This passive speaker is made out of concrete and wood. There's no wiring, no electronics, it's a pretty simple build, so it's a perfect project if you're looking to make your phone sound a lot better.

I find the concept of a passive speaker really appealing, because it means I can just put on some music or a podcast on my phone, immediately place it in the speaker holder, and that's it. No need to pair it to a bluetooth speaker, and no rechargeable battery to do deal with. Simple. I decided to combine concrete and wood, in some form of a cube shape, that way the speaker wouldn't take up a ton of counter space.

Step 1: The Molds

I'm using a stem glass to create the indention for the mold. At the Dollar store I got a bunch of nice plastic wine and cocktail glasses. I decided to make two new molds, and decide later on what shape was the best.

I'm using my woodworking tools to create the pieces for these molds, but another idea would be to use the bottom of a half gallon milk container or something like that - or why not make a mold using foamboard and glue. There are lots of ways to accomplish this.

Step 2: Drilling the Holes

Next, I marked out the center of the bottom piece and drilled a hole that was about the same as the thickness of the stems. Then I marked out where the screw holes needed to go, pre drilled, and connected the molds together. To prevent the concrete from sticking, I brushed the plywood and outside of the plastic glass with vegetable oil.

Step 3: Mixing the Concrete

I mixed some mortar, which is nice and smooth without any aggregate, so it's what I often use for these types of projects. I created a pretty loose mix, kind of like oatmeal.

Then I placed the glass in the middle, coming through the hole and then added the concrete mix around it. And I tapped it down a couple of times to remove any air pockets and level the concrete.

Step 4: Removing the Molds

A few hours later I unscrewed the pieces, removed the plastic cups, and was left with two pretty nice looking concrete forms. They both were nice, however I decided to use the mold with the wine glass indention.

Step 5: The Wooden Surround

I want to create a wooden surround, I'm cutting up a piece of mahogany, however any type of wood, would be great, whatever you have. I'm creating mitered corners here for a nice framed in look.

The idea is to have the concrete cube in the middle and the wood surround on all sides.

Step 6: Gluing the Support Pieces

I want the phone to sit just above the hole which will direct the sound outwards, so I'm making a mark and then I'm cutting up some scrap pieces. And I'm gluing those one with polyurethane glue which attaches well to concrete.

Step 7: Chiseling the Slot

Next I'm marking out where the phone should sit within the top wooden piece. First drilled some holes to remove as much wood as possible. And then I used a chisel and mallet to clean it up. To round the edges a bit, I used a file and then sandpaper.

Step 8: Glue-Up

Time to glue the wood together. And I'm using this simple technique where masking tape is acting as clamps. Then I wanted to confirm that the concrete piece was going to fit inside, which it did perfectly.

Step 9: Gluing the Concrete to the Wood

Once the glue dried, I sanded the wood, and then glued the concrete cube in place here, again with some polyurethane glue. I also have a piece for the back here and I glued that in place as well.