An acoustic amplifier is a bit of a misnomer; the sound isn't actually amplified but rather all of it directed in one direction, making it appear louder.
This is a simple project that takes advantage of that principle and is achieved with some basic wood working tools.
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Step 1: Making the Plywood Tube
Start by cutting all of the plywood discs. You can use any thickness of plywood but the thicker the plywood the less pieces you'll need to achieve the height you want. My final height was 150mm.
Use a hole saw to cut the discs. I used a 100mm hole saw. This gave a pleasing height to width ratio.
Once you have cut the outside diameter, cut the inside. I used an 70mm diameter hole saw for this. It gave me a 15mm thick wall and I wouldn't want to go thinner than that.
To hold the disc in place briefly cut the inner circle with the disc in your hand being VERY CAREFUL. This will show where you can clamp the disc. Then clamp it in between 2 pieces of wood so that the centre of the hole saw can exit the bottom of the disc and cut out the centre.
Once that's complete glue all of the discs together.
Step 2: Making the Top and Bottom
Start by lightly cutting a piece of wood with the hole saw. In my case I used a piece of laminated oak from a bench top. This score will show where the centre of the disc is and this will be the guide so you can mortise out a hole for the phone. Don't worry, that's just a technical terms of "making a slot".
Draw a rectangle the size of your phone somewhere on the top piece. I placed mine just in front of the centre hole. Then using a chisel cut the wood along that line.
Next, I used a router to remove the material but you could also do this step with only a chisel. You'll need to make a shallow grove for the phone to fit in, then drill through where ever the speaker is. I created this for an iPhone so half of the groove had to be drilled through.
Once you've made the slot you can go back to the hole saw and finish cutting through.
Step 3: Sanding
Make sure to sand the inside of the tube now, you won't have another chance later on.
Glue and drive a dowel into the top and bottom to close the centre hole from the hole saw.
I used a belt sander to sand the outside of the tube because this removes a lot of material quickly.
Sand all the pieces.
Step 4: Final Assembly
Cut out a piece of the tube. I removed about 40% but anything around that number would work. The main thing is to keep the saw straight so the 2 cuts are parallel.
You could also experiment with drilling holes on one side of the tube. I thought about doing this but went with cutting instead, only because that was my original idea.
Now glue the top and bottom to the tube, making sure the slot is on the correct side. I also glued a small piece of wood onto the top to allow the phone to lean back slightly and have something to rest on.
Step 5: Finish With Some Finish
You're almost done. The last step is to apply some finish then you're all set to go. I applied oil but you could use varnish, paint, anything goes here.