# Diaphragm / Membrane Bass Traps



## icor1031 (Mar 24, 2011)

I have various questions about diaphragm resonators, but the first ones:



Is there a cheaper alternative to Mass Loaded Vinyl? What about PVC "closed cell expanded" sheets, dB-3 Acoustical Barrier @ Home Depot, or Torch Down Roofing?

If I put insulation in them, they'll give a moderately wide absorption band, right?

Can they be useful down to 16Hz?



Thanks.


----------



## phazewolf (Feb 5, 2012)

Some of those may work but nothing I know will get down to 16hz. The are by nature not broad spectrum devices but adding insulation to them widens the scope do which they cover. Again not by a lot if I recall maybe be a half octive each side of the peak.

One other thing your going to most likely need several of them to treat a issue not just 1 or 2.


----------



## bpape (Sep 14, 2006)

Unless you want to go 36" thick, you'll be lucky to get to 30Hz. 

The mass of the membrane, the thickness of the absorber, the density of the absorber all determine how low it will go and where it will cut of on the top end. For instance, FSK as a facing will start to cut maybe 600-700Hz. Pond liner - maybe 350. 1/2 lb MLV, about 250Hz


----------



## icor1031 (Mar 24, 2011)

bpape said:


> Unless you want to go 36" thick, you'll be lucky to get to 30Hz.
> 
> The mass of the membrane, the thickness of the absorber, the density of the absorber all determine how low it will go and where it will cut of on the top end. For instance, FSK as a facing will start to cut maybe 600-700Hz. Pond liner - maybe 350. 1/2 lb MLV, about 250Hz


250Hz for 2lb MLV, perfect! A specific estimate 

The other gent mentioned a half octave away from the peak, does that sound about right - below the target? i.e., if I target 40Hz, the resonator is only good to ~30?



Thanks.


----------



## phazewolf (Feb 5, 2012)

If I recall ruffly the box would be around 9 1/2 inches deep with 2lb limp mass on the front to hit 40hz I am at work so i will look later


----------



## icor1031 (Mar 24, 2011)

phazewolf said:


> If I recall ruffly the box would be around 9 1/2 inches deep with 2lb limp mass on the front to hit 40hz I am at work so i will look later


Yeah, the excel file I'm using says 9.01" (internal), so - you're close.


----------



## phazewolf (Feb 5, 2012)

I have 2 rolls of 4x10 2lb limp mass sitting at hone that I still need to try one. The issue is getting it tuned just right which a lot of people have issues with. I would think using 8 2x2 traps would be a fairly good amount of traping but never got to building any yet. 

When if ever my basement gets setup that is where they will go.


----------



## bpape (Sep 14, 2006)

If you're building a sealed membrane then yes, it's more narrow in scope. If a limp unsealed membrane, then bottom up to X with X defined by the mass.

And the 250Hz was for 1/2 lb, not 2lb.


----------



## icor1031 (Mar 24, 2011)

bpape said:


> If you're building a sealed membrane then yes, it's more narrow in scope. If a limp unsealed membrane, then bottom up to X with X defined by the mass.
> 
> And the 250Hz was for 1/2 lb, not 2lb.


Oops.

Any idea on the high Hz for 2lb?


----------



## icor1031 (Mar 24, 2011)

Does any one know where I can get 4lb/ft2 membrane? 

*ALSO*, can I build the box with varying depth? In other words: 
The very center of the box would be deepest. The sides would be most narrow. (To fit snugly into a room's corner, while increasing depth.)

If so, I take the average depth and put that into a calculator, right?


----------



## bpape (Sep 14, 2006)

You're still talking 2 different things. If it's a non-sealed damped membrane, then the volume of the box doesn't matter, only the thickness and it should not have a back on it at all. If you ARE doing a sealed box with a back then it tunes narrowly.

No idea on 2lb high cut, sorry. 4lb MLV, not heard of it


----------



## icor1031 (Mar 24, 2011)

bpape said:


> You're still talking 2 different things. If it's a non-sealed damped membrane, then the volume of the box doesn't matter, only the thickness and it should not have a back on it at all. If you ARE doing a sealed box with a back then it tunes narrowly.
> 
> No idea on 2lb high cut, sorry. 4lb MLV, not heard of it



My mistake, you mentioned sealed and unsealed earlier. I should have clarified: yes, I mean sealed. Here's an image to help explain the kind of box I'm talking about:











The brown is the wood, black is the membrane, and gray is some insulation. The rest is just air. 
The membrane is pressed tightly to the wood, to make an air-tight seal. The box would have a top and bottom too, of course - thus, it's sealed. 

Most of the box is shaped like a triangle, to fill up a corner of a room and (hopefully) increase the apparent depth. 


The depth in the triangle area varies from 0" (left and right side of the triangle) to, let's say 8", at the center. 
So, how would the depth be calculated in the triangle portion? Would it be 4"? (I need to put that into the calculator, to find the tuned frequency.)




If it's still not clear, I apologize.


----------



## bpape (Sep 14, 2006)

That's going to tune VERY narrowly. Figure the volume of that triangle plus the rectangular space and relate that to the volume of a rectangle the size of the face (gotta work backward).


----------



## icor1031 (Mar 24, 2011)

bpape said:


> That's going to tune VERY narrowly. Figure the volume of that triangle plus the rectangular space and relate that to the volume of a rectangle the size of the face (gotta work backward).


The example above is 4" of insulation (though unstated and unscaled, lol). And unless I screwed up,
8" in that triangle should give *4"* of volume if it were a rectangle the size of the face.


The blue one should mimic that.










The green one is an alternative design, with 200mm (8") insulation, with the triangle beginning 10" out from the corner. (5" worth of volume)



Does it look like I followed your directions correctly?  
Thanks for your help!


----------



## icor1031 (Mar 24, 2011)

I really like these two:


----------



## bpape (Sep 14, 2006)

and those are limp membranes, not sealed ones which is why they are broader. Damped limp membranes will be even broader on the bottom side but still roll off on the top side.

In the end, what you do should be based on what the room requires. Do you know that part yet?


----------



## icor1031 (Mar 24, 2011)

bpape said:


> and those are limp membranes, not sealed ones which is why they are broader. Damped limp membranes will be even broader on the bottom side but still roll off on the top side.
> 
> In the end, what you do should be based on what the room requires. Do you know that part yet?


Ah, . I don't want unsealed, because of the fibrosis possibility from the insulation. *Unless putting 2mil plastic over the insulation won't ruin the affect in this kind of design? *
I don't suppose you know of a calculator that shows the response like this does, for a sealed membrane resonator?


Also, no - I don't know what I need yet. I have a proper measurement mic. It's calibrated, not that it needed to be. However, I'm moving in about a month - and I'll be able to take measurements shortly.


----------



## bpape (Sep 14, 2006)

Other than mine that I use to design our Scops Traps, nothing - but the formulas are available on the web.


----------

