Concrete speakers

Hi

I would like to know if there is someone with expereience in this field.

I currently have a pair of "working prototype" speakers. They are made from 12 mm plate, wich is rather thin. After assembling the cabinet I cast ~30mm concrete on each side. So the finished cabinet is a rather solid construction.

They are 90 cm tall and 25*20 cm floorstanding loudspeakers, so they are actualy not that big. They are 2-way with vifa d27tg tweeter and a vifa M18wh woofer (6.5").

The cabinet is dead at even loud spl. All who have listened and seen the speakers think they are great (they are veneered with teak). But then i tell them about the concrete they say I'm eccentric, at least.

Never the less I'm thinking of improving the design with the experience made on the 1'st pair. I would also like to hear from anyone with any experience in this sort of teknique before i start.
 

Brett

Member
2002-01-07 6:02 pm
Hi,
I'm not sure how you want to improve them. With a 30/12/30 sandwich for the enclosure, it should be very inert as you say it is. And then with some nice veneer on the outside to make it look more domestically acceptable, it sounds like you have some nice speakers.

I think concrete is an excellent way to go for enclosure material as it can be extremely inert if done right, has high mass for the driver to work against and the high pressure generated by the driver(s) especially if they are large and/or high spl will be well contained with little/no flexing of the walls. However, it's also bloody heavy and makes mucking about with optimising placement hard.

Concrete can 'ring' though. Pick up a bit and hit it with a hammer to see what I mean. Perhaps if you build another enclosure, you could try using some aggregates and/or different mixes to minimise this. I'm sure a web search will turn up 'tons' of info on on different concrete formulas and thier performance. Also, concrete gives you the flexibilty to shape the outside any way that pleases you, and the inside in odd shapes to minimise standing waves, subs could be integrated into the house as parts of internal gardens, built as extensions to the fireplace, window seats etc, thin face bricks/granite/marble used on the outside to blend them in to the decor, or any of hundreds of possible finishes that have been developed for other purposes. There are also special flexible plywoods and steel, aluminium or cardboard tubes that can be used as forms to get the shape how you want it.

I have an old G.A.Briggs book on speakers that uses (mono) corner enclosures built out of bricks and getting good performance out of them.

Two other construction techniques to consider:
- timber shells with a constrained layer of dry packed sand in between. Harder and more expensive to make than concrete, but has to possibility of being able to pour out the sand for house moves.
-autoclave concrete blocks, often called Hebels. These are very inert, and come in a range of block sizes, can be cut with a special saw, and are glued together. Very popular in parts of Europe and becoming more so in Australia. I am building a set of "false corners" for my Klipschorns out of them. www.hebel.com

HTH,
Cheers
 

jwv3

Member
2002-03-04 2:12 pm
DE
JoeBob said:
I don't have experience, but I've heard of talk about making spherical enclosures with concrete. Using say a ball and submerging it in concrete and then removing the ball after it's dried. I'd like to try it myself, but I gotsta wait until summer :(

How about a fiberglass sphere? It would be pretty easy to wrap a beach ball (or what ever size) in fiberglass. Cut a hole for the driver and voila. Wouldn't the 'equal pressure on all sides' theory of the sherical shape apply to any material?
 
Thanks guys!

Chris, Bryan:

I think Darren Kuzma's site was the most interresting. It seems like he did a lot of research on this. He talked about adding rubber or metal pieces to change the damping characteristics , and i think i will look into that in my next project.

JoeBob: Go for it! ;) I will wait untill summer also...

Brett:

They are nice speakers indeed! :D
Being the 1'st speakers I have buildt, I think there should be room to make an improvement or two. In my initial contribution I did'nt say anything about what kind of improvement I had in mind though. But now I know my needs better, I think I will add another woofer to make them 2½ ways. I will make them bi-ampable, external crossover, etc. - Basicly "normal" improvements not related directly to the concrete itself.

I still have a block of concrete i cast in a used milk-carton as a test. I gave it a few knocks and heard the ringing. Before starting on the next pair, I will make several tests using different materials as aggregates. As you mentioned, I might be lucky to find something about this on the internet too.

The inner shape of the walls are something I want to look into also. On my 1'st pair I just put a piece of wood under the bottom end of the cabinet to make the concrete a little thinner in that end. Doing that on all four sides, the inner walls tilted slightly inwards towards the top of the cabinet, thus making the front bafle thicker where the speaker units are mounted.

Hopefully i will be able to take pictures of the construction as I begins and post them too along with the "aggregate test".
 

Havoc

Member
2002-02-06 9:16 pm
You can use polyurethane foam plates to shape the interior as you wish. Then build a plywood case around it and pour in 1 go. Break the foam away after drying. Best would be to pour on a vibrating table to eliminate air bubbles, but having access to one is not easy.