Putty Pads - Theoretical Acoustic Isolation Effectiveness

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endorka
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Putty Pads - Theoretical Acoustic Isolation Effectiveness

Postby endorka » Tue, 2019-Oct-22, 08:13

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.
2019-10-22 10.58.42.jpg

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
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Starlight
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Re: Putty Pads - Theoretical Acoustic Isolation Effectiveness

Postby Starlight » Tue, 2019-Oct-22, 09:59

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.

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Re: Putty Pads - Theoretical Acoustic Isolation Effectiveness

Postby Soundman2020 » Tue, 2019-Oct-22, 15:00

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. :thu: 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 -

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Re: Putty Pads - Theoretical Acoustic Isolation Effectiveness

Postby Avare » Tue, 2019-Oct-22, 17:54

Great stuff. Thank you.
Good studio building is 90% design and 10% construction.

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endorka
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Re: Putty Pads - Theoretical Acoustic Isolation Effectiveness

Postby endorka » Wed, 2019-Oct-23, 08:15

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

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endorka
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Re: Putty Pads - Theoretical Acoustic Isolation Effectiveness

Postby endorka » Thu, 2019-Oct-24, 18:35

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


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