| Re: Pi push pull slot loaded subwoofers? [message #60774 is a reply to message #60771] |
Sat, 05 September 2009 13:14  |
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Wayne Parham
Messages: 18786
Registered: January 2001
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Illuminati (33rd Degree) |
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The only subwoofer I offer with push-pull drive is the 12π hornsub.
I thought about making a seven π bass bin with push-pull drive, but as I studied the concept in detail, using measurements to guide my design choices, I realized it wouldn't offer as much benefit as I first thought. I initially thought the application was a perfect fit since the drivers are hidden from view, so the exposed magnet of the reversed driver would not be seen. But the truth is the woofer in the seven π isn't a sub - it's used up into the lower midrange - and in that range, shorting rings work better.
The bass bin of the π cornerhorn is really designed to cover the range from the Schroeder frequency up to the crossover point with constant directivity, and in this range, the higher quality drivers with shorting rings offer very low distortion, and it's getting a little bit too high in frequency for summing between drivers in a push-pull configuration to provide distortion cancellation. So in reality, even though the cornerhorn bass bin operates lower than the Schroeder frequency where push-pull driver might offer some benefit, I think probably the benefits would be limited. I prefer to use shorting rings for this application.
Push-pull drive is best suited for deep bass, subwoofer territory. The three π (direct radiating) sub would be a good canidate, and you could easily implement push-pull drive in a pair of cabinets that size. You could even take a couple three π cabinets, reverse mount one driver and face them together. This would work just fine.
Crossover for a system like this should be limited to about 100Hz. There's no penalty for going higher, say to 150Hz or so, if used as flanking subs in a multi-sub configuration. But beyond that, you're not going to get any reduction of distortion from push-pull drive. You can go higher, as I said, without penalty, but distortion will rise because coupling isn't as good up high. So it is probably better to crossover to a driver with a shorting ring.
If used as flanking subs, it would be best to overlap up to the lower 100's or so, blending the subs with the mains. More distant subs should be low-passed at lower frequency, of course, to prevent localization problems. You never should be able to identify a sub; It should seem to be "invisible" to the listener. If you don't have the impression that the sound is coming totally from the mains, then the subs aren't setup right.
Cooling plugs and plates can be installed in the three π subwoofer whether configured push-pull or not. You start to gain a benefit from them at about 100 watts, approximately 108dB/M and louder. They reduce core temperature, which in turns reduces electro-mechanical parameter shift and power compression.
I think most people don't really need the plugs and plates for home hifi or home theater, but I suppose if you push them hard, they'll help. But you'll definitely be rattling the house at those levels. On the other hand, if the subs are used for prosound, then I would definitely suggest the cooling plugs be installed.
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