It takes a lot of science and a lot of magic to make a good pair of speakers. From hard calculations with Thiele/Small parameters to ideas like 'transparency' and 'presence,' a lot of decisions make up a good speaker design.

One important factor is the material that you use for your enclosures; concrete has some distinct advantages structurally, acoustically, and aesthetically. It is very stiff and heavy--great for supporting the speaker drivers without changing their vibrational dynamics. It also damps out vibrations well, so it doesn't contribute unwanted rings and unpredictable tones to your sound reproduction. Equally important for something that sits in plain view all the time, it has a unique and interesting visual appeal.

So dust up on your concrete mixing skills and lets get to work!

Step 1: Find the Molds

Choose some outer molds that are the right size and will let your speakers balance as you want them. Shiny plastic molds release the concrete well. The ones in the photos are affordable but nicely designed melamine bowls from a department store.

For the inner molds, a kids' rubber play ball works great. This one is especially good because the nubs prevent it from touching the outer mold in a large area. You can adjust the internal volume by adjusting how far the inner mold sits down into the outer mold. Put sand in the ball so that they don't float in the concrete.

Also, don't forget that the molds are about to get very heavy, and will be difficult to move when they're full. Think ahead about where you set up your work. The surface should be as level as possible because the top of the wet concrete will level itself, and you want it to match to the height of the mold all around.

Step 2: Make Forms for Terminal Openings

To make the rear terminal opening, use two small craft foam cylinders. Cut each one so that the rubber ball sits atop it at the correct height in the outer mold. For more volume, set the ball lower in the mold by making the foam shorter and adding extra sand in the ball. Don't make the walls too thin. For less volume, make the foam taller, and put less sand in the ball.

Wrap the sides with masking tape and spray the bottom sides with foam-safe spray glue.

Peel the tape off and stick the gluey ends into the bottoms of your molds.

Step 3: Apply Mold Release

Give the insides of the molds a nice even coat of mold release spray or wax. Don't leave any pools or droplets. You don't need to coat the rubber balls.

Step 4: Mix the Concrete

Use whatever type of concrete you have access to. The speakers in the photos are made of a glass fiber reinforced mixture with no large aggregate (rocks.) Make sure you have enough concrete to fill both molds. Because you should mix a little extra, have some miscellaneous smaller molds or forms on standby to use up the remainder. Bonus prizes!

Step 5: Fill the Molds

Fill the bottom of the mold without covering the foam plug. Smear concrete up to cover the side surfaces of the mold, this will help achieve a smooth surface finish with as few bubbles as possible. Then carefully place the rubber ball inside. Fill the rest of the mold evenly to the brim, trying not to let air get trapped under the ball. Some shaking can help even out the concrete and eliminate bubbles, but make sure you keep the ball centered.

Step 6: Wait

Concrete takes a while to cure, so think about something else for a while. Maybe make a mold for your next project.

Depending on your mixture your wait time will vary. This mix can be de-molded in 24 hours; some might need to wait for a week. Don't be hasty, hmm?

Step 7: De-Mold

When the concrete has cured, pour out the sand. The ball should collapse and peel out easily once it's empty. The hard outer mold can be more difficult; blowing compressed air into the edge sometimes works. You can also try tapping, vibrating, shaking, and prying, but don't crack your concrete!

Step 8: Clean Up the Shell

Remove the foam plugs and punch through any thin wall of concrete that got between the ball and the plug. Remove any lumps that climbed up onto of the ball and sand the fronts flat with a belt sander.

Check for pin-holes where the ball may have touched the outer mold. You can tape over the outsides of these holes and then fill them from the inside with JB Weld.

Step 9: Cut Out the Front Panels

Trace the cleaned-up speakers onto some wood of your choice. Don't use wood that's too thin, or it will vibrate in an undesirable manner. Cut out the panels with a band saw and then bevel the edges using a sander with an adjustable support plate.

Trace, measure, or download your speaker hole pattern and mark it onto your front panels. These are positioned off-center to avoid internal resonances (plus they look a little bit like fried eggs that way.)

Apply a finish that goes well with the colors of your concrete and your speaker drivers. A clear coat will also protect the wood from moisture and scratches.

Step 10: Assemble the Front Panel

Seal the back of the speaker driver mounting faces and install them into the panels. Attach the panels to the concrete shells using a good construction adhesive. 

Some polyfill stuffing will help further reduce resonances inside the speaker. Make sure everything is well-sealed, because any leaks will show up later as hissing, whooshing, fuzzing, and all kinds of other annoying air noises.

Make sure you put the speaker wires in where you can find them from the back. The wires should be long enough to come a little way out the back of the speaker.

Step 11: Make the Terminal Block

Measure the concrete shell and cut some discs of wood to fit--one inside the hole, one covering over the top. Glue them together with wood glue and clamp them until they're set up. Find some speaker wire terminals that you like and install them into the discs.

These speaker wires have spade connectors that slide onto the nubs on the back of the speaker wire terminals. To be sure that nothing vibrates loose you can use hot glue as a backup. Use silicone caulk to glue the speaker terminal plugs into the back of the concrete. Make sure the smaller wooden disc has silicone radially between it and the concrete.

Step 12: Wait Some More

Don't try to play any music until the glue is dry. The speakers can generate significant air pressure inside your well-sealed enclosures.

Step 13: Listen

You're done! Sit back and enjoy the sonic radiation.
this <a href="http://www.liztek.com/products/bluetooth-speakers/" rel="nofollow">Bluetooth Speaker</a> connected via desktop and laptop?
<p>Wow good ible, great speakers!! thanks for sharing!</p>
This is amazing!
<p>Can't find a bowl like it, but perhaps it is possible to 3d print one, by modifying a Icosahedron model? http://www.thingiverse.com/search/page:2?q=Icosahedron&amp;sa=</p>
<p>Looking everywhere for these bowls or something similar, does anyone have any ideas at all? Thanks!!</p>
as the molds you used are no longer available, do you know of any comparable alternatives? I've hit a wall!!
What speaker did you use, or what are the specs so Ik can find one myself
Hey would these be good drivers for this design? <br>http://www.seismicaudiospeakers.com/Titanium-Horn-Tweeter-p/sahg207b.htm?utm_source=googlepla&amp;utm_medium=CSE&amp;utm_campaign=CA&amp;utm_content=productAdSAHG207B&amp;CAWELAID=1912445395&amp;catargetid=1911682958&amp;cadevice=c&amp;cagpspn=pla&amp;gclid=CMi6kf6Dz7kCFRDl7AodplcA9w
That driver is only designed for high frequencies and would probably not be what you are looking for (unless you are looking for only high frequencies). The driver we selected has a range from below 100hz (we are talking 3db down here, they have a fair punch and response below 60) to above 20Khz.
Ah ok, so something like the 4&quot; full range drivers that Dayton Audio has?
Me gusta!! Hagan más instructables de esto!
This is great, man. Amazing.
Just wanted to say congratulations on being a finalists in the Concrete &amp; Casting Contest! This was a fantastic instructable! Good luck!
Found the link for the bowls. I just ordered them in black..... <br> <br>http://www.target.com/p/room-essentials-melamine-geo-serve-bowl/-/A-14511968#prodSlot=medium_1_2 <br> <br>I also thought I might try this one in the future but I don't know if it is deep enough: <br> <br>http://www.webstaurantstore.com/red-get-b-790-rsp-1-9-qt-angled-red-sensation-catering-bowl/375B790RD.html
The 9-quart angled red catering bowl looks boring. It will produce a shape that looks like a slice off an egg. It also has the disadvantage of being angled, so it would have to be supported on an inclined plane during the concrete pour.
I like the shape of the bowl, but that's my personal opinion. If it will affect the sound though, I will take that into consideration. Thanks!
The best speakers would be to mount the drivers in a rigid, soundproof wall with an infinitely large space behind the wall. Good in theory, but not always practical.
Can you tell me where you were able to find the bowls you used for the molds? <br>
looks like maybe the bowl is from this set: http://jennskistudio.blogspot.com/2012/07/geometric-dinnerware-at-target.html which is no longer available at Target.
It looks like they have a picture of a blck version here: <br>http://www.target.com/c/dinnerware-kitchen-dining-home/-/N-5xtrv <br>But when you click it they don't have it listed anywhere...
I found the black ones, they are smaller dipping bowls: <br>
You got the source for the bowls right, picked them up a few weeks ago so they might still be on the shelf even if they show &quot;no longer available.&quot; We would encourage you to always keep an eye out for cool looking molds, you could do something very similar with almost any container.
No resonating I say ! Should sound very neutral without dips or peaks
great idea!!!, So, could you comments any idea to use them in gardens or external places? Many thanks.
This is a really great instructable. Thank you! Can you share some cost estimates for how much this project cost you?
Mainly depends on your budget for the speakers, you can spend as much ($200+) or as little ($30 for a decent pair probably) as you want.
Thanks! I was hoping you could shed some light on the cost of the other pieces (i.e. what did you spend on the cement, molds, release spray, etc.).
I don't see any use of concrete, cement yes- did you mix anything in the cement?
It would be nice to see some grills on these - no protection = $$ down the drain! <br>I built a set of sub enclosures back in 1990 for car sound offs made of concrete. <br>I would have made a fiberglass enclosure for that type of driver used. <br>The terminal cups are also exposed, they should be recessed in the enclosure! <br>I do like the angles of the enclosure itself. <br> <br>+1 for appearance <br>-2 for protection <br>
Could always be more cautious, but we are happy with the way they turned out. Might design some grills for the next set!
You mentioned design criteria - where is your observance of T-S parameters here? <br> <br>I love the cabinet - and I see a tightly sealed driver that I'm sure sounds quite nice, but I don't see any enhancement based on T-S, low end extension, efficiency gains, etc., etc. <br> <br>Concrete used to be the suggested way to utilize the 30 inch woofers from ElectroVoice, that were available from ~1955 through 1985. But even they have very specific requirements for dimensions of the port and the depth into the concrete 'cabinet' in order to maximize low frequency response. <br> <br>Have you made any with true T-S design parameters in mind as of yet? <br> <br>Also, did you consider utilizing a driver that could withstand the rigors of outdoors, given the cabinet material? <br> <br>Last, although obviously much more difficult, a reverse throw or a simpler downward firing addition (using a PP or other polymeric cone) and nubs for enough height to throw out the wave, would make for some impressive outdoor (really, only one is needed) subs that fit the parameters and are omnidirectional, at bass frequencies very difficult to obtain for the average mortal!!
No attenuation circuits or anything fancy, just selecting a nice driver and sizing the volume to suit. We might try some 2-ways in the future and those would have a little more math involved. <br> <br>Don't tempt me with comments about making subs, if we did they would be absolute monsters!
What kind of speakers are these? They look awesome. And hopefully sound awesome too.
We got them from parts express, but they seem to be out of stock. There are a lot of great options, just depends on your budget and application.
You will save concrete using another bowl instead (they're stack-able, thus no ball is needed)
Stacking the bowls would leave the wall too thin, unless the &quot;bottom&quot; were very thick. The angle isn't right. If the sides were approx 45* to the bottom, stacking would be a good option. Even though the concrete is sturdy, it will resonate if it gets too thin. To be honest, saving concrete isn't much of a priority; once you are all set up to mix a little, you might as well mix a lot.
Any reason to not make the front plate from concrete as well? You could color it for contrast.
Front face out of concrete would be nice, but it is difficult to size it exactly right, and sanding/polishing the edges to size would affect the finish, but I'm sure it would still look good! It also complicates the speaker mounting a little bit, but nothing we can't figure out.
Nice! How do they sound? Also, I wonder how making these a vented enclosure would help too.
Vented enclosure wasn't something we were looking for on these. They are desk or bookshelf speakers, have nice even response with very clear mid/lows. &quot;Help&quot; is a relative term, it depends what you are looking for in the sound, we didn't need super strong low end, looking more for clarity.
So, how did you calculate for the volume inside the box?
The interior volume was verified with a plastic bag full of water. There are many online calculators that can help you determine what interior volume you want, and many drivers are rated with a recommended enclosure volume. The full calculations and methodology for how to match a driver with an enclosure are beyond the scope of this instructable.
Really cool idea, So many people back in the day, thought solid wood enclosures would be a sign of quality. In fact, wood is the worst material for speakers due to resonance. Same reason musical instruments are made from wood. Most speakers that appear to be wood, are in fact particle board. Concrete is much better, and avoiding parallel surfaces in the enclosure is a big plus.
What I do to prevent staying air bubbles inside the concrete i put my mold with concrete on a vibrating table (just two plywood boards connected with four springs and a 12V old scooter motor with added weight out of axis and we have simple vibrating table!) <br> <br>I love the idea of making these! I'll make a set for my new room, maybe a bit smaller and with rubber paddings but similiar design. <br> <br>cheers! you have my vote!
No vibration needed with the glass fiber reinforced mix I learned from Brandon Gore!
really? that's awesome!
Looks great. Nice job. Thanks for sharing!!!
Is there a brand name on the bottom of the bowls or somewhere online they can be purchased?
looks like maybe the bowl is from this set: http://jennskistudio.blogspot.com/2012/07/geometric-dinnerware-at-target.html which is no longer available at Target.

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Bio: April Brooks www.4brooksdesign.com , Will Gibson , Andrew Harmon www.andrewsharmon.com , Melanie Morgan , Drew Morgan , Nate Hood
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