DIY Horn Speakers - Step 4: Cutting the Circles

I determined the width of the horn near the throat by measuring the driver mounting and approximating how wide the base of the horn should be so I would have enough room to mount the compression driver (JBL 2446), and then mount that piece to the rest of the horn through bolts and threaded inserts.  Because of this mounting process, the base of wooden horns should be wider than plastic/metal/fiberglass horns with metal mounts unless you plan to use a special adapter.  The first seven circles of the stack would be the same width.

I've found the best way to cut perfect circles is by using a plunge router mounted to a circle-cutting jig.  There's no need to buy one, a simple scrap of baltic birch or MDF will work well.  Just measure from the inside of the straight router bit to the radius anywhere on the jig and drill a hole the width of a screw.  Drill the screw through your jig and into your baltic birch sheet and you're ready to make perfect circles.  You probably won't be able to cut through the whole piece in one pass.  I took 2-3 passes with the router for each circle.  I cut out 19 pieces per horn, so by the end, the brand new Freud (Italian made) carbide bit was bogging down a bit, requiring more passes.  Whiteside (US made) also makes great bits, and are even higher rated than Freud, although they weren't readily available at the local hardware store when I needed to replace an older bit that wasn't up to the task.

Tip: Save your lungs, use dust collection for this task.  I made my own attachment out of a free conference Nalgene bottle give-away.  It worked really well and collected 99% of the dust.  I probably collected ten gallons of fresh sawdust from the 38 circles that I cut.