Here is a top view of a resonant speaker I have built. I used a piece of 2 inch PVC for the enclosure and made a wood piston to tune it with.I am not a math major so I just built it with a variable chamber to find the resonant frequency. I then glued and sealed the piston in place then cut off the remainder of the PVC pipe. As you can see on the bottom there is another chamber made from a coupler which also increased the sound pressure level. It takes a few of these to find which worked the best. Just start cutting away a little area at a time until you find the strongest output. I used a microphone and a oscilloscope to tune it with.