Author Topic: Soundproofing in under 2 inches  (Read 1483 times)

Erin S

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Soundproofing in under 2 inches
« on: May 08, 2019, 04:28:30 AM »
Hello, I am trying to add soundproofing to a wall separating a newly built in-law apartment from the adjacent space. My contractor wants to use 3 layers of 5/8 drywall. He says this will be 'plenty to block the noise.' From my little bit of reading, it sounds like Green Glue would be a great thing to add between the layers of drywall. Do you agree? If so, should the Green Glue go between both layers of drywall? My other question is what to do about the soffit (if that's the right word) that you can see in the picture. It contains electrical wires. Do we need to soundproof that too, and if so how? We only have 1 7/8 inches to work with because there is a return vent at floor level we need to work around. Thanks so much for any advice you might have!!

Randy S

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Re: Soundproofing in under 2 inches
« Reply #1 on: May 08, 2019, 03:29:38 PM »
replace one of the layers of drywall with cement board 1/2" 3lbs sqft to increase the weight. you need to double the wall mass ( 2 layers 5/8" - 4.6 lbs x 2 - 9.2 lbs ) 9.2 lbs is the goal.

you must treat the soffit the same way.

is that return vent on the wall you are trying to soundproof?

Randy S.
760-752-3030
Randy Sieg

Super Soundproofing Co
www.soundproofing.org
888-942-7723
Ph. 760-752-3030
Fax.760-752-3040

Erin S

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Re: Soundproofing in under 2 inches
« Reply #2 on: May 08, 2019, 04:09:31 PM »
Hi Randy,

No the vent is on the wall just perpendicular to the wall in question. I'm not sure if it leads into the same air space or how that works.

So you are saying 2 layers of 5/8 drywall and 1 layer of 1/2 inch cement board. Should I use any green glue in between? Thank you so much for your answer!

Erin 


gc4

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Re: Soundproofing in under 2 inches
« Reply #3 on: May 08, 2019, 09:10:39 PM »
I would be very skeptical of your approach. My office was terrible for STC (aka airborne) sound transfer. The contractors decided to put up a layer of rubber sheeting on top of the existing drywall, then homasote, then another layer of 5/8 inch drywall. Instead of sounding a foot away, the voices now sound like they are maybe 5 to 10 feet away. But I can still make out what the people are saying through all of that. And if they're at the perfect angle, then its no change - clear as day.  Caveat the building is a converted loft made of wood but I wouldn't bother doing this again to any rooms, it wasn't worth it.

If I were trying to kill STC sound, I would cut off the existing drywall on one side of the wall. Pack the joists with rockwol safe and sound or something similar (perhaps two layers - it isn't that expensive and is pretty easy to do yourself -- see youtube) then I would use Quietrock drywall (probably have to hire someone to hang it) and, if that didn't do it, try green glue and a layer of 5/8 inch drywall on top. If that doesn't do it, you're probably SOL, consider moving.

Quietrock is about 10x the cost of normal drywall but sounds legit from my reading. You should call Quietrock and talk to the guy there (forgetting his name). He is an awesome resource. Very helpful and into educating people on realities of sound transfer.

As for vents, I can't help much there. I've watched some videos on soundproofing vents and such but haven't done the same research. At some level, you should probably managed expectations on how much can be accomplished. As I've been told by many vendors on hours of calls about all of this, sound is like water, it finds a way through.

Randy S

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Re: Soundproofing in under 2 inches
« Reply #4 on: May 08, 2019, 10:52:39 PM »
I will be removing your link to quietrock , we dont permit flogging on our forum

 look up Acoustic "MASS LAW" now that you know every doubling of mass = around a 5db drop which is about 40% reduction of human speech. You have to understand that at a minimum you must double the wall mass.

MASS is MASS and as long as you have the space I dont mind what materials you use that are very heavy to achieve doubling of mass or more...I have even done mortar and chicken wire between studs.

Not a big fan of quiet rock as I have had to redo many of those jobs due to lack of mass.

Quiet Rock EZ snap - 2.6 lbs sqft around $50 a sheet  / Duroc 2.6 lbs sqft $18 a sheet
Quiet rock 510 - 2.13 lbs sqft / 5/8" drywall is better
525 - 2.7 lbs sqft  hardie backer or permabase cement board both exceed 525 and 527
527 - 3.0 lbs sqft
530 RF is for faraday cages / this one I have used in a few projects
and the only one that actually is something is 545 at 6.25lbs sqft but Ive seen upwards of $200 a sheet.
Which I can make myself for $60 a sheet.
Now if you saying the the magic stuff in the middle is doing the work dont forget you are driving screws right back into the frame sending sound and vibration into the studs unless you are on complex structure.
This is why I have seen green glue jobs not yield desired results and now I only use it for low frequency response of studios and theaters.

And the biggest mistake I have seen with quiet rock is the installers forget to butter the edges with acoustic caulking during install.

If you are trying to achieve higher then a 5DB drop based on mass law then you would have to go into complex structure ie. double frame/ clip and channel / resilient channel and you would still need to double or even triple the mass to get greater decibel reductions.

remember we can not talk through 6" concrete but if I hit it with a hammer you are going to hear it.
so it is important to decide what type of sound and how much reduction you are trying to reduce in order to design the assembly correctly.
Also keep in mind anytime you are doing single partition only you will have some value of flanking noise around the soundproof partition. this value I find to be unknown until the wall is complete.

And finally, yes every layer from start to finish has to be installed airtight / water tight without fail.


feel free to call me direct.

Best Regards,

Randy S.
760-752-3030



Randy Sieg

Super Soundproofing Co
www.soundproofing.org
888-942-7723
Ph. 760-752-3030
Fax.760-752-3040

 

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