Howdy crew:
I have an assembly I'm looking to do, using 2 pairs of isobaric subs, mounted at a right angle, very close together. The magnets do not touch but there's enough magnetism to repel each other somewhat.
The concern with this is that it's going to act like a weak bucking magnet, and create some extra motor strength on the interior pairs. This, of course, creates an asymmetrical setup, where the inner driver in either pair will be somewhat stronger than the outer.
Now, the question is-
IS THIS A PROBLEM?
Given that the motion of one cone translates into motion in the other cone, a well-sealed clamshell mount may not actually have a problem here, as the suspension nonlinearities will still track each other.
On the other hand, the whole point of isobaric is to reduce nonlinearity and I don't wish to introduce another source of distortion.
I've been going back and forth on this, and can't seem to reach an internal consensus. Accordingly my plan is to create spacers of 3/4" ply. The cabinets are already built (surplus M&K) so I'm stuck with the format (don't really want to deviate from using them as they're quite nice).
But I'd welcome any feedback from the crew on this.
I have an assembly I'm looking to do, using 2 pairs of isobaric subs, mounted at a right angle, very close together. The magnets do not touch but there's enough magnetism to repel each other somewhat.
The concern with this is that it's going to act like a weak bucking magnet, and create some extra motor strength on the interior pairs. This, of course, creates an asymmetrical setup, where the inner driver in either pair will be somewhat stronger than the outer.
Now, the question is-
IS THIS A PROBLEM?
Given that the motion of one cone translates into motion in the other cone, a well-sealed clamshell mount may not actually have a problem here, as the suspension nonlinearities will still track each other.
On the other hand, the whole point of isobaric is to reduce nonlinearity and I don't wish to introduce another source of distortion.
I've been going back and forth on this, and can't seem to reach an internal consensus. Accordingly my plan is to create spacers of 3/4" ply. The cabinets are already built (surplus M&K) so I'm stuck with the format (don't really want to deviate from using them as they're quite nice).
But I'd welcome any feedback from the crew on this.
Hi hits:
What you're talking about is a bucking magnet. The thing is that typically, the magnet interactions are consistent amongst the drivers in the system, where in this arrangement, it's only affecting the inner drivers, the outer ones of each pair don't have that issue, and it's that asymmetry that's the concern. In the case of "push push" subwoofers, where they're opposed for force cancellation, the interaction of the fields is actually a benefit, as it uses some of the stray field and should slightly increase flux density. But the effect is symmetrical for each woofer in that arrangement. My concern is that the inner portions of the drivers would have a slightly stronger magnet system and that would create asymmetrical behavior between inner and outer drivers. Isobaric would seem to rely upon good matching to perform its best, but the effect may be swamped by the other effects.
What you're talking about is a bucking magnet. The thing is that typically, the magnet interactions are consistent amongst the drivers in the system, where in this arrangement, it's only affecting the inner drivers, the outer ones of each pair don't have that issue, and it's that asymmetry that's the concern. In the case of "push push" subwoofers, where they're opposed for force cancellation, the interaction of the fields is actually a benefit, as it uses some of the stray field and should slightly increase flux density. But the effect is symmetrical for each woofer in that arrangement. My concern is that the inner portions of the drivers would have a slightly stronger magnet system and that would create asymmetrical behavior between inner and outer drivers. Isobaric would seem to rely upon good matching to perform its best, but the effect may be swamped by the other effects.
The stray field is "wasted" magnetism, and compared to the strength concentrated in the magnetic gap is of no consequence.
The nonsymetrical cabinet layout will have far more acoustical consequence than the magnet proximity.
Thanks for your input, certainly the effect would be fairly low level, perhaps I'm chasing too far down the foxhole.
Care to elaborate upon the nonsymmetrical layout? I don't understand your concern.
I'm not concerned, but if you want to chase down foxholes, the interior cabinet reflections from the different isobaric pairs will not be symmetrical, so the cancellation of even order harmonics will not be as effective as a symmetrical layout.
That said, the difference in production driver parameters from unit to unit probably would exceed that difference.
And the effect you room has will make any of these tiny differences look like ants on a mountain.
Don't worry, be happy.
Why should symmetry of cabinet reflections play a role? The coupling of the inner driver with the outer driver is the method for which the even order distortion cancellation takes place, by virtue of equal but opposite tilts in BL/Suspension curves. While I can see how the airload/reflections on the diaphragm would be different for the inner driver vs. the outer in a given pair, I'm not following how this would mean anything between one pair and the other, since there's no mechanism by which the nonlinearities are coupled from one clamshell to the next, for cancellation.
I appreciate the input, certainly you're right in that I'm being too anal about it, but I'm sure you know how it is- once the gears start cranking...
I appreciate the input, certainly you're right in that I'm being too anal about it, but I'm sure you know how it is- once the gears start cranking...
You are correct regarding even order distortion cancellation.
The non symmetrical loading airload/reflections on the inner diaphragm of one iso pair compared to the other would change frequency and phase response compared to a symmetrical loading, and probably not change the second harmonic distortion signature.
Oh, agreed, though I'd not expect this to be a concern at the lower freq XOs I use for subs, thanks for following up.
That was my thought too but I keep thinking about bucking magnets, and how much influence they can have (as much as a 2 dB and a couple tenths of a point of Qes) , but I suppose they're not contstrained by the presence of the motor (which limits the stray magnetism dramatically)
Thanks for all the feedback guys, I think I'll limit myself to one driver spacing ring, just to buy a little extra space, rather than doing 2.
You could always attempt to measure a difference in Qe between front and back pairs. It would be interesting to see if a difference of supposed theoretical effect did actually poke through the tolerance between free-air measurements.
Maybe you could even optimize driver positions.
If they were smashed back to back there would undoubtedly be something more to see than being 90 degrees off with some space.
Maybe you could even optimize driver positions.
If they were smashed back to back there would undoubtedly be something more to see than being 90 degrees off with some space.
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Measurements (in some other thread) show that the two drivers of an isobaric pair coupled by air do not in real installations work exactly identically. The inner driver has less excursion than the outer. How much depends on the volume of air between the drivers versus the volume of the cabinet. When the cabinet is large and space between drivers is small, the pair work in symmetry. When the cabinet is small (which is why you make an isobaric) and the volume of air between drivers large (if you didn't use flat-format short-clearance-depth automotive sub drivers) then the inner driver has less excursion when the driver motors are identical. SO...having the inner driver a bit more powerful actually makes it work more like the "ISO-Baric" constant-pressure theory.
Making the inner driver a bit more powerful will avoid the comon isobaric problem of the outer driver reaching xmax before the inner one (though output is still limited by the outer one reaching xmax). Not that both bottoming at the same time is really better...
And box shape barely matters at sub frequencies, only the volume; especially a stuffed sealed box where all sound is absorbed (in a perfect world) anyway.
Are the magnet corners almost touching? Will the magnetic force at the front plate be symmetrical? Will there be more force on one side of the magnetic gap? Got me there.
Making the inner driver a bit more powerful will avoid the comon isobaric problem of the outer driver reaching xmax before the inner one (though output is still limited by the outer one reaching xmax). Not that both bottoming at the same time is really better...
And box shape barely matters at sub frequencies, only the volume; especially a stuffed sealed box where all sound is absorbed (in a perfect world) anyway.
Are the magnet corners almost touching? Will the magnetic force at the front plate be symmetrical? Will there be more force on one side of the magnetic gap? Got me there.
Interesting stuff, it intuitively makes sense. The magnet corners are almost touching, like less than an inch, yes. I have to assume that the steel would normalize any field variance within the gap. Certainly any 3d measurement of flux density in the gap is well beyond my pay grade.
The box in question is about 3.4 cubic feet, and it's a clamshell mounting, with a 1/4" spacer between drivers, to allow excursion without surrounds touching each other, so the compliance of the coupling volume should be pretty low, relative to the compliance of the box volume.
As one cone goes in, the other goes out. If the surrounds don't extend past the mounting gasket, no spacer is needed.
They're very close to the plane of the mounting gasket, and surrounds don't perfectly track cone motion, so the spacer seemed appropriate. They're already made and it's only a 1/4" increase so no sense not using them. Better to have it and not need it....
My latest isobaric has 12" drivers clamshelled, with a 1" thick baffleboard cutout between them. In a few days I'll test a bit and maybe find out whether I should have routed a recess into the baffleboard to get them closer. I suspect I'll want the strength. It would have been interesting to make aluminum baffle boards.
It would also be pretty interesting to take two of the low-profile automotive subs and glue the two cones together with some spray foam or foam ane a toilet-paper roll or something.
Inverted butyl surrounds (concave to the front) like my old Wharfedale W60-e drivers could be a nice feature too. Sometimes when I re-foam drivers I put the surrounds on backwards (concave) without much if any discernable ill effect (contrary to some simulations of surround sound radiation). I guess they have concave tweeters...sound radiated from the surround is probably terribly non-linear anyway.
Anyway, good luck and post pics.
It would also be pretty interesting to take two of the low-profile automotive subs and glue the two cones together with some spray foam or foam ane a toilet-paper roll or something.
Inverted butyl surrounds (concave to the front) like my old Wharfedale W60-e drivers could be a nice feature too. Sometimes when I re-foam drivers I put the surrounds on backwards (concave) without much if any discernable ill effect (contrary to some simulations of surround sound radiation). I guess they have concave tweeters...sound radiated from the surround is probably terribly non-linear anyway.
Anyway, good luck and post pics.
"It would also be pretty interesting to take two of the low-profile automotive subs and glue the two cones together with some spray foam or foam ane a toilet-paper roll or something."
This was eventually the problem for me in deciding to build an isobaric sub. At some point in this sort of "optimization" you wind up with a double heavy cone and motor, single driver in a smaller box.
Other than that it does remain a decent way to make two fairly "wimpy" drivers actually work in a box without the Q being impractically high.
It might be reasonable to say that the highest isobaric performance would be obtained when using light motored but otherwise well built woofers, since box volume ultimately determines efficiency at the bottom of the band. Two IB designed woofers in a fairly large box for example. But most people going for isobaric design are looking for some kind of volume reduction.
This was eventually the problem for me in deciding to build an isobaric sub. At some point in this sort of "optimization" you wind up with a double heavy cone and motor, single driver in a smaller box.
Other than that it does remain a decent way to make two fairly "wimpy" drivers actually work in a box without the Q being impractically high.
It might be reasonable to say that the highest isobaric performance would be obtained when using light motored but otherwise well built woofers, since box volume ultimately determines efficiency at the bottom of the band. Two IB designed woofers in a fairly large box for example. But most people going for isobaric design are looking for some kind of volume reduction.
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