolation Between Rooms 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 (R), the laboratory measurement of the sound insulating properties of a material or building element in a stated frequency band is gven 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

Apparent Sound Reduction Index (R'), field measurements of the sound reduction index, are calculated using the L1 - L2 + 10 lg (S/A) formula detailed above and include flanking and any other 'on-site' acoustic limitations.

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 propagation transmission loss.

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 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 added to DnT,w or Dw or Rw takes into account low frequency noise

Standardized Sound Insulation :

Standardized 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. Standardizing 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 • normalized sound insulation and weighted sound insulation


Standardized Impact Sound Pressure Level (LnT) is the impact sound pressure level (Li), standardized 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.

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

See also • other impact levels

Standardized Level Difference (DnT), difference in sound level between a pair of rooms, in a stated frequency band and standardized 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.

Standardized 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.


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 propagation transmission loss..

Transmission Loss 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


Weighted Sound Insulation Procedure :

To establish a single figure rating descriptor the normalized or standardized 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.

** 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 sound insulation and standardized sound insulation


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.

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.

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, 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'').


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

See also • other impact levels


Weighted Standardized 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.

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.
 
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