Acoustic FAQ’s


How does insulation provide acoustic benefits?

The heavier the mass of a layer of plasterboard, and the greater the cavity depth between two partitions, the better the sound isolation provided. Insulation within the cavity will also provide excellent sound isolation.

The “mechanical connection,” in other words the studs used between the two partitions is also important.

Twin studs typically provide the best acoustic performance, followed by staggered studs, then timber or steel studs.

The indicative performance of each stud type is as follows:

  • 2 to 4 dB increase in Rw for timber stud partitions;
  • 5 to 7 dB increase in Rw for steel stud partitions;
  • 8 dB (insulation on one side) to 14 dB (insulation on both sides) increases in Rw for twin steel and timber stud construction; and
  • 25 dB increase (insulation both sides) for Partiwall.

Depending on the system, it is best to fill the partition cavity at least 75%, and oftentimes as high a 100%.

Does acoustic insulation completely stop noise transfer?

Not quite. Whilst insulation significantly reduces sound transfer, typical sources of sound leakage will impact the overall acoustic atmosphere of a building.

Typical sources of sound leakage include the following:

  • Lightweight panels above doors;
  • Doors;
  • Air leaks through gaps, cracks or holes;
  • Sound transmission via suspended ceiling partitions;
  • Common ventilation systems without sound absorbent treatment;
  • Common floor ducts;
  • Electrical outputs and service pipes;
  • Lightweight mullions or mullion/partition closers;
  • Continuous runs of ducting; and
  • Partition performance.

It is important to seal all possible paths for sound leakage, or “flanking noise.” Noise not only passes through a shared wall, but finds “flanking paths” through poor joints between the wall and floor, poorly sealed penetrations, ceiling spaces, floor spaces, ductwork, and even other walls.

How can I reduce leakage/flanking?

There are a variety of ways to prevent sound flanking. All floor/wall junctions must be sealed along the perimeter with a flexible fire/acoustic sealant. Skirting alone is not sufficient to seal gabs. A correctly installed cornice can also provide an acoustic barrier to minimize flanking, as long as an airtight seal is achieved and maintained.

If the gaps around the wall perimeter are not sealed correctly, the wall system would be degraded. For example, a Rw=62 dB wall system would be reduced to:

  • Rw = 50 dB if there is a 0.01 mm gap left around the perimeter*; and
  • Rw = 25 dB if there is a 2 mm gap left around the perimeter*.

* Based on a 6mx6m wall area.

The purpose of an acoustic system for walls or ceilings is to reduce the transmission of sound, both airborne and structure-borne. This can be accomplished by sealing all leaks that would otherwise permit the transmission of airborne sound, and through the use of proper construction at wall and floor intersections, prevent the transmission of structure-borne sound.

What is a ‘Weighted Sound Reduction Index’?

A Weighted Sound Reduction Index or Rw, is the rating used to measure the level of sound insulating abilities of walls, floors, windows and doors. It is expressed in decibels (dB), and is used for a partition or single component only.

The higher the Rw figure, the better the sound isolation that is provided. It must be noted, however, that the Rw figure is not reliable for sounds which contain a significant amount of low frequencies – for example music or sounds emanating from home theatre systems.

What does Rw+Ctr refer to?

The end results for the Weighted Sound Reduction laboratory tests are subject to a “spectrum adaption term” (CTR). This accounts for distinct types of noise sources, such as traffic, or music which has a large low-frequency component.

This is always a negative number, typically between -1 to -15. A large negative number means a large dip in sound insulation performance in relation to the performance in other frequency bands. A large negative number does not necessarily mean poor low-frequency performance, however.

Rw + Ctr is Rw with the addition of a low frequency sound correction factor Ctr (always a negative number remember). Rw + Ctr is used because of the increase in low frequency sound sources such as surround sound systems, drums or bass guitars, and of course traffic or aircraft noise. Two walls can have the same Rw rating, but have different resistance to low frequency sound, thus a different Rw + Ctr.

What does a Noise Reduction Coefficient (NRC) represent?

The Noise Reduction Coefficient is a scalar representation of the amount of sound energy absorbed upon striking a particular surface. An NRC of 0 indicates perfect reflection; an NRC of 1 indicates perfect absorption. Because of the formula used for the NRC, the coefficient is not a percentage, and therefore values larger than 1 are quite common.

What does αw represent?

αw is calculated in accordance with ISO 11654 using the sound absorption coefficient αp values at standard frequencies, and comparing them with a reference curve.

The practical sound absorption coefficient αp is the average of the three 1/3 octave αs values centred on the octave band frequency and rounded in steps of 0.05. The reference curve is shifted downwards in increments of 0.05. This shifts the reference curve to the point where the sum of the negative deviations from the measured values ≤ 0.10 in relation to the values of the reference curve. If this is the case, then the value of 500 Hz on the reference curve is theαw value.

αw is used by all suspended ceiling suppliers in Europe because it is the method which has been adopted as the norm for CE marking of suspended ceilings.

Sound Absorption
αwAbsorption class
(as per VDI
Approximate NRC
A0,90; 0,95; 1,00Extremely AbsorbingNRC >=0,75
B0,80; 0.85Extremely AbsorbingNRC>=0,75
C0,60; 0,65; 0,70;
Highly Absorbing0,5 <=NRC <0,75
D0,30; 0,35; 0,40;
0,45; 0,50; 0,55
Absorbing0,5 <=NRC <0,75
E0,15; 0,20; 0,25Hardly Absorbing0,25 <=NRC <0,5
Not Classified0,05; 0,10Reflecting

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