volume adjustment for cabinet lining

cast55

Member
2010-09-07 10:40 am
I recently purchased some damping material which I intend to use for lining the walls of a ported mid bass enclosure. Specs are here:

Sonic Barrier 3/4" 3-Layer Damping Material w/PSA 18" x 24" | Parts-Express.com

My enclosure will require about 480 cubic inches of this material. I do not know how to appropriately account for the loss of cabinet volume due to this lining. Being a porous acoustic foam composite, I presume that the actual volume loss will be somewhat less than if I considered the lining as a solid, but I am unaware of any specs or rules of thumb to guide this estimate. I would appreciate a response from anyone who has used this material in their own designs, or who could suggest how I might best account for the lining volume.
 

Ron E

Member
2002-06-27 10:41 pm
USA, MN
I haven''t used this, but if I had to guess, I might subtract the thickness of the back two layers (to the darker strip) and ignore the thicker foam layer. you might model the cabinet with the volume reduced thusly and decide whether it makes any appreciable difference.
 
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good news, it's bigger!

My enclosure will require about 480 cubic inches of this material. I do not know how to appropriately account for the loss of cabinet volume due to this lining.

Cabinet stuffings such as fiberglass and foam actually increase the cabinet apparent volume rather than reduce it. There is a conversion from "adiabatic to issothermal" conditions (I'd explain it if I understood it!) that give a theoretical 40% increase in apparent volume. In practice I've never seen that much increase but would use maybe 20% as a factor. That means your 480 cubic inches looks like 576.

It won't make a huge difference in your simulation but it is :)in the right direction.

David S.
 

cast55

Member
2010-09-07 10:40 am
I have heard of this for sealed enclosures, but I am using the material only to line the walls of a vented enclosure - not to stuff it. The 480 cubic inches is not the enclosure volume, but rather the calculated volume of the installed lining, when considered at 100% (i.e. as a solid). The enclosure itself has a total volume of 1900 cubic inches. I want the lining to assist damping of the panels, as well as to eliminate standing waves and internal reflections. The application is a PA mid bass (55 Hz tuning) - this is in contrast to the adjacent sealed midrange chamber which has no lining but is loosely stuffed with fiberglass insulation. In that case, I could see the stuffing increasing the apparent volume, but the frequency response of my midrange driver within its active passband is particularly insensitive to the enclosure volume, so I ignored any effect that the fiberglass has.

As far as the PE Sonic Barrier material is concerned, you think that this would increase the apparent volume of a vented enclosure? Looking at the description, it is a 3/4" laminate of 1/4" of foam, an impermeable barrier, and another 1/2" of foam, so considering it at 50% of its outside volume seems reasonable, as a component to be subtracted from the enclosure volume to get the effective volume. If it increases apparent volume however, then I should just ignore it completely?
 
I have heard of this for sealed enclosures, but I am using the material only to line the walls of a vented enclosure - not to stuff it. The 480 cubic inches is not the enclosure volume, but rather the calculated volume of the installed lining, when considered at 100%

I didn't say it would be a big factor but a proper stuffing material will increase apparent volume. It doesn't matter whether we are talking about vented or sealed. +20% for the actual volume of stuffing is a good approximation and that is where the number 576 came from (480 looks like 576 or +96 cubic inches).

The inner vinyl layer is of minimal volume (a mass layer), so the total material is primarily compressible foam.

David S.
 
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cast55

Member
2010-09-07 10:40 am
I received the foam today, so I made the modification. Each of these enclosures consists of two separate ~30 L chambers, one of which has an 18Sound 10M600 midrange driver in it, and the other side has been empty pending expansion. My shipment from Parts Express arrived today, so I made the modification to the second chamber, cutting a hole and adding a 2.75" ID x 3.375" port out the back. (This position isn't ideal, but there was no room on the baffle for it.) I lined everything except the baffle, the port tube, and a few insignificant spaces around the cabinet handle. The cabinet is 3/4" MDF with the exception of the baffle panel which is 1", and is coated externally with about 3/32" of polyurethane bed liner. It passes the knock test to my satisfaction, and sounds sufficiently dead inside. These chambers will carry 18Sound 10MB600 drivers.



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