The thing that always concerned me was that portable basshorns are driven below their flare frequency, where they begin to act as direct radiators. They just aren't big enough to hit the low notes operating as a horn unless they are used in groups or near boundaries. And even in the best conditions, something this size really needs to have a lot of attention paid to motor performance in the lowest frequencies, in my opinion.

Rather than ignore or evade the fact that the horn is driven under the flare frequency, this design effort seeks to address it. At the lowest frequencies, the system acts sort of like a pseudo-bandpass speaker. Increased harmonics from low frequency signals enter the throat and are acted upon by it. If this were a low-budget system, that would be one thing. But when it is to be the best it can be, as should be the case for a loudspeaker system of this cost and complexity, then I expect the performance at the lowest frequencies to be optimized. For that reason, I sought to have a flux stabilized woofer made that would perfom well and offer reduced distortion in direct radiator mode.

But there is another way to do it, and I have changed my focus in this direction. With two drivers being used, they can be connected in a push-pull configuration so that they pressurize the front chamber with opposite sides of the cone. A positive pressure would be created by the front side of one cone and by the rear side of another. Any asymmetries caused by magnet non-linearity, flux modulation, or suspension non-linearity would then cancel in the front chamber. This offers great promise for reduced distortion basshorns.

I am working on two models right now, a version that uses a 12" driver and another that uses a larger 15" driver. The prototype will likely be built with a LAB12 woofer, and the LABhorn can then be used for comparison, to determine the reduction of harmonic distortion provided by this horn configuration. Several people make higher power 1000 watt 15" drivers, and this makes possible a second version with higher average output. They would be crossed over to large format conical midrange horns loaded with 15" drivers at 100Hz, again to 10" conical horns at 500Hz and finally to 1" exit compression horns at 1.6kHz.

12 π folded horn

12 π folded horn, front view

The 12" version would likely be aproximately 18-20" in width, whereas the one with 15" drivers would need to be 26-30" wide. In this way, the 15" version would actually have a slightly higher flare rate but potentially much higher output capacity. Both horns would be the roughly the same in the other two dimensions, approximately 40" to 48", with the 12" verson slightly smaller because of its smaller back chamber volume. The smaller horn would also probably have nipped corners.

15 π folded horn

15 π folded horn, front view

Both the 12" and 15" push-pull plenum horns provide cancellation of second-order harmonics in the front chamber, so I expect they should offer much lower distortion levels than other designs. The front chamber will be larger than other designs, which will act as a low-pass filter. Improved performance in the form of reduced distortion is what motivated me to undertake this effort. But the possibility of using different drivers having increased power handling and output is a welcome improvement too.

I was disappointed when the long wait for the B12 woofers ended without an improved, low-distortion woofer but am now excited to see progress in other areas. I had made push-pull cornerhorn designs, so I'm not sure why I didn't consider it as an option for a folded horn before now. It looks like an obvious and attractive way to build a dual-driver basshorn, with the front chamber serving as a harmonics-cancellation plenum. I am quite anxious to build a working prototype for testing.

The front chamber volume is larger than some others, but it can be partially filled with solid material to reduce volume, if necessary. The front chamber acts as a low pass filter, so on a basshorn, it isn't a bad thing. That will serve to reduce harmonics even further, just like the horn folds do. All the other characteristics of this "push-pull plenum" horn are the same as any other horn. It's just the harmonics cancellation feature that's different, and that's done with a very simple mechanism of turning one driver around.

You probably didn't think of it before because you've been trying to simplify and reduce costs in your systems. Using more drivers puts the price up and makes them harder to build. Always the engineer.

Build them and they will come.

Grant.

The black area in the front chamber shows how space can be displaced to make the volume smaller, if required. These can be placed behind, above and below the driver. There is a lot of space surrounding the drivers that can be easily filled with the use of filled blocks. It is not a large extra step to add these, so if the front chamber must be made small, that's one way to do it.

Gar.

It is long been established that the main reason for loudspeaker distortion is asymmetry from the motor. It literally goes further in one direction than it does in the other, with the same amound of driving voltage applied. The reason is that the magnetic flux from the voice coil pushing the cone in one direction adds to the fixed magnet, and it subtracts from the fixed magnet in the other. This difference causes the motor to generate slightly less force in one direction than the other. That'swhat the shorting ring corrects. That's why it is called a flux stabilization ring, because that's literally what it does. Asymmetry is the cause of 2nd order harmonics, and reduction of second order harminics is done by restoring or improving symmetry.

When mouting the radiators as shown in my drawing, the two drivers pressurize the chamber together. One driver is the "weaker" one and the other is the "stronger" one on each half cycle. So each half cycle is equalized by the fact that it always has equal amounts of drive, one strong and one weak. It is a very simple mechanism to use.