Limp Mass Build Question
Posted: Thu Jan 05, 2023 8:57 am
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.
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.