Concrete Anyone? Subwoofer et al (1 Viewer)

Hello All,

Although I briefly mentioned this in another thread, I thought I would place it in a thread of its own and not little someone else's thread.

The idea is using Quikcrete readi-mix concrete in a form to fabricate, in place, a subwoofer cabinet that is about as static as physically possible. This would be for the DIYer with no intentions of ever moving it.

My HT room is in my basement. The stairwell has a landing halfway down (where it makes a u-turn), and the cavity underneath that landing forms a square box that is about 3.5' on the side/elevation.

The space is pretty much useless for any and all other regards, and considering many have said and I've read that the idea behind a subwoofer cabinet is to make it as acoustically static as pissible.

I have not heard of any DIYers that have done this, but I have seen pictures (wish I had their links now) of concrete-formed baffle systems for professional environments.

I think the most difficult portion would be either forming the cabint in place and making necessary accomodations for the driver opening, etc., or making in a form and sliding the end result into place.

I've thought about it actually... then I realized I would never get this to fly past the Mrs... so that plan went by the wayside.

However... one of my favorite speaker designs uses pre-formed concrete sewer pipes and may give you a little insight on working with concrete.

I think my kind of personal application would not have a problem with WAF as it would be hidden in a cavity not in the HT room (or any room for that matter) and visually covered via acoustically transparent material.

I take it your application does not have this option.

Brae,

Keep in mind that the Quikcrete tubes do not come larger than 12" in diameter (at least not at the local HD or Lowe's).

I never said anything in this thread about tubes.

Oops! Sorry about that! :b

I think I could make a form from cheap OSB, with an inner liner, but the problem I am seeing is one where I could do 5 of the six sides of a cube, but the 6th side can't be installed as I would need a way to pull said liner out.

I suppose I coule make it in such a way as that the 6th side is the side the driver is actually mounted on, and then fabricate 1-2" of MDF faceplate. Still, I suppose one would have to be pretty accurate to get the box's volume right as adding to or chiseling away from a concrete cabinet would be messy, labor intensive, and just not worth the consideration.

Also, for the beta box cabinets ... what does one do with the throwaway beta cabinets?

If I hadn't seen it, I wouldn't have believed it. Aristocratic Sewer Pipes. Ingenius!

One man's trash is another's treasure.

IDEA: can you burn the inner wooden form out? After the concrete is hard, pour some lighter fluid inside and toss in a match. But, maybe this'll damage the concrete.

Or you could reach in there with a sawzall and break up that inner box just enough to get it out the driver hole.

Hmmm.. or, you could just leave the inner box in there.

Actually, making the 5-sides of the concrete cube is probably better in that it'll make for ease in terms of installing the driver.

I haven't really gone into any deep thought on this, but doesn't concrete have a very high Q ... which may affect the sound of the subwoofer? I was under the impression this was one of the reasons why MDF was preferred for it's heavy mass but low rigidity?

Just thinking out-loud...
Rob

Interesting idea... obviously concrete has high mass so it would lower the resonant frequency quite a bit, but it also has poor internal damping- sound travels through it quickly and easily, so it would be more prone to ringing (albeit at a very low frequency). Seems like it would best be used in a constrained layer design with mdf, etc.

Everyone, I have no experience in DIY acoustics. Furthermore, I have extremely little (almost none) experience regarding audio in general. So, please understand that what I am about to ask comes appropriately in nature:

What is 'Q' and what function does it control and whats the good, bad, and ugly of it?

Aaron, should we put together a real thought experiment? I'm certainly game. If I wanted to use the best 15" bass driver that didn't cost more than say $300 for the driver, how big of an internal volume would on need for a perfectly square cube? Would the mass of the concrete box dictate the resonant frequency? QuikCrete is cheap. How about a box with 4" thick walls?

If you are worried about the weight of concrete, you might look up the concrete canoe competitions on then internet that civil engineering students participate in. The build about 20' long canoes that are light as a feather. You could probably use this type of material. I don't know how the concrete would stand up to all the internal air movement. You could put a nice stuco finish on it or look at some of the canoes. I remember my senior year at LSU in CE, the canoe that the students built had a pretty smooth finish with a fine paint job. Hope this helps.

Daniel T

Brae,
This is a good site for explaining Thiele Small parameters. Understanding Thiele Small Parameters

Assuming for the moment that the location is actually a good placement for a sub, it sounds as though you have enough potential Vb to make an infinite baffle (IB) sub, so the enclosure could be built to be a very strong closet or storage area and used as a sub until such time as the need changes. It will need to have enough volume to allow 10x Vas/driver. If it's not big enough to be even 4x, then either a sealed LRT or vented design is probably best, and if there's too much volume for the chosen design, book/mags/newspapers/old clothes/etc., can be used to reduce it.

The driver(s) would be mounted to the door, so the jambs/door sill would need to be sufficient for proper gasketing and allow retaining it using bolts/threaded metal inserts. The door should be a UL approved commercial grade solid wood, or three layers of 3/4" MDF or Baltic Birch plywood, or a combination of the two, laminated together to get the desired rigidity/damping.

The walls of the cab can be concrete, MDF, BB ply, whatever 'floats yer boat', it just needs to be sufficiently thick based on the material's MOE/density to make it acoustically inert in the desired BW. Concrete is good for getting below the BW and BB ply is best for raising it well above it. MDF requires more thickness for a given performance level WRT BB ply, but its cheap, so in this case three layers of 3/4" glued to a 2x4 framework is probably the easiest/cheapest way to go.

If you want to use concrete 'just because', then the thickness needs to be based on the lowest frequency you expect will excite it, the internal dims of the 'cab', and the density of the type you want to use, i.e. Portland Cement is 150lbs/ft^3.

GM

Brae,

Since many people use Sonotube to build their subs, you could go that route, except in concrete. Culvert could be used for the "tube", and double or triple MDF used for the ends.
As far as it being porous, to whatever degree, seal it with something like the spray- or roll-on bedliner. Fill with your favorite stuffing and tell us how it sounds.