"Force Cancelling" Woofer Configuration - Hype or Real Benefit

[auracle]

I've read a fair amount of promotional material (possibly hype?) decrying the benefits of mounting woofers opposite one another to achieve an alleged "force cancelling effect". Has anyone actually built something like this and measured the difference in cabinet vibrations with an accelerometer between running the woofers in such a configuration in phase (opposing directions) and reversed phase (woofers travelling in the same direction)? Given the physics involved in suspending woofers with a fixed connection (KEF Uni-Q) or not - it seems to me that the more important forces making the cabinet vibrate are ultimately the small changes in pressure or pressure waves travelling through the cabinet - not the transmission of force from the moving diaphragm assembly to the baffle through the basket mounting surface. It would seem to me - especially if you were able to install bracing from one driver to the other - that operating the woofers out of phase (acting somewhat like a passive radiator/woofer pair would) would minimize the pressure fluctuations in the cabinet and thus reduce mechanical enclosure resonances throughout the enclosure structure - not simply in the vicinity of the basket mounting area. Does this make sense and if so, why isn't the out of phase connection setup considered the defacto "standard" instead of the in phase connection? Lynn Olson apparently has a design using this "force cancellation" method although I haven't had a chance to read up on it as yet.

Most of my enclosures have used solid veneer core plywood with MDF inner and outer skins and elastomeric adhesives between the layers. This has been very effective in silencing cabinet vibrations but obviously, particularly with triple layer construction, gets quite heavy. So, before I take the time to experiment with Isobaric or push/pull configurations, I was wondering if anyone has tried different setups and compared the results.

[Flint]

I haven't done anything quite as fancy as your accelerometer idea, but will comment that force cancellation totally does work for subwoofers. It just keeps the cabinet from shaking. It's noticeable for subs largely because the cone mass is high and excursion is also high. At midrange frequencies it would be more difficult to demonstrate the difference.

[auracle]

I haven't done anything quite as fancy as your accelerometer idea, but will comment that force cancellation totally does work for subwoofers. It just keeps the cabinet from shaking. It's noticeable for subs largely because the cone mass is high and excursion is also high. At midrange frequencies it would be more difficult to demonstrate the difference.

I have no doubt that it does something - how effective is the $64k dollar question. I have been toying with the idea of using two 15 or 18 inch subs with a high Vas in a push-pull configuration side mounted in a tower with double wall construction. There has been some discussion about inefficiency of Isobarik configurations but I think that presupposes a fairly small baffle. A push-pull in a tall tower with fairly deep sides is not going to excite the average room in the same way as a small cube version will. So the blanket statements I've seen about poor efficiency may not be all that accurate. As Johnk pointed out in a DIY thread, there are two sources of enclosure resonance to be concerned with - that arising from energy transferred to baffle by the basket and that caused by pressure fluctuations in the cabinet itself. The configuration where both subs are in phase electrically is supposed to help reduce basket related resonances while the out of phase (push-pull) should reduce the air pressure related resonances. Without having tested both situations in the same cabinet - my hunch is that the latter is more effective in reducing real world enclosure resonance. It would be nice to see someone's test results of that theory cuz without actual test data, my hunch could be completely wrong.

[Paul Carmody]

I don't own an accelerometer, but I did try it, and it really does work. The cabinet vibration went down to almost nothing, except for frequencies coming from the ports. Take a look at the thread, and you might get some more leads...
http://techtalk.parts-express.com/sh...d.php?t=213731

[jonasz]

My Phoenix W-baffles with Peerless XLS 12" does hardly vibrate at all even at loud passages. This is in big contrast to the Orions tens that makes the whole speaker vibrate like a tuning fork!

Dunno if it makes a real world difference but my gut feeling says no (or very, very little) vibration is better than a lot of it.

[auracle]

I don't own an accelerometer, but I did try it, and it really does work. The cabinet vibration went down to almost nothing, except for frequencies coming from the ports. Take a look at the thread, and you might get some more leads...
http://techtalk.parts-express.com/sh...d.php?t=213731

Great link. Thanks Paul.

I think Mayhem13 hit on a good point in that thread - the importance of the through bolt back to back approach. Ultimately, I think that might be what gives the push-push setup the edge over the push-pull setup. If the rest of the enclosure is fairly well braced, it resolves the remaining weakest link that would normally occur behind a single driver. In that case, the "back wall" would logically be the most vulnerable location for vibration to go unchecked since you normally wouldn't want to restrict air flow away from the driver with bracing all around it. This may be why KEF and others have settled on the push-push setup.

[Pete Schumacher ®]

I have no doubt that it does something - how effective is the $64k dollar question. I have been toying with the idea of using two 15 or 18 inch subs with a high Vas in a push-pull configuration side mounted in a tower with double wall construction. There has been some discussion about inefficiency of Isobarik configurations but I think that presupposes a fairly small baffle. A push-pull in a tall tower with fairly deep sides is not going to excite the average room in the same way as a small cube version will. So the blanket statements I've seen about poor efficiency may not be all that accurate. As Johnk pointed out in a DIY thread, there are two sources of enclosure resonance to be concerned with - that arising from energy transferred to baffle by the basket and that caused by pressure fluctuations in the cabinet itself. The configuration where both subs are in phase electrically is supposed to help reduce basket related resonances while the out of phase (push-pull) is should reduce the air pressure related resonances. Without having tested both situations in the same cabinet - my hunch is that the latter is more effective in reducing real world enclosure resonance. It would be nice to see someone's test results of that theory cuz without actual test data, my hunch could be completely wrong.

A single sub driver vs. two in the same cabinet will behave almost identically. Of course the two driver version has the displacement advantage. If you wire the woofers to fire out of phase in the same cabinet, now you have more of a dipole style, and greatly reduced bass output.

The benefit to mounting the woofers in opposition is for inertia to cancel. The benefits are quite palpable, even without measuring equipment. The tactile benefit alone is obvious. Unless you plan on building a truly massive enclosure, inertia cancellation is a very good choice to keep size small without having a cabinet that wants to walk around the room.

[auracle]

Yeah, unless the side baffle is very deep and tall, it's definitely going to have dipole characteristics so efficiency would be an issue.

[Dennis H]

it seems to me that the more important forces making the cabinet vibrate are ultimately the small changes in pressure or pressure waves travelling through the cabinet - not the transmission of force from the moving diaphragm assembly to the baffle through the basket mounting surface.

There's no need to guess about that. The formulas are in any mechanical engineering textbook. You need the mass of the cone, the mass of the box, Vd of the driver, Vb of the box and the modulus of elasticity of the box along with its dimensions.

Short answer, for a sub where the moving mass approaches a pound, inertial cancellation wins big time over pressure cancellation. One you can maybe hear, the other walks the box across the floor.

[auracle]

There's no need to guess about that. The formulas are in any mechanical engineering textbook. You need the mass of the cone, the mass of the box, Vd of the driver, Vb of the box and the modulus of elasticity of the box along with its dimensions.

Short answer, for a sub where the moving mass approaches a pound, inertial cancellation wins big time over pressure cancellation. One you can maybe hear, the other walks the box across the floor.

You could fill a tractor trailer with the assumptions you just made. One standout is the "modulus of elasticity of the box" - as if this were uniform or predictable in any sense of the words. I can point to numerous tests of speakers that have exhibited cabinet resonances which were no where near the baffle where the driver's were securely mounted - but were in fact in locations where large unsupported or unbraced expanses of enclosure existed. Most folks who've built enclosures can attest to this phenomenon. Rap on a cabinet in the middle of the sides where there is little or no bracing and you get a high level low frequency echo. Rap on the same cabinet near a corner and you get a shorter duration, lower level, high frequency sound. This phenomenon is an integral part of Speaker Building 101.

[Dennis H]

You could fill a tractor trailer with the assumptions you just made. One standout is the "modulus of elasticity of the box" - as if this were uniform or predictable in any sense of the words. I can point to numerous tests of speakers that have exhibited cabinet resonances which were no where near the baffle where the driver's were securely mounted - but were in fact in locations where large unsupported or unbraced expanses of enclosure existed. Most folks who've built enclosures can attest to this phenomenon. Rap on a cabinet in the middle of the sides where there is little or no bracing and you get a high level low frequency echo. Rap on the same cabinet near a corner and you get a shorter duration, lower level, high frequency sound. This phenomenon is an integral part of Speaker Building 101.

Ah, Villastrangiato, you're finally showing your true self with a 'well duh' post.

Obviously any competent mechanical engineer knows the bracing points matter. I even know that from the engineering calcs I've submitted with the plans of houses I designed. Where's Noah Katz when we need him? He does vibration analysis for a living.

[Flint]

Ah, Villastrangiato,...

Ooooooh. Okay.

[Æ]

My reply might not be completely applicable, but this thread did remind me of Jeff B. who used two opposing passive radiators on opposite sides of the enclosure for motion cancellation. One of his commercial designs.

[badman]

A single sub driver vs. two in the same cabinet will behave almost identically. Of course the two driver version has the displacement advantage. If you wire the woofers to fire out of phase in the same cabinet, now you have more of a dipole style, and greatly reduced bass output.

Um... NO. 2x the woofer needs 2x the cab for equivalent alignment.

[billfitzmaurice]

There is no 'force canceling'. The same internal pressures will exist if the drivers are on a single baffle or opposed. The advantage of opposing drivers lies in the baffles being smaller, therefore stiffer, and the reduction in forces that otherwise might cause a cab to 'dance' across the floor.

[Flint]

Years ago I owned a set of Infinity Overture speakers, which had a bipole design in the bass. They had 6.5" woofers front and back, operating below 300 Hz. I'm not entirely sure if that was intended to balance the vibrating forces or as a BSC strategy, or what.

Later generations of Infinity speakers started having side-mounted woofers, which obviously abandoned the Overture concept (of course Infinity has always abandoned their most interesting ideas). I also owned a few sets like that, from the Interlude line. They were better speakers in the midrange (due to the new ceramic coated cones vs. the old poly mids) but less refined below 300 Hz, in my judgment. That was evidently related to being designed for maximum boom, more of a theater effects speaker than a music speaker.

At any rate the Overtures were quite nice speakers in many ways, and may have owed some of that to the vibe cancelling bipole design. Sorry that's a little vague and subjective. FWIW my latest project in the works has an elaborate vibration control strategy that involves tie rods going through the cabinet. I think this kind of thing has a lot of potential benefits.

[auracle]

There is no 'force canceling'. The same internal pressures will exist if the drivers are on a single baffle or opposed. The advantage of opposing drivers lies in the baffles being smaller, therefore stiffer, and the reduction in forces that otherwise might cause a cab to 'dance' across the floor.

I'm inclined to agree. Considering a popular cabinet construction type - like the NHT 3 - it's not very clear to me how the forces on one side of a tall, wide side baffle are going to cancel those of the other side. If they were linked with cross bolts like the KEF blade - yeah, I can definitely see how the net vibrational force imposed on each basket mount would oppose one another and effectively cancel any resulting force on the cabinet. But the cross bolt affair would have to be a requirement in my mind for that to take place.

[Taterworks]

Force canceling would be most useful in a tower-style bass module for a bookshelf/standmount speaker, because it would prevent the speaker's enclosure or spikes from buzzing against the bass module cabinet when rockin' along. For a unitized design it probably wouldn't have any benefits other than preventing vibrations from being transmitted to the floor.

[benchtester]

Force canceling would be most useful in a tower-style bass module for a bookshelf/standmount speaker, because it would prevent the speaker's enclosure or spikes from buzzing against the bass module cabinet when rockin' along. For a unitized design it probably wouldn't have any benefits other than preventing vibrations from being transmitted to the floor.

I am starting on such a design. It will be a TMWW with the woofers on the front and the back. I am hoping that an added benefit is that the sound will sum up in a shorter distance. (Zaph discusses this limitation of a TMWW front firing in the ZDT3.5 project.) However I am a little concerned that the average acoustic center (for both woofers) is moved back to the center of the bass module. Fortunately the wave lengths are long.

[dlr]

Ooooooh. Okay.

I thought I recognized a pattern, but hadn't picked up on whose it is.

dlr