## Sound Insulation • definitions • terms • 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 etc..

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 sound absorption value or standardized reverberation time.

The sound 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 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 transmission loss.

See also • normalized • standardized 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

*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 adaption term 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

**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**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 Transmission** 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)**, a

**single number rating**on the effectiveness of a floor construction to reduce the impact noise from below.

**Impact Sound**is 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 sound pressure level 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

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 y 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 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**, 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 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** and in all cases where it is uncertain whether results are obtained without flanking the normalized impact sound pressure level should be denoted by 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

**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. 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

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

*Single Figure Ratings*under the weighted levels

**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)**is 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 (m

^{2})

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 propagation transmission loss.

**Apparent Sound Reduction Index (R')**, field measurements of the *sound reduction index*, include 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 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 may 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 = the level difference

T = the reverberation time in the receiving room

To = the 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. Also known as sound insulation and sound reduction index, but 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 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)**, 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.