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.