Flooring installed with acoustic underlay is useless without this 1 thing. ( Source: Pliteq.com)

Flooring Installed With Acoustic Underlay Is Useless Without This 1 Thing.

So you’re installing, designing or purchasing a flooring system which includes acoustical underlay? No matter which type you choose one of the key details to pay close attention to is the edge (or perimeter) of the floor where it touches or more specifically should NOT touch the wall. .

Why is this a problem you ask? Well in this article we will demystify the perimeter detail and why acoustic underlay is useless with out this one thing – perimeter isolation strip.

At Pliteq, we manufacture and sell acoustic flooring underlay and perimeter isolation strip. Because of specific project details you may not need either of these products but the only way you will know is by learning about it. So read on for more details!

How Does Sound Move Through Floors In Buildings?

When vibration passes through the structure of your building, it will travel 10 to 15 times faster than it does through air. This gives you an idea of how sneaky and difficult it can be to control noise and vibration in buildings.

When it comes to sound moving between spaces, whether directly or in-directly, it looks for the weak points to move between spaces via the shortest possible path.

What is Direct Sound Transmission?

Direct Impact sound travelling through a floor with no acoustic underlay
A wooden toy block being dropped on a floor and the direct sound travelling straight down through a floor into the space below

Direct sound transmission is straightforward enough, it is sound taking the shortest possible route to travel from one space (whether this is through a floor, wall, ceiling , window or roof construction) to another space.

A simple example of this would be the noise of hard soled shoes on a floor. As someone walks across a floor the vibration caused by the impact of the shoe on the floor will pass directly downwards into the concrete or wooden structure below and turn into noise in the space below.

As we have discussed in this article we can make it harder for sound to travel via this direct sound path by introducing an acoustic underlay beneath the floor.

This will reduce the amount of vibration passing into the structure. But where else does this vibration travel other than down? 

What is In-Direct Sound Transmission?

GenieMat PMI transmission 02 2
A wooden toy block being dropped on a floor creating vibration which then travels in all directions, and unless stopped, into adjacent spaces.

Well the answer is in all other directions!

In-direct sound transmission is one that travels via the long route as opposed to the short route of direct sound to get from one room to another.

So when something is dragged or dropped (keys, toys, coins, foot steps, chair shifting) on a floor it creates vibration which passes into the floor finish and the vibration will then pass along it horizontally and will only stop once it runs out of energy, the floor comes to an end or it is stopped by something resilient, like a perimeter isolation strip.

The building industry calls this in-direct sound flanking transmission or flanking noise.

The most common flanking noise issue in a floor assembly occurs when the floor meets the wall without either a physically break or resilient perimeter isolation.

Flanking Noise – Acoustic Underlay With And Without Perimeter Isolation Strip

So now onto how what we have described above effects the the acoustic performance of a floor, potentially rending the acoustic underlay useless.

Acoustic underlayment is installed to reduce the downward path of vibration caused by an impact. But if we do not stop the vibration caused by the impact on the floor (which eventually causes the noise in an adjacent space) from going into other spaces by indirect routes then we can expect to see problems.

This could be the acoustical underlayment not living up to IIC/Ln,w performance expectations which in turn could mean building code failures and nuisance for adjacent neighbours what ever the building type.

So what we are saying is, if you get this detail wrong there is almost no point in using or installing the acoustic underlay!

What Does A Bad Perimeter Isolation Strip Detail Look Like?

GenieMat PMI transmission 03 2
Incorrect Perimeter Isolation Strip Detail – The green arrows show the direct sound path and the red arrows show the in-direct (flanking noise) path.

The above image shows a bad perimeter isolation strip detail for an acoustical underlayment installed directly under a typical floor/screed.

As we can see, even with an acoustic mat installed dealing with the direct vibration passing straight downward, the floor is in direct contact with the wall. Therefore any vibration in the floor finish will pass horizontally along it into the wall and then on into other adjacent spaces.


Any performance improvement provided by the underlay will be limited by the vibration passing around it.

What Does A Good Perimeter Isolation Strip Detail Look Like?

GenieMat PMI transmission 04 2
Correct Perimeter Isolation Detail – Like before the green arrows show the direct sound path and the red arrows show the in-direct (flanking noise) path

The above image shows a good perimeter isolation strip detail for an acoustic mat installed directly under a typical floor/screed.

There is now no direct contact between the floor finish and the wall.

The acoustic underlay is still dealing with the direct vibration passing straight downward but now the perimeter isolation strip is stopping the vibration which was previously going around the underlay and into the rooms below.

The vibration now has no where to go!


So hopefully anyone who thought that having perimeter isolation strip was an extra step, an extra detail to monitor on site or an additional cost should have now read the above and maybe reconsidered!

So This Perimeter Isolation Detail Is Important?

Yes! To recap a correctly installed perimeter isolation strip will stop two parts of the building from touching e.g. flooring and wall. In doing so it breaks that path of vibration.

As such a perimeter isolation strip:

  • Reduces flanking transmission
  • Maximises the effectiveness of the acoustic underlay
  • Protects the investment in acoustic insulation of impact sound

So doing this 1 thing can make a big difference!

Posted by VK Acoustics:( Source: Pliteq.com )

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