Dual opposing drivers cancellation issues?

[hornloaded]

I am planning on building a subwoofer in the near future, likely with 2 Mach 5 IXL 18" woofers in a sealed enclosure. I have an old 3'x2'x2' .75 inch thick MDF enclosure that I plan to recycle. Basically I want to put each driver on opposite sides of the enclosure, NOT isobarik.

Can anyone think of any problems with the drivers mounted back to back on opposite sides of the enclosure?

The highest frequency it would be playing is around 100hz, which with a wavelength of 11.28 feet seems like I shouldnt have any phase cancellation issues (obviously I will have the polarity on each speaker opposite another) will I? It will only be about 1/5 wavelength difference between the two drivers at 100hz? If that is correct, how much cancellation would I get at that frequency in decibels?

[soho54]

If you mean say a square enclosure with the drivers firing out of two opposite sides. With magnet structures pointing towards each other in the enclosure it's the same as having them on the same side.

You get zero phase issues, +3dB, and less vibration. Wire the drivers in phase, not out of polarity.

[hornloaded]

I attached a (not to scale) example of what I want to do. You are saying that the two drivers would need to be wired with the same polarity if hooked up that way? I dont think I understand why that is, I would assume that since they are facing opposite directions, the polarity would have to be switched on one.

[Joachim Gerhard]

One idea whould be to mount one driver with the magnet sticking out and wiring it in antiphase. Acoustically they are in phase but mechanically they are out of phase.
That reduces K2 distortion by a factor of 3 because unlinearities in the suspension cancel.
i have measured that effect on an open baffle but it should work in a closed cabinet too.
Mechanical vibration also cancels. See my origigal Audio Physic Avanti for an example of the topology you plan and look at the Orion from Siegfried Linkwitz ( Linkwitz Lab - Loudspeaker Design) for the distortion canceling arangement in an H dipole.

[soho54]

Originally Posted by hornloaded

You are saying that the two drivers would need to be wired with the same polarity if hooked up that way?

Yes.

[planet10]

Mounting the woofers this way (or inverting one mecahnically as Joachim suggests) is a very good idea. I do it whenever possible. The better you can get the mechanical coupling between the drivers, the more active vibration cancellation you will achieve.

2 18" in a sealed 3/4" enclosure could well suffer from ballooning thou. Hope it is well braced.

dave

dave

[sumaudioguy]

Jochim is right-

If drivers are mounted on opposite side then there are a lot of issues. Can the woof be in a corner? How about 2 15s on the same side instead? On opposite side the crossover effects get pretty muddy. Yes it makes bass but will it match into the next higher frequency driver? Tough question.

[planet10]

Originally Posted by sumaudioguy

Jochim is right- If drivers are mounted on opposite side then there are a lot of issues.

I don't read that in Jochim's post. There are big advantages to mounting woofers in pairs push-push. Done right the active vibration cancelation can make big improvements in the reduction of energy reaching the box walls.

At the frequencies involved, the drivers radiate omnidirectionally, it makes no difference from that POV which direction they face.

Jochim suggests taking it a step further and mechanically inverting 1 driver to also cancel suspension non-linearities. This makes it harder to achieve really tight mechanical coupling and leaves the issue of the back of a basket sticking out (and the magnet and maybe an ugly vented polepiece). If your driver is really good (ie symmetrical suspension) this last step gives you little (Nick @ Llambda asserted you would gain nothing with his drivers)

If possible i'll always use 2 smaller drivers push-push instead of a larger single driver for bass.

dave

[Joachim Gerhard]

Yes, mechanical connection of inverted drivers is not easy ( Kef did that with a metal tube in the old days) and dstortion of the suspension is low in a closed cabinet anyway because they work on an "air spring" that is more linear then a mechanical suspension.
I tested that with my Klippel Laser Analyser and it is true.
So go for the simple Push-Push version like i call it.
Yes planet 10, two small drivers are faster then one big one but the parameters can make trouble to get deep bass. One solution is the Linkwitz transformer that allows wide choice of synthetic parameters.

[Dr.EM]

To couple the drivers effectively, wouldn't it be a simple and cheap option to mount them together and to the cabinet with long threaded rods? Especially on a driver with numerous mounting points.