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12π and 15π horn loudspeakers [message #45732] Thu, 04 November 2004 22:17 Go to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18787
Registered: January 2001
Illuminati (33rd Degree)

As many of you may recall, I had sent a requisition to Eminence for an improved low-distortion "B12" subwoofer that could be used as a direct radiator in small 2ft3 vented loudspeakers or in a large folded basshorn. Eminence has decided that the device would be cost prohibitive to build at this time. They would be willing to consider continuing to work on the project if 1000 pieces were ordered and a substantial price paid to them to re-tool. Otherwise, they are not particularly interested in adding the larger flux ring required to reduce distortion at low frequencies. That leaves other options to consider, for anyone interested in an improved system.

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.


Re: 12π and 15π horn loudspeakers [message #45734 is a reply to message #45732] Fri, 05 November 2004 09:11 Go to previous messageGo to next message
Zeno is currently offline  Zeno
Messages: 122
Registered: May 2009
Master
i am very keen with this design

Re: 12π and 15π horn loudspeakers [message #45735 is a reply to message #45734] Fri, 05 November 2004 11:51 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18787
Registered: January 2001
Illuminati (33rd Degree)

I like the concept too. Honestly, I can't find any disadvantages. If a basshorn is driven by a pair of radiators, it makes sense to connect them this way so that the front chamber serves as a harmonics cancellation plenum. It's the perfect application for this, since the front chamber partially contains the pressures from the two drivers and distortion causing asymmetries should be cancelled quite well.

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.


Re: 12π and 15π horn loudspeakers [message #45736 is a reply to message #45732] Fri, 05 November 2004 13:23 Go to previous messageGo to next message
pgolde is currently offline  pgolde
Messages: 21
Registered: May 2009
Chancellor
Would there be a difference in chamber pressure from a design like the Lab where the drivers both fire towards each other? How would this affect the horn loading?

Re: 12π and 15π horn loudspeakers [message #45737 is a reply to message #45735] Fri, 05 November 2004 13:44 Go to previous messageGo to next message
Bill Fitzmaurice is currently offline  Bill Fitzmaurice
Messages: 335
Registered: May 2009
Grand Master
Good concept, similar to a humbucking pickup. The distortion prone components are out of phase to cancel out the HF distortion that's not in the passband anyway, leaving clean fundamentals.

Re: 12π and 15π horn loudspeakers [message #45738 is a reply to message #45737] Fri, 05 November 2004 14:25 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18787
Registered: January 2001
Illuminati (33rd Degree)

Thanks, Bill. I'm real excited about it and eager to make a prototype to test the concept. I'm not sure why I didn't think of it before, when I was having Eminence work on the B12 woofers. This is probably a better idea, because the cancellation plenum won't lose effectiveness at low frequencies like a shorting ring does. Shorting rings are great and allow 2HD reduction in a single driver. But when two drivers are used in a horn anyway, this would appear to be a good configuration to use. We'll see.


Re: 12π and 15π horn loudspeakers [message #45739 is a reply to message #45738] Fri, 05 November 2004 14:39 Go to previous messageGo to next message
GrantMarshall is currently offline  GrantMarshall
Messages: 77
Registered: May 2009
Viscount

Hi Wayne.

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.


Re: 12π and 15π horn loudspeakers [message #45740 is a reply to message #45736] Fri, 05 November 2004 14:43 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18787
Registered: January 2001
Illuminati (33rd Degree)

The cancellation plenum is the horn's front chamber. It acts as a low pass filter and is one of the parameters used when modeling a horn. The magnet of the rear-facing driver protrudes, which tends to make the front chamber volume larger. But there is no reason why you can build up the internal area around the cones to displace volume if the horn design requires a small front chamber.

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.


Re: 12π and 15π horn loudspeakers [message #45742 is a reply to message #45732] Fri, 05 November 2004 23:04 Go to previous messageGo to next message
GarMan is currently offline  GarMan
Messages: 960
Registered: May 2009
Illuminati (2nd Degree)
Hi Wayne. Need some help understanding this. Are the two drivers wired in phase or out of phase from each other?

Gar.

Re: 12π and 15π horn loudspeakers [message #45743 is a reply to message #45742] Fri, 05 November 2004 23:34 Go to previous messageGo to previous message
Wayne Parham is currently offline  Wayne Parham
Messages: 18787
Registered: January 2001
Illuminati (33rd Degree)

The drivers are connected so that each will pressurize the front chamber in phase. There is also an isobaric arrangement that is sometimes called push-pull, but that's not what I'm trying to do here. What I want to do is to drive the chamber with the front side of one driver and the back side of another. If both drivers are the same, then any asymmetries caused by the motor or suspension will equalize in the throat.

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.


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