How To Soundproof Existing Walls
Jun 27, 2022I have been asked several times in our Soundproof Your Studio Community how to soundproof walls that are completely finished without having to take them down to the studs. I always wish I could tell them there was a way, but the truth is you simply will not get the results you want by adding drywall to an existing wall or adding a second wall in front of an existing wall without first taking that wall down to the studs.
Why You Must Remove The Existing Drywall
So there is some lab tested science that explains why you must remove the drywall in an existing wall before you can soundproof it. If you look at the diagram below we can see there is a much bigger improvement in the STC rating when you remove the drywall from the exisitng wall.
What Is An STC Rating?
STC stands for Sound Transmission Class and it is a numerical value that helps to define the ability of an assembly to reduce sound transmission from one side to the other. In normal english that means the higher the number the better a wall is at reducing sound passing through it. For recording studios an STC of 55 is generally suitable for keeping enough sound out so as not to ruin your sessions.
Understanding Different Wall Designs
In the diagram below you can see the different STC ratings of varied wall designs. 
Graphic derived from Home Recording Studio: Build It Like The Pros by Rod Gervais
Notice that a regular wall without insulation has an STC of 33. This is not great and at an STC of 33 you can hear loud speech but not understand it. If you look at the two walls with drywall on the inside and outside you notice you get an STC of 40. At this STC loud speech becomes a murmur, but it still is not good enough for a recording studio. If you look all the way to the right you see that leaving the drywall off the interior and instead adding a layer to each outside wall increases the STC to 63. With an STC of 60+ you can barely hear an outside noise and this is considered excellent soundproofing. (Data from the Soundproofing Company)
Conclusion:
You cannot simply build your soundproof wall alongside an exisiting wall. You also cannot simply add more drywall to an exiting wall because you need to decouple the drywall from the wall. Although it is not what you probably want to hear, you need to take all walls down to the studs in order to properly soundproof your walls.
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