Soundproofing an Existing Interior Wall

AngelArs

Always open for interpretations and facts

That's a blanket statement Ted and leaves out many factors. No one buys MLV because it is or isn't better than other materials of comparable mass. People buy it because it's easier to install, cheaper, and has less inherent issues than installing another material of comparable mass. For example; MLV is wear resistant and perfect as a carpet under-layment to dampen sound from basement areas, or first floor sound entering the second floor, etc. Go ahead, try using drywall under a carpet for sound proofing and see what kind of mess you have a month later As I said before, every material has its application. Some MLV has around the same mass as a comparably sized piece of lead. That means MLV offers a lot of bang for the buck. Also not all MLV is equal. Some MLV is double weight which makes it even more desirable. If I was building a new house I would have no problem using MLV inside the walls. After a house is built however I do not see any advantage to using it unless it's used under carpets areas.

I don’t want to get too techie here but essentially sound is a form of energy. Energy can't be destroyed, only converted to another form of energy, so to dampen sound energy you have to convert it into heat. The simplest way to dampen any sound is to simply put a solid wall in its way. As you you double the mass of a wall, you roughly halve the amount of sound that is transmitted. But it’s not just about mass, because as a sounds frequency is reduced, the isolation provided by a structure also reduces. Basically for every octave drop in pitch, the sound isolation is halved. This is why it’s easy to muffle the sound of your neighbors annoying canary, but it’s hard to muffle the sound of their home theaters subwoofer. High frequencies are not too much of an issue to dampen, but deep bass is usually very hard. That is precisely why we need to know what KIND of sound the OP is trying to dampen. Just telling them to use this or that is like using a bazooka to kill an ant problem. Sure it might work, but there’s probably a better way to do it.

That's another blanket statement. Not all drywall weighs the same. Not all MLV weighs the same. Drywall does NOT always give better results.

I've never seen anything like that. In fact, I've seen just the opposite. Feel free to post your links to prove your statement.

That's not the point! We're on a DIY site and if the OP wants to muffle the sound of a leaky faucet then closed cell insulation is a cheap, easy and quick way to achieve that.

No one is suggesting to use spray insulation in the walls of a recording studio

Not for the application it was drawn for. Every application is different.

Ted White

I appreciate your patience and demeanor, and hope this sort of discussion isn’t outside the bounds of this forum. Generally people find this stuff interesting, I’ve found, so hopefully it’s OK.

Anyway, I think we’re just talking about a guy’s wall, not his floor, but I’d certainly agree you would not want drywall in a floor. I’d also agree that MLV under a carpet + pad is a great help if overall floor height can’t be increased much. That’s a great point I would also agree that after a house is built, there’s not much use for MLV, however if I were building a new wall, I’d start with 5/8” drywall, and if I needed more mass, I’d just add another layer of 5/8”. More mass for cheap.

Categorically, the great uses for MLV come from it being very thin and / or it being flexible and also being dense. That comes in mighty handy in some circumstances. Another fun fact is that due to it's flexibility it has an extremely high coincidence frequency . We'll save that for another day.

You bring up yet another good point in that it’s not all about mass. For higher sound isolation you would introduce decoupled framing, make sure there’s a little absorption (cheap fiberglass insulation) in the wall, and look to damp the mass. You quickly reach a point where the continued addition of mass alone becomes a diminishing return.

You mentioned about doubling the mass, which adds 6 STC points an assembly (rough lab rule of thumb). As far as how much sound the doubling of mass will affect, the answer is very frequency-dependent. Doubling the mass could very well stop 99% of frequencies above a certain point, while barely affecting lower frequencies, or any coincidence frequencies.

AngelArs

Next time I'm at Loews I'll have to check and see how much Quietrock 510 cost. I would think it would be cheaper than two pieces of standard drywall.

Ted White

Given that all of these pre-damped drywall panels are made from standard drywall and cement board that you can buy yourself along with readily available damping compound that you can buy yourself, it's no surprise that you can field assemble a much heavier, better damped panel for a fraction of the cost. And the seams will be stagered from the first layer to the next so better assurance against seal failure.

This is an area where the DIYer will always do a better job for a lot less. Good news