Putty Pads - Theoretical Acoustic Isolation Effectiveness

#1

In my studio design thread Stuart recommended lining the electrical and signal wall socket boxes with putty pads. The added mass helps heal the acoustic gap made in a drywall partition when a recessed socket box is put on it. I'd never come across these before as they don't seem to be used in domestic properties in the UK. They do however seem to be mandatory in some commercial applications as a measure to slow the spread of fire, so are readily available.

There are many brands available at a significant range of prices. It's much cheaper per unit to buy in boxes of say 20. Buying a handful of "singles" will likely cost just as much as such a box.

There are a lot of sockets to treat, so I did some research to figure out the most efficient and cost effective approach before getting to work. Hopefully this will prove useful to others, and if my layman's postulations are off, please let me know.

The problem is that removing a chunk of plasterboard to fit a recessed socket box causes an acoustic hole in that partition. This comes from two aspects 1) There will be holes in the socket box, its fittings, the join between the box and plasterboard, and where the wires go through and 2) The mass per area of the socket box is much less than that of plasterboard, so reduction of bass will be severely affected. As demonstrated by theory and practice, even a small percentage gap of the total surface area can give a huge loss of sound transmission reduction.

The idea is that you apply the putty pads around the back of socket box and onto the surrounding drywall, moulding it thoroughly over all cracks & holes and up against the wires. This effectively seals the gap, but you require access to the inside of the wall to do it. If that's not possible they can be applied inside the socket box in a similar manner. It will be more difficult to seal the cracks where the socket box and drywall partition meet, but I think this may be possible in some designs if the socket box can be pulled out from the partition and putty applied around it, then pushed back in. The better designs of socket boxes have a lip around the edge that will help make such a seal more effective.

Here are some I've bought. The Hilti pads on the left are in boxes of 20 and are for double gang sockets. There are loads of these on ebay in the UK at the moment, going for as little as £30 per box including postage, so £1.50 per unit cost.

They are rectangular shaped so require cutting into crosses to fit. They are a little thinner at ~3.5mm than the "standard" thickness of around 4mm, so I bought another type to compare. I figured I could always use the offcuts from cutting it into a cross to pad out the thickness a bit to make up for this if it was significant. No doubt the offcuts will be useful in other areas anyway!

The cross shaped ones are "Firespan Socket Intumescent Putty Pads" and are for single gang sockets. They are supplied precut into cross shapes and are closer to 4mm in thickness. More expensive - £23 including postage for 10, so unit cost £2.30

I also have a box of 20 Pfc Corofil two gang cross shaped putty pads on order, £25 including postage for 20, unit cost £1.25. They've now arrived so I've added their data below. At this price they offer the best value for money in terms of cost per mass.

I weighed and calculated the surface area for both and derived the following mass per area;

Hilti is 5.5 kg/m2
Firespan is 6 kg/m2
pfc Corofil is 5.9kg/m2

Probably not a big difference in practice, especially since the offcuts from the Hilti could be used to beef it up a bit.

Here's where it gets interesting: 12.5mm plasterboard is 8 kg/m2. This makes sense as putty on average tends to be denser than plasterboard: 1600 kg/m3 vs. 640 kg/m3.

I don't have a recessed wall box to weigh at the moment, but figures online give the weight of 2 gang recessed wall boxes as no less than 50g. Including the area of the side walls, this gives a mass per area of ~2 kg/m2, not including front cover. Presumably the front cover will have a similar attenuation effect too, as long as the socket has these little doors that close automatically for the plug terminals.

Combined with the putty pads this should give a mass per area of more than 8kg/m2, as good as the plasterboard. So assuming a perfect seal, the acoustic hole has been sorted.

I await your comments on the theory, and I'll measure the effectiveness of the application of these too when I install them.

Cheers,
Jennifer

#2

Your thoroughness is appreciated, Jennifer, as this is something I will need to deal with in a few months. You have made it easier to know what to look out for. I chuckled when I saw the Hilti boxes as one of my neighbours drives a red Hilti car so I know where to ask if I cannot find any putty pads where I am.

#3

Great work, Jennifer! As always, with you!

That's an excellent post about the issue, and what you found. Good job! You are spot on with all of your comments.

About the only thing I'd add, is that you can use caulk around the edges too, where the putty meets the wall, if the seal doesn't seem to be good. But test your caulk and putty first, to make sure they play nice together, and stay stuck.

- Stuart -

#4

Great stuff. Thank you.

#5

Glad you found it useful folks. I love it when the theory adds up, the designers of these pads obviously knew what they were doing.

Cheers,
Jennifer

#6

The pfc Corofil pads have arrived and I've added their measurements to the original post. They were the best value of the three.

Cheers,
Jennifer

#7

This may or may not be related. I was thinking about puddy pads and the complexity of using them in walls that are in-accessible from behind. Filling the internal box crossed my mind, but I was not sure if that would be acceptable to our NEC. I was then thinking of the alternative of using the pad as a backer to the faceplate. I was thinking that if density and a proper seal was at issue, it fullfilled the density and could come close to forming a seal depending on the receptacle/switch penetration of the face plate.
So here is the question. Would the use of a putty pad on the faceplate be an acceptable alternative to back-sealing the box?

The benefits could be many:
smaller footprint of putty (less use, is less expensive)
closer seal to the mass layer (IE, the putty pad is contacting the faceplate and drywall

The graphic below shows...

Light switch plate (White), Red putty pad, drywall (white), electrical box (blue)

#8

That is a good question!

As you say, I think adherence to code and manufacturer's fitting instructions is crucial in these matters. Filling the internal box is one of the recommended methods from the manufacturer, so I was happy with that. The other was of course fitting round the back of the box.

I've no idea how code applies to fitting on the back of the faceplate, but two things cross my mind. The first is that there is not a lot of blank faceplate behind a typical UK socket as most of it is taken up by the workings;

: Capture.PNG (541.41 KiB) Viewed 43141 times

: Capture.PNG (541.41 KiB) Viewed 43141 times

There may well be more in other parts of the world though.

The second is that you'll get a red putty coloured line all around the perimeter of the socket. On one socket with the internally fitted putty I applied it completely to the edge of the socket, and it resulted in said coloured line. It looks rubbish! Fortunately hidden behind an acoustic panel :-)

Cheers,
Jennifer

#9

The manufacturer suggestion to enclose the box makes sense, as their intent is primarily fire blocking, not acoustic. The sketch I have is crude, in reality, the putty on the inside of the faceplate would be in fact completely enclosed and would not show once installed on the wall. I also see a problem in this method as you point out, where not all outlets have a simple 'faceplate', but rather are the 'guts' of the switch or device. I will also have this issue on a few of my intelligent switches.

#10

That would work, I reckon.

Another option might be replace the face-plate with a thicker metal one. Aluminum, for example, is about four times more dense than drywall, so a 3mm thick aluminium face-plate would have the same surface density as 12mm of drywall. Steel is even more dense: about 3 times more dense than aluminium, or 12 times the density of drywall. So a 1mm steel face-plate would have the same surface density as 12mm drywall, an 2.5mm steel (1/10") would be about the same as two layers of 16mm drywall.

Of course, getting special metal face-plates made up for all your outlets and switches might be a little expensive!

The other issue with just putting mass on the face-plate, is air-tight seals: I'm not sure how well you would be able to seal the plate to the wall, or seal the tiny gaps between the plate and the outlet or switch rocker. That might be an issue.

- Stuart -

#11

Apropos to Stuart's post above, some rooms in my house have screwless brushed metal faceplates on the light switches and sockets. I took one apart to measure and examine the construction. Single gang, UK type;

The metal faceplate portion alone weighs 66g, so 8.5 kg/m2. Just by itself that is more than the 12.5mm plasterboard, very impressive.

The plate behind that is also substantial, and has a plastic gasket around the edge. This inner plate gets screwed flush to the socket backbox, pulling everything tight into the plasterboard. If the backbox hole is cut properly there is considerable overlap between the faceplates and plasterboard providing a good seal. For extra surety the backbox could be sealed into the plasterboard with caulk or putty. I filled the cable holes in the backbox with putty as a precaution.

Once everything is screwed into place the faceplate attaches onto the assembly with strong a strong spring latch. It's a good fit, needing a screwdriver to prise off.

These are off the shelf, my apologies I forgot to check for a manufacturer. They are more expensive than standard types, but it's nice to see the benefit is not just cosmetic for the extra cost. They're also easier to paint round - just remove the faceplate before painting and put it back after to avoid splashes.

Cheers,
Jennifer

#12

Thanks for that, Jennifer! Those look pretty good, actually, and would probably perform very much like the rest of the wall, in terms of isolation. They keep the surface density consistent, and have good seals on them, it seems, so they should be pretty good.

Question: Is the type of switch you have in your studio, or just in the rest of your house?

If you don't have them in your studio, you might want to consider swapping out whatever is there right now, for something like this... Even with your beefed-up boxes, it wouldn't do any harm to have a little extra mass and seal on the front.

- Stuart -

#13

That's a really good idea. These switches (and similar for other types of sockets) are throughout most our house, alas not in the studio and a couple of other places though! They were that way when we moved in :-)

Next time I open one up I'll try to figure out the manufacturer.

Cheers,
Jennifer

#14

Here's another interesting idea: There's a product called "draft stoppers" for electrical switches and outlets:

They are meant for stopping air drafts through light switches, I guess from badly done electrical installations. I just came across a pack I had in my workshop while I was looking for something else, and realized they would also work to provide a decent seal for acoustic purposes. It's basically a thin, soft, closed-cell pad of some type, so not much mass in it, but it should work fine to seal an already massive metal face plate. I'm not sure if you have anything like that in the UK, but you probably do. These specific ones come pre-cut for "standard" US sized boxes and face plates, for both switches and also outlets, but they could easily be trimmed for a smaller size face-plate or larger outlet.

Anyway, just another idea that might work... Might give you some brainwaves on other ways to mass up and seal your electrical stuff!

- Stuart -

#15

All corners are usually a good place to start with the bass trap, even the ceiling corner triangle shape for the corner.
Also, use more dense material for the bass trap, 8 lb for example
Acoustic panels for the side should be 3 to 6 lb, this is "best practice", but anything is better than nothing.

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