> I personally like equiangular and nearly equiangular folding.

I think its one of the most efficient/smallest/waste-free folding methods too, at least out of the various ones I've tried.

About the woofer cutout hole, I don't think its shape or proximity to the apex are terribly important because the frequencies you'll be using the horn for are so low. Cut it round or square, whichever is easiest for you to do. I would position the woofer(s) near the apex and cut a hole for each woofer, located centrally, just over the dust cap. That will make the hole less than a foot away from the apex, which will be just fine. The wavelengths you're dealing with are much longer than that.

The same applies to horns. All of your diagrams are two-dimensional, which infers that your thought process behind them is also two dimensional. When you lose that second driver you halve the required space for the rear chamber, space that can be added either to your pathway for a lower Fc or to the mouth area for a higher SPL or both, or just subtracted from the box for net size savings.

Don't sweat standing waves in basshorns; they don't exist where surface to surface dimensions are 1/4 wavelength or less. A quarter wavelength at 100 Hz is about 2.8 feet; you do the rest of the math. Also forget both reflectors and damping; both the Tuba 18 and Tuba 24 don't use either, with no degradation in performance. Standing waves for the most part only exist in the minds of advertising executives.

The poster who noted that SPL was the same from an indentical box with either one or two drivers was correct to a point. While wattage sensitivity renains the same, voltage sensitivity does go up by 3dB, as the halving of impedance doubles the wattage in for a given voltage. By the same token total output power is up 3dB, due to the doubling of PMax. However, what you don't get is the 6dB voltage sensitivity/power output gain that direct radiators acheive from doubling drivers. That because doubling cone area in a direct radiator acheives an effciency gain of 3dB from the doubling of radiating area, but in a horn the radiating area is the horn mouth, and since doubling or quadupling or googleplexing the mumber of drivers at the throat does not change the throat size the horn efficiency remains the same, only the input power capacity changes.

As for standing waves, you're right about doing the math. No matter how it's folded, if you have three sections of approximately three feet each, I'd expect the horn to work well up to about 100Hz. So as long as output is limited to 100Hz, it should be alright.

Still, one thing that's attractive to me about the equiangular spiral is that the radius is constantly increasing. When a horn is folded to approximate this shape, each section's length is different and that tends to break up the standing wave modes along the long dimension.

The equiangular spiral I agree is the best folding method from a variety of standpoints for basshorns. It's the foundation for my Tuba sub series for just that reason.

• BillFitzmaurice.com

I too considered something like this, but seems like a lot more work - mostly because I dont know how I would draw it up in CAD. Now thats a basshorn which really needs a lot of thinking! ~could it be more space-efficient than the spiral method?

Are you sure that doing a seperate hole for each woofer is OK? What you are saying makes sense, just want to make sure - I guess the horn wouldn't know where the drivers at this low freq's + close spacing.

Can't help you with the CAD thing- I don't have it. I'm just at the point of giving serious consideration to a graphics program so I can finally retire the pad and pencil.