Wall Soundproofing

Wall Soundproofing deep dive

The basic principles

All you need to know when soundproofing a wall

It can be a minefield when comparing soundproofing systems as there are many ways to display how effectively a wall system has performed. Also, the dB scale is a logarithmic curve which complicates things further (I will explain this in more depth below). I will go into each part of how to soundproof a wall below to help you understand the science behind soundproofing so you can compare systems and make an educated decision on which soundproofing method is the best.

How and why the sound is coming through the wall

All sound is a form of vibration. Let’s take TV noise as an example: the speakers vibrate the air, pushing particles around the room so you can hear the TV. The sound vibrations hit the shared part wall and vibrate the wall. These sound vibrations travel through the wall and the wall then emits the sound into your room.

What is the performance of my current wall?

A standard double-skin brick wall with plaster on either side should perform at around 45 dB.

A single skin of concrete block may be closer to 40 dB.

A cavity wall with concrete blocks will be approx. 60dB

A timber or metal stud frame with 1 layer of plasterboard on either side should be 35dB.

The dB scale explained

The human ear perceives every 10dB as a halving of the sound. This means a 50dB reduction is twice the reduction of 40 dB. Other useful markers are:

• 1dB is the smallest difference you can hear

• 3dB is a noticeable difference

• 6dB is considered a significant difference

• 10dB is a halving of the sound

When we say a halving of the sound, most people think, ‘I will still hear half of the sound’. This isn’t quite true. If you reduce the sound below the background sound level, the sound will then be audible. I will explain more below.

How much do I need to improve my wall?

Let’s give an example. In this example, you have a double skin, red brick wall, and this is performing at 45dB (it is likely between 40-50dB). Your neighbour has the TV on and typically that would be 90dB. This means you will get 45dB of the TV sound coming through into your room, as the wall has reduced the sound by 45 dB.

Background sound level is the level you will perceive as silence and in most cases, it is around 40dB (a fridge runs around 38dB and you don’t notice this)

So the difference between the sound coming through the wall at 45dB and the background sound level at 40dB is 5 dB. This means you only need to reduce the sound by a further 5dB to bring your neighbour’s TV sound below the background sound level of 40dB in this example.

This shows that if you improve your wall by 10dB, you won’t hear half the TV noise as the neighbour’s TV will be coming through your wall at 35 dB. This is because you can’t hear the sound when it goes below the background sound level.

Mass law

Mass law is one of the key aspects of soundproofing. It dictates that if you double the mass of a ‘partition’ then you reduce the sound by 5dB. This is shown with products like Tecsound and MLV. The 5kg membranes will have a performance figure of 25dB and the 10kg equivalents will perform at 30dB (Tecsound is 26 and 31dB but still has a 5dB difference)

From here, it gets significantly more complicated; many other factors come into play such as material, density, thickness, resonant frequency, de-coupling, dampening, flanking, etc.

The reason mass law is important is it shows why adding acoustic plasterboard directly to a brick wall will not work. A double-skin brick wall is around 80-120kg/m2. A 15mm acoustic plasterboard is approx. 15kg/m2. You can see that even if you add 2 layers of acoustic plasterboard to this wall, the relative additional mass is minimal and will not make a significant difference. The additional 30kg/m2 (additional 30%) may give you an extra 1-2 dB which is not a noticeable difference.

• Decoupling mass from the wall

Although 2 layers of acoustic plasterboard would only give you a 1-2 dB improvement when fixed directly to a wall, the same 2 layers of acoustic plasterboard can give you an 8-12dB improvement when mounted on an isolation clip and channel system.

If you remember that all sound is a form of vibration, then it is easy to understand that attaching ridged materials to a surface that is vibrating, will also vibrate. However, when you mount the same layers onto the channels that are now floating off the wall then the vibrations must travel through the rubber, into the metal bracket, into the channel and then try to vibrate 30kg/m2 of plasterboard. The sound vibrations at this point have lost a lot of the energy travelling through the different densities of materials and therefore struggle to vibrate the floating mass.

Sound absorption in a cavity

You always need to use sound absorption material within a cavity. This is the same with all soundproofing. If the cavity is left open then it can cause cavity resonance.

Cavity resonance is when the sound is stuck in a space surrounded by hard surfaces which allows the sound to bounce around and resonate like an acoustic guitar or a drum. This allows the sound to also travel freely around the space and amplify. The bigger the cavity is, the worse the baser frequencies can amplify. Think of the difference between a violin and a double bass.

Higher-density materials are typically better at absorbing the base frequencies, which is where you usually get the sound issue. Also, the thicker the absorption is, the more base frequencies will be absorbed. This is due to the wavelength of the base frequencies.

Vibration dampening

There is a range of vibration-dampening materials on the market but the most popular of these is the Tecsound range. This should be the self-adhesive version when installed on walls for ease of installation.

Vibration-dampening materials are efficient when soundproofing as all sound is a form of vibrations. When you use Tecsound between 2 layers of acoustic plasterboard, you reduce the sound transfer between the layers as well as add mass.

Tecsound vs Green Glue vs MLV  (membrane and de-coupling options for between your 2 layers of plasterboard)

Tecsound and Green Glue are both viscoelastic materials which means they are designed to reduce the transfer of sound vibrations. MLV is just there as a slim line way off adding additional mass.

Tecsound is the best option for a wall as it adds both mass and vibration dampening. It is a self-adhesive, vibration-dampening membrane but it can be difficult to fit!

MLV only adds mass and doesn’t commonly come self-adhesive. It is harder wearing than Tecsound, so is great for floors.

Green Glue doesn’t add mass and it is unclear how much vibration dampening it will give. If you apply the Green Glue thick enough to de-couple your 2 layers of the board, then you will have cavities between the plasterboard layers and you could get cavity resonance. If you apply the Green Glue thinner, then the plasterboards will be touching and there is no de-coupling; there may be something I have missed here but the before and after performance tests all have multiple variables. Therefore, you could attribute the performance upgrade to the resilient channels used in the aftertest or similar.

Independent stud wall vs resilient channels on a wall vs isolation clip on a wall

Installing an independent stud frame is the ideal situation to completely de-couple the plasterboard from the wall the sound vibrations are transferring through. The main issue is that it can take up a lot of space and if you have a large wall then you will need a thicker timber frame. Also, the price of timber has increased significantly over the past few years. Try using an isolation strip around the perimeter of the frame where possible.

Resilient channels are the cheapest upgrade available. However, the upgrade is relatively small. This is because you are attaching metal directly to the surface which is vibrating. This will also then vibrate the metal channel. Resilient channels should not be attached directly to a brick wall as they don’t give you enough space for mineral wool. Resilient channels are mainly used on timber or metal stud frames but you need to be very careful not to short-circuit the channel and fix into the channel and back into the frame. This is a common issue as the channel is only attached on one side, causing it to move when trying to fix into the channel.

Isolation Clip systems offer the best upgrade for the space they lose. They give most of the performance of an independent frame, whilst losing 50% or less space (typically 40mm for a clip and channel compared to 80-120mm for a stud frame). They will be the same thickness no matter the height of the wall with no need for perimeter isolation strips to reduce flanking.

Plasterboard vs acoustic plasterboard vs PLY/OSB

Acoustic plasterboard is a cost-effective upgrade when soundproofing a wall. At 15mm the acoustic plasterboard is around 15kg, compared with standard plasterboard or timber which is approx. 10kg/m2 at the same thickness. Ply or OSB can be useful to replace the first layer of acoustic plasterboard if you know you are going to have multiple fixings into the wall such as wall-mounted TV, lights, shelves or kitchen units.

Flexi Slab vs RWA45 vs RW3 vs RW5 (mineral wool/cavity insulations options)

All of these products are part of the Rockwool Range. The higher-density mineral wools are better at absorbing the base frequencies which is important. When you hear sound coming through a wall, it is usually the mumble from a TV. The mumble is the base frequency coming through. Most walls and materials are very good at reducing the high frequencies, so you don’t need to worry about these too much.

I would usually recommend the 45-60kg/m3 mineral wool. Which is the RWA45 or RW3 as these give a good performance and are easier to work with compared to the 100kg/m3 RW5.

APR vs RS45 vs RS60 vs RS100 (mineral wool/cavity insulations options)

APR stands for acoustic partition roll. APR is 10kg/m3 density, so although it is advertised as an acoustic product, it isn’t the best at absorbing base frequencies. The rest of the Knauf mineral wool range is self-explanatory. RS45 -45kg/m3, RS60 – 60kg/m3 and RS100 – 100kg/m3.

Again, the higher density of the mineral wool, the better it is at absorbing the base frequencies.

How much mineral wool should I put in the cavity?

The ideal situation is to fill a cavity 60-80%, but it commonly comes down to what is available to you. You should avoid compressing mineral wool too much between de-coupled surfaces. If you used a sound isolation clip system at 40mm, you should avoid squeezing 75mm of RW3 in this cavity as you could start transferring sound vibrations from the original wall to your new floating wall.

How to fit RW3 or RS60

Both RW3 and RS60 are 60kg/m3 mineral types of wool and will perform at a very similar level at the same thickness. Both want to be cut with a bread knife or similar. When fitting this between a stud frame you cut the material slightly bigger than the gap to friction fit this between the studs. When fitting with an isolation clip system, you want to try to cover most of the wall. The mineral wool can be held behind the channels if there is enough space, but some clip and channel systems require you to glue the mineral wool to the wall as the channels are too close to the wall.

Sealing any gaps and why

You should seal all gaps in the layers where possible. Soundproofing is a little like waterproofing: if there is a gap, the sound will get through. An example of this is when you are in a car and the window is open just a little bit vs fully open; most of the sound still comes in. You get the same if you have music on in a room. If you leave the door slightly ajar, it is almost the same as the door being fully open.

Plug sockets on a soundproof wall

You can install plug sockets on a soundproof wall. However, you are far better at surface mounting plug sockets on a wall. This allows you to have a small hole to pull the wires through and seal the hole. For the same reason you wouldn’t put a cat flap or letterbox on a soundproof door, you want to avoid putting a socket box in a soundproof wall.

You can get socket box inserts but they are trying to fix a weak point. You are better off not having the weak point.

Wall mounting a tv, shelves or a radiator on soundproofing

This does require planning when soundproofing. You will commonly replace a section of the first layer of plasterboard with Ply or OSB, as these allow a stronger fixing point. This can also be done by fixing the timber to the back of the first layer of plasterboard, but it will need to go between the channels.

You can also add extra channels if you know exactly where the fixings are going to be then fix the TV bracket or similar directly into the channel as this will be stronger than just fixing into just the plasterboard.

Fitting pictures

You can fit pictures into soundproofing with no issues in most cases if these are lightweight, and you use a shorter fixing. You can also consider the self-adhesive strips to put pictures on a soundproof wall. If it is a heavy frame or mirror, then you may need to consider one of the methods above.

Fitting kitchen cupboards on wall soundproofing

When fitting kitchen cupboards to floating soundproof walls, you will either add a layer of timber to the soundproofing layers or replace 1 of the layers of acoustic plasterboard with the Ply or OSB so you can fix it into the wall with confidence. You will add additional rows of soundproof clips and channels when fitting a kitchen to soundproof walls to take the additional weight.