In this case, I do agree with John Brandt, where he says: "Question everything. Especially people who want you to spend a lot of money when common sense tells you otherwise. Beware of unscrupulous merchants."
OK, here's the straight truth from somebody who isn't trying to sell you stuff, and just designs studios for a living:
What is MLV? It is Mass Loaded Vinyl. A flexible rubber-like sheet, just a fraction of an inch thick, that is very heavy. And very expensive. As the name suggests, it is made mostly from vinyl, but also includes some extremely heavy additives mixed in to the gunk when it is manufactured, to give it much more weight. Often that is some type of barium compound. That makes it very massive while still allowing it to be thin and also flexible. Those are the two desirable characteristics of MLV: It is thin while also being heavy, and it is flexible.
Does it work? Yes, it does. Like all massive, heavy, dense building materials, it can be used in several different ways. One of those is to increase the mass of a leaf (wall, ceiling, floor, door, etc.) However, it is very,
very expensive! Here's a link to a current add on Amazon:
https://www.amazon.com/Loaded-Soundproo ... B007N3356S That's US$ 210 for a hundred square feet roll of 1 pound-per-square-foot MLV (1 PSF). Here's a link to Home Depot for ordinary 5/8" drywall
https://www.homedepot.com/p/Sheetrock-F ... /100321591 That's US$ 21 per sheet, which is 32 square feet. So three sheets would be about 100 square feet (same as the roll of MLV), and would cost US$ 63. However, the drywall is MORE THAN TWICE AS HEAVY, at roughly 2.2 PSF. So to get the same mass by using MLV, you'd need to double up, using two layers: So that would cost you US$ 420 to cover the same area with the same mass as three sheets of drywall for US$ 63...
Here's the thing: sound waves are not snobs, nor elitist: They can't even read the price tags on your materials, so they really don't care if you paid SEVEN TIMES MORE than you needed to. All that the sound waves care about, is how much mass you put in their way to stop them. That's what they react to, not how much you paid.
So yes, MLV does work, and is excellent stuff... if you happen to be the shop that sells it!
You'd make a wonderful profit from telling everyone that it is magical stuff, made with pixie dust, and it stops sound waves fantastically... while NOT mentioning that a sheet of drywall would do the exact same job and cost only 14% of that price, ...
Does it have uses in acoustics? Yes it does. Having said all of the above, there are actually some places where it is justifiable. MLV has two big advantages over drywall: 1) it is flexible, and 2) it is thin for high mass (in other words, very dense). #2 means that any time you need a lot of mass and have to fit it into a thin space, then MLV makes sense. For this reason, I often use it as part of the "sandwich" of materials that make up the front baffle of a speaker soffit. I could use drywall, but that would make the baffle much thicker, and I don't want to do that, as it means that the speaker has to be cantilevered too far out of its mountings, which places too much weight on the front mountings, and tends to make it lean forwards. So I use MLV which is high mass in a thin package. The other advantage (#1 above) means that you can use it where using drywall would be complicated or impossible. I often use MLV to isolate water pipes and sewage pipes in the ceiling space or walls around studios. You wrap soft insulation around the pipe, then wrap MLV around that, and seal it. MLV is flexible and high mass, so it works very well to stop the pipe sounds getting out, in that application.
You could also use it in the way John Brandt shows in that link, as a limp-mass "curtain", hanging inside the wall, and that can certainly work acoustically... but that creates a third leaf, so you would have to compensate for that in the wall design, and it also creates the potential for another issue that John didn't mention: rot. fungus. mold. Inside the wall! Since MLV is an impermeable sheet, it will act as a vapor barrier inside the wall. And since it is NOT pressed up tightly against the warmer leaf, it will remain cold, so humidity held in the air inside the wall will condense on the MLV in cold weather, running down as liquid water, and providing the ideal conditions for mold... as well as wetting the sole plates, and providing ideal conditions for wood rot.... So I would not use it like that either.
One other use is in adding mass and damping for sheet-metal parts, such as the car door and roof panels, and also on HVAC ducts: that can work quite well, when done properly.
That's in the actual construction of the studio, but MLV also does have uses in some types of
acoustic treatment, namely: limp membrane traps. There's a type of resonant bass trap called a "panel trap" that works by having some type of mass placed across a sealed air-tight chamber, and you can tune that to absorb certain low frequencies, very well. Often the "mass" is just a sheet of plywood or MDF of a specific thickness, and that works, but there's a sub-class of panel traps, called "limp membrane" traps, where you can use something like MLV as the mass: You place it across the front of the sealed chamber. It works very well in this application, even though it is hard to tune the device properly. But if you do tune it well, then it works great. The cost is justifiable here, because it allows you to build a low-profile trap that works at very low frequencies, and fits into a thin space.
In those three applications, the high cost of MLV is justified: wrapping pipes, adding mass when very thin structures are needed, and for limp-membrane acoustic traps. It works very well for those three, and the cost is justified, since no other material works the same.
However, the application described in that link you provided is pure garbage. Adding thin battens to an existing wall then adding MLV and drywall on top of the battens, with no damping, could very well REDUCE your isolation on that wall, instead of improving it! Why? Because you built a panel trap! That's exactly what a panel trap is. And with a thin un-damped air gap inside, it will resonate at a fairly high frequency, thus TRANSFERRING that frequency INTO the wall! Where it will flank across to the other side, and be heard loud and clear. This is the infamous "three-leaf" problem, that is so well known in studio design, and is always avoided wherever possible. If you really did want to build a panel trap, that's not the way to do it, as it would trash your isolation.
Also, as John points out, if you sandwich MLV between two layers of drywall (without the battens), then that negates one of the most important properties of the MLV: limpness. If you hold it rigidly between layers of drywall (or any other far-more-rigid material), then it is no longer "limp mass": it is just mass. Or rather, to call it by its correct technical name in that application: "grossly exorbitantly priced mass"... and we get back to the point of sound waves not being able to read price tags....
Some vendors try to convince you that MLV will act like a CLDM (Constrained Layer Damping Material) in that application: not true. It won't. Or rather, it might act slightly as CLD, but the effect will be minimal, and certainly not worth paying seven times the price.
Green Glue Compound is way cheaper (although still expensive) and really does work as a very effective CLD, if that's what you need. (MLV can be used as a CLD in some applications, but it's not easy to use it that way, and there are better methods).
So that's the plain truth abut MLV: It works, yes. It is extremely expensive (about seven times the cost of drywall, pound for pound). It has a few very limited uses in studios. Adding mass to an MSM leaf is not one of those uses. You would get the exact same benefit for 1/7th the cost, by just adding another layer of drywall.
Any place that tries to sell you MLV for that purpose (adding mass to a wall leaf) is a place from which you should run, very fast, while checking your wallet carefully to make sure all the bills are still there.... It is not honest to say that MLV is the best product for that application...
The second post in that thread, by Andre (highly respected acoustician!) sums it up: "There is no advantage to using MLV in a wall. Regular gypsum is about the cheapest. If you want to add more than just Gypsum board use Green Glue between layers of gypsum board."
- Stuart -
