DIY Concrete speakers
As some of you may know I am a young audiophile, I am currently in my last year of A-levels in the UK and have started my exam project in DT. It is quite an arty course but I like to bring my technical skill into it anyway. I have decided to create a concrete speaker for it as the fluidity of concrete will allow me to make an "arty" type mould, and the fact that because concrete is so dense I will create a very inert cabinet. So I was wondering if anyone had any ideas for mix ratios and what aggregates to use for a smooth uniform finished concrete piece. I've heard of people making a cement mix instead but there are no pictures of how they turned out. If anyone could help me with this I would be very grateful. I do know that large aggregates like stones should be left out because of them sticking out of the surface, but I don't know to what extent. Thank you in advance, you guys have been great on the forum, you are very good people. 😀
Never worked with concrete in a non-structural situation, and it's always been delivered or from a ready-mix bag, so, here goes.
You'll need some sort of structure (wire form or multi-piece mold) to be able to shape the concrete to the end result. And the piece needs to be thick enough to withstand the vibrations without cracking. (There's a lot more to it than that, but let's let that be one of the first things.) And since you are going to be doing what would be considered an art piece out of concrete, your aggregate should be either sand or fine chip stone. That's the kind that looks like the little bits left over on the ground when you smash a rock into "not quite dust". That'd be your call as to what you want.
Next I'll be giving some links, since I am not sure what the "statue people" use.
Concrete basics - a good primer for construction of wells. While not a speaker enclosure, it does have merit.
How to properly mix concrete - good for the home do-it-yourselfer.
Proper mix proportions - self explanatory.
Best mix for decorative / art concrete:
concretedecor.net - determining the best mix for your use
madebybarb.com - mixes for crafting
Google / Pinterest - Pinterest search for the above types of mixes / tips.
I know there are some more craft mixes out there that have added polymers that make it easier to shape, etc., but I'm not up on that. It's best to find out more.
Last thing. Slump is important for how easy the concrete holds it's shape when wet. The lower the number, the stiffer the mix. What you want depends on your choice between free-form sculpting or use of a mold. It is explained in one of the links above.
Above all, take your time and be patient. Concrete is a medium you cannot rush to the final cure. And have fun experimenting with it!
Thanks for the help Charlie, I should have mentioned this but I'll be making a 3d printed mold so I don't need to worry about it being sculptable, thanks for all the information that should be everything I need to get my project started...
Actually just one more thing, I want this to be as resonance-free as possible (one main reason I'm using the concrete), any minimum thicknesses you recommend for a -4db point of around 40-60hz? Many thanks. Elliott Bridge.
That's a darn good question. I am not sure what the stresses would be, but I think they'd be much more with a larger driver with good or extreme XMAX. I mean, I've had concrete stuff my mother-in-law made that had edges around 3/4" to 1" that lasted outside for almost 30 years, but they were static pieces not subject to such vibrations. So I'm kind of on the unsure side. I would think that at least 1" for smaller drivers and up to 2" for the bigger ones. That's just a W.A.G. based on what I've seen and think from seeing construction materials. Ultimately, it will probably come down to how well you reinforce, form, and cure. Cure being one of the most important aspects. Concrete reaches design hardness in about 30 days. So, like I said above, patience is necessary when working with concrete.
At this point, you may want to look at some others who have done this, such as SoundBlab or DIYPerks. There's a few on YT.
Yeah I saw the videos, not to useful though they just show the build process (no mix amounts or details etc.). As for speaker size, it is a 5.25" driver, however it has 20mm displacement (xmax) so it packs a punch: i.e. 3db down at 40-60hz depending on the Q of the enclosure I decide on, I think I'll go for 0.8.
Yeah, I know about the curing it is different for each mixture, you are meant to use the least water possible (to an extent) for the highest strength and then keep spraying with water when taken out of the mould... It is the thickness I'm worried about because I need the smallest cabinet possible whilst still needing the internal volume for the damping of the speaker. I am using a bunch of fibreglass and polyfill though so that will make the enclosure "seem" bigger to the driver. Thanks a lot for your help Charlie. Hopefully people will find this post and it might help them if they want to do something similar. Once I have done my project I'll upload my build log and everything, but I can't do that until results day though because of the exam board. 😀
That much? I'd err on the side of caution and go with a minimum of 1-1/4" to 1-1/2". I can't give you any calculations, just what I see in my head at this point. The info on lateral / vibratory sheer stresses are not easily located.
Actually, let me look in my archives. I have some old engineering programs that may have something in them.