Sub Enclosure made out of concrete

[BassAwdyO]

hmmm... you know finishing fiberglass would probably be alot easier if you made it with some sort of a mould where the showing side would be made where the mould touched and would be smooth and prime for sanding

[AndrewT]

Hi,
Gilbert Briggs investigated sand filled panels for full range speakers. His information was from a time when deep bass was not even recorded never mind able to be reproduced. There may well be more recent research that supercedes his work.

I believe this does give a dead sounding box and that will apply at any frequency. The inside could/should be braced just like any other comparable speaker space.

The little Acoustic Energy AE1 (mk1 & 2) uses plaster (of paris?) as a thick coating inside the box. This deadens the panels and adds weight (cheaply). I don't have any info on the current mk3 or the bigger brethren.

Fibre reinforced concrete will improve the tensile:compresive strength ratio above the 1:10 (approx) ratio inherent in standard concrete. Another major advantage of reinforcing is crack control, fibres will reduce the tendency for major cracks to form in the shell as curing /shrinkage proceeds, to be replaced with micro cracks that holefully will be inconsequential. But adding fibre changes the mix from cheap to quite expensive, but nowhere near as expensive as GRP. Getting a gas tight mix may be difficult, particularly if you use foamed aggregate although a painted on sealing coat could solve this.

[Drew Harty]

Hello,

I made an ACI Titian subwoofer using concrete. It was simple to make and much, much stiffer than a box enclosure. I purchased two laminated wood tubes (available on the internet from several vendors), then sleeved one tube inside the other and glued a MDF plate on the bottom pre cut for the driver. I poured a concrete epoxy slurry in the cavity between the two tubes--about 1.25" and then glued on a top plate cut from MDF. I purchased the exterior tube prefinished with a wood veneer and placed the plate amp in a small box outside the driver enclosure. It weighs about 80lbs, a guess, and is extremely stiff. Placing one tube inside the other and filling the cavity with something stiff creates a stress skin construction which is very ridged. You could also pour a high density foam into the cavity, which would make a stiff enclosure but wouldn't have the weight.

Drew Harty

[BassAwdyO]

It was simple to make and much, much stiffer than a box enclosure.

Maybe simpler... Obviously you havent seen my lastest subwoofer enclosure!

[mantraxl]

Made with no rebar, 1.5" thick walls, Sackrete 5000+ pre-mix. No internal bracing. This is the first attempt. A friend and I are working on testing samples of different mixes including concrete, ground rubber, and structural fiber in varying proportions. The mold for the 2nd version is already in the works.

Subjectively, several people including my girlfriend have guessed about a 3db in-room gain, compared to the 1.5" thick mdf box that the same guts (Rythmik DS12) were swapped from. Presumably, this is due to reduced backwave bleed-thru causing cancellations. I will run the Audessy EQ and compare the levels for the sub and report back.

PS: 200 lbs, in case you were wondering.

[MaVo]

I remember a post from Tom Danley. His company made a BT7 special build with doubled wall strength for a customer. yet, when measured, they didnt perform any different than the normal ones. for me this does mean, concrete wont have a gain in acoustic performance but a loss on money side.

[paulspencer]

Concrete is a novel idea and an interesting project to try. It definitely has downsides. Firstly, it will always shrink - I'd be wary of a construction that does not allow for this. Secondly, very heavy. This makes for more difficult construction, and you have to factor in the cost of chiro visits after! It's probably the heaviest way to get a given result, so it justifies itself more on the basis of novelty and the "why not factor" than it's technical merits. I read about one guy making concrete kitchen benchtops - an inspirational effort but the effort involved put me off.

Did I hear the word "easy" in the first post? That word definitely doesn't belong on this thread!

The Rythmik kit is a great choice (I have 2 of them). If you are going to go to this kind of effort in making the box, you should make sure the sub itself is worth the effort. Often the driver itself is the limiting factor and putting too much into the box doesn't make sense in that case.

Want the most cost effective and simple way to get a solid, stylish curve box? Laminate 3mm MDF sheets together. Use the internal bracing to create the curve, bend the sheets around to create curved sides. Similar to B&W subs in appearance. The cost isn't too bad and the complexity of moulds and mixing materials and construction methods is avoided.

This is shown in the image minus the side curved walls. The top in this one is a bit more difficult!

Ed mentioned foam - I'd be considering this perhaps before concrete. Geddes talks about a combination of a fibreglass skin with foam being better for enclosures than MDF (however I suspect this applies more to fullrange than subs). Lots of work to do it this way. In effect you create it twice. By the time you have created a mould you could have finished the MDF version.

An alternative. Built it using the curved MDF approach. Create a few layers to give an internal skin wrapped around the bracing. Now put in some spacers so that you get a big gap between the sheets - 20 to 50mm (depending on how extreme you want to go). Now continue to wrap a few more layers of 3mm MDF. Pour in urethane foam (choose a dense version, not the stuff you buy in a spray can). Let it ooze out the top and bottom, or put holes in the inside skin and let it expand and ooze inside. You can cut it back shortly when it stops expanding.

[tb46]

Hi Rafal,

This sounds like an interesting field of investigation, as a general comment-if you haven't yet-google for:

- ferrocement,
- fabric cast concrete, (e.g.: http://www.umanitoba.ca/cast_buildin...no_Article.pdf),
- concrete stepping stones (http://www.concretegardenleaves.com/concrete-leaf.htm)
- hypertufa,
- Lafarge Ductal (shells seem to have been cast down to a minimum thickness of 10mm), steel fiber and PVA fiber reinforced concrete.

The way I see it, for your standard subwoofer use standard construction methods and materials.

For your concrete subwoofer I would suggest stepping away from the beaten path, and trying to come up with something a bit different in shape and assembly, e.g.: as the individual parts of a concrete enclosure will be quite heavy, it might be interesting to see if an enclosure can be build that can be knocked apart and assembled reasonably quickly; e.g.: you could combine wooden edges that fit together with cloth forms into which you shape your concrete.

For build-in horn enclosures (including subwoofers) concrete can be quite impressive, just make sure that your model is correct, and the foundation strong enough :-) (http://www.audioavid.de/projekte6.htm).

Regards,