Why acoustic flooring can fail soundtesting
For an acoustic floor to deliver on its promise and minimise the risk of noise issues for homeowners and tenants occupying a domestic property, it has to be tested to comply with the Building Regulations relating to ‘resistance to the passage of sound’. Hence why new and converted dwellings have to be tested for levels of sound insulation offered by suitably qualified testers.
An acoustic floor test, also referred to as a sound insulation test, is needed to check that the design of the floor and the combination of materials specified actually performs in-situ. The key reason why this is important is because, even with the best intentions, there is no guarantee that the acoustic floor design will meet the sound reduction targets if installation is not completed in line with the recommendations of the manufacturer or if unforeseen issues come into play as work is undertaken.
Acoustic testing has the check for two types of sound transmission – airborne and impact.
Where separating floors are concerned, we usually think about the test being concerned with impact noise, usually caused by issues like footsteps on a hard floorcovering and furniture being moved. But airborne sound must also be considered to minimise the potential for noise issues resulting from activities such as people talking, playing music or watching TV.
How is an acoustic floor test done?
To conduct a sound test, we have to replicate these types of activities in one room or property and measure what level of sound can be detected in the adjacent room.
Acoustic tests are carried out by qualified acoustic engineers who are accredited to conduct sound testing. They use state of the art equipment, including a Class 1 sound-level meter, to ensure levels of sound transmission can be completed with a high degree of accuracy.
In simple terms, with an airborne sound insulation test, loudspeakers are placed in the ‘source’ room, and these are used to play white or pink noise at a very high amplitude (95 – 110 dB(A)). The acoustic engineer will then go to the adjacent room (with the loudspeaker still on) and take a series of measurements within the ‘receiver’ room. These measurements are adjusted to account for the reverberation time within the room and background noise to increase their accuracy, giving us a figure referred to as DnT,w + Ctr.
Impact sound insulation tests use a ‘tapping’ machine placed on the floor and measurements are made in the room below. As with airborne sound testing, these measurements are adjusted for reverberation time and background noise in the ‘receiver’ room to calculate what is referred to as the L’nT,w.
What performance does the Building Regulations require for acoustic floors?
In England & Wales, Approved Document E – or Part E – sets the minimum performance standards for sound insulation. This applies to new buildings, alterations to pre-existing premises and buildings being converted to flats often classed as material change of use.
Acoustic floors in new build must achieve a minimum DnT,w + Ctr value of 45 dB and in material change of use projects this figure is at least 43 dB. Impact sound reduction is expressed as a maximum figure, which means that in new build projects the L’nT,w level must be no higher than 62 dB and 64 dB for material change of use.
These figures differ from those required under Section 5 of the Technical Standards which apply in Scotland where acoustic insulation levels have always been regarded as more stringent, largely due to a higher proportion of people living in ‘connected’ properties. For example, in new build dwellings, that means a minimum airborne sound level of 56 dB is required.
If the acoustic floor test records figures below the minimum levels for airborne sound and above those for impact sound, the test is declared a ‘fail’ and corrective work will be required ahead of a retest.
What are the most common reasons for acoustic test failures?
There are a wide variety of reasons why acoustic floor tests fail. However, the majority can be attributed to one or more of four areas: quality of the workmanship, incorrect installation of acoustic materials, missing some of the sound insulation products or using inadequate products – including substituting them wrongly, and failure to address the risk of flanking noise.
Poor sealing
Sound behaves like water in that it can leak through any gaps that exist around the edges of the acoustic floor, junctions, or penetrations such as pipes or cables. All these things allow sound to leak through. The risks can be reduced by proper sealing of the acoustic floor, which is why products such as Hush Acoustic & Intumescent Sealant and Hush Seal 20 Perimeter Seal are so important – relatively low cost but essential components within an acoustic floor.
Incorrect product installation
The devil is in the detail when it comes to acoustic products, so it is crucial that they are installed in line with the manufacturer’s recommendations.
Products such as Hush Bar resilient bars or Hush Slab mineral wool sound absorber slabs are extremely effective at preventing sound transmission through floors, ceilings and walls, when used correctly. But, if there is any deviation from the installation methods prescribed, there is no guarantee they will deliver the expected performance.
Incorrect product selection
The acoustic floor will always be doomed to failure if you choose the wrong products to start with. Hence why it is best to seek advice from an acoustic product manufacturer, distributor or acoustic consultant at an early stage.
One common mistake made in acoustic product specification is choosing an acoustic floor panel that lacks the required density and mass to block sound waves. This is a particular issue in lightweight construction such as timber frame new build housing, a type of construction that has very different acoustic properties compared to a stone built house, for example.
Use of the wrong products, missing products or poor quality substitutes
Do not assume that all products will do largely the same thing. Take, for example, an acoustic infill such as Hush Slab, which is often used as part of an acoustic floor between timber joists.
This is a mineral wool composition optimised for acoustic insulation, meaning it is not the same as similar looking products that may be designed to achieve thermal insulation goals. The thickness of these products is very important too – if the specification requires an acoustic absorber slab with a 100mm thickness, using a 50mm slab is never going to achieve the same performance.
The same applies to rubber barrier mats which are commonly used for reducing impact sound through absorption as well as by adding density. Hush Mat, for example, is available in thicknesses of 12mm and 15mm – both of which offer different levels of acoustic performance.
Finally, remember that every material or component specified has been included in the acoustic floor design for a reason. If one is omitted due to it being unavailable at the time, that could compromise the acoustic floor’s performance.
Flanking
Many sound test failures are due to flanking noise. This results from the structural elements, such as the timber joists and sub-floor, being physically connected to the floor surface. In the case of a ceiling, this could be the result of attaching the plasterboards directly to the floor joists without any attempt of de-coupling.
But it can occur in more subtle ways through bridging. This may be a floating floor being in contact with skirting boards or where mechanical fixings are used to screw down a floating floor. Connections like these provide very effective sound transmission paths that bypass the main insulation materials.
Poorly detailed junctions between walls and floors or between different structural elements can also result in flanking.
Unforeseen consequences of ceiling lights
The highest performing acoustic floor designs treat both the floor above the structure and the ceiling below it. Hence, the acoustic ceiling part of the design must follow the same principles as the floor above.
This is why ceiling lights can cause serious issues. If you are fitting recessed lights into a ceiling, which are commonly referred to as downlights or spotlights, consider what the impact would be of creating the necessary holes. The acoustic insulation could be compromised if no thought has gone into ensuring that the recessed light is properly insulated, or if too many recessed lights are incorporated into the ceiling.
How to overcome a failed sound test – a case study
As disappointing as a failed sound test can be, it is often possible to modify the acoustic floor, ceiling or wall to achieve the minimum standards required under the Building Regulations. And the remediation may even be relatively simple, providing you know where the issue lies, and which products will best address it.
One example is a residential development in East Sussex. Here, the developer sought help from Hush Acoustics after a separating wall between the bedrooms of two self-contained units failed a sound test. Although this was an acoustic wall issue rather than specifically to do with the acoustic floor, the resolution illustrates why it is so important to consider how the walls and floors interface to prevent flanking.
The construction involved a timber stud wall with a thickness of 170mm, and a concrete screed floor. The initial airborne sound test, conducted with no floor covering present, revealed a level of 31 dB DnTw+Ctr, indicating a significant flanking issue. After investigation, it was clear that this was due to sound passing under the separating wall via the floor screed.
After reviewing the design and the materials used, Hush Acoustics recommended installing Hush Mat 15 rubber matting which was laid over the floor screed in both rooms. Following its installation, a second test was carried out with the result being an Approved Document E compliant 44 dB DnTw+Ctr.
