Limp Mass Build Question

[gabo]

I'm thinking of building some limp mass traps for use at a couple of frequencies.

If you're familiar, it's a rectangular sealed box, with a heavy membrane on the front, tuned to a specific frequency. I'm thinking of making some that are 3' x 6' with a depth of 10"

The general formula being fo = 170/sqrt(m x d) where m is weight in lbs/sqft and d is the depth of the panel in inches. These days, you can buy Mass Loaded Vinyl (MLV) easily online from many sources. Most of it is 1 lb/sqft but you can also find 2 lbs/sqft easily as well which is what I plan on using. From the formula you can make the depth shallower for the same frequency with a heavier membrane.

If you fill the box with fluffy pink stuff, making sure it doesn't touch the membrane, then the formula is fo=143/sqrt(mxd).

So if the depth is 10" and I use 2 lb/sqft MLV, the center frequency of the trap is 143/sqrt(2x10) = 143/sqrt20 = 32hz. Which is where I have a mode that wreaks havoc.

I have two questions for anyone who has built these things.

1. Is there an easy way to measure the frequency once you build it? I'm considering making the back panel of the box movable, such that I would put it together, then measure and possibly slightly tweak the back panel to exactly center the correct frequency. Maybe someone has a clever way to measure the fo.

2. How tight to make the front membrane? I've read that is needs to be loose (after all it is a "limp" mass). I've also read that it needs to be tight but not like a drum, just snug. Still others seem to think it needs to be just a bit tight. I know if it's too tight it will not be effective, I'm not sure if it can be too loose. Without better information I'm just going to try putting it across without wrinkles, so not tight at all but not sagging.

Any help or real world experience would be helpful.

Thanks in advance.

[gullfo]

generally speaking, you would use a "test bed" approach:

1) a room with a known response (e.g. an REW measurement as the baseline) -- see some of the test labs and standards for what this looks like.
2) build your absorbers - assume the calculations are correct
3) place the absorbers in the room and measure
4) compare the change -- did the expected frequencies get attenuated?
5) repeat.
6) once you have the absorbers working as expected, make sure your materials sourcing is consistent to ensure repeatability in additional units.

you can in the interim also place some mics on the surface of the unit and inside it and measure resonances and so on. they can tell you if the unit is resonating at the design frequency. however, actual absorption is empirical data.

[gabo]

generally speaking, you would use a "test bed" approach:

1) a room with a known response (e.g. an REW measurement as the baseline) -- see some of the test labs and standards for what this looks like.
2) build your absorbers - assume the calculations are correct
3) place the absorbers in the room and measure
4) compare the change -- did the expected frequencies get attenuated?
5) repeat.
6) once you have the absorbers working as expected, make sure your materials sourcing is consistent to ensure repeatability in additional units.

you can in the interim also place some mics on the surface of the unit and inside it and measure resonances and so on. they can tell you if the unit is resonating at the design frequency. however, actual absorption is empirical data.

Thanks Glenn. Yes I understand that measuring the room after I put the absorbers in is the ultimate test of the system. However, that's a bit challenging and I would prefer not to build 4 of them, then find that they all need modifications.

Testing with mic's and doing some REW work in the shop is what I was looking for. It's not really possible to do any of that from inside the box as it has to be sealed. I guess I'll have to do that via a bit of trial and error. I was hoping someone had done this before and come up with a clever way to do it in the shop. If I can build one, tweak it to make sure it's meeting the design criteria, then I can confidently build the others.

From what I've read, the calculations for the absorbers many times tend to be a little bit lower than actual performance. But I don't know if that's due to the physics of the absorbers or if it's something else.

If I have a good way to test them, then I can also resolve my first question of how tight to install the membrane. It's an interesting build. There seem to be a lot of people building studios, a lot of people recommending limp mass absorbers, but very few details of people actually building them. I guess everyone gains a bit of knowledge and then wants to keep that to themselves.

Thanks, G

[gullfo]

you could build a box (think drum) which would enable you to set the depth of the box (via backside frame and plate + insulation) using some screws, and a top which is the MLV stretched (like a drum head) and tighten (or loosen) as needed. you could use a drum head gauge to assess the tension used when you arrive at the desired specifications...
http://www.runnel.com/outbound/limp%20m ... %20box.zip

[gabo]

you could build a box (think drum) which would enable you to set the depth of the box (via backside frame and plate + insulation) using some screws, and a top which is the MLV stretched (like a drum head) and tighten (or loosen) as needed. you could use a drum head gauge to assess the tension used when you arrive at the desired specifications...
http://www.runnel.com/outbound/limp%20m ... %20box.zip

That's an idea, how did you know I was a drummer

I'll probably build one of these in the shop and possibly video the build and the tests to post to my Utube channel. If nothing else it would be good documentation for myself in case I ever build another one.

gabo


Also: I no longer get pictures on this web site. All the sticky posts and user posts no longer show any picture, just the name of the picture file listed where there is suppose to be one, such as pic1.jpg, pic2.jpg, etc.

[gullfo]

That's an idea, how did you know I was a drummer

everybody is a drummer! (some are better than others, just like singing lol)

[gabo]

That's an idea, how did you know I was a drummer

everybody is a drummer! (some are better than others, just like singing lol)

Haha, True.