The XSD Speaker

X, as you know, over on the OB thread one well known and respected OB aficionado opined that SLOB is effectively a waste of time.

Based on no experience using it in an OB, the time it takes is well worth it. If only for the force reaction cancelation. Because the edge is not clamped, an OB suffers most from the “box” being subject to ringing. Removing most of the energy that would get that going seems significant to me. Xs implementation has push-push pairs of drivers (althou not optimally oriented) and that is where the magic happens. He did not maximize force cancelation, the orientation used in these makes more sense in that respect. https://www.diyaudio.com/community/threads/39-hz-4-x4-compound-parallel-qw-pipe.385425/

I know early on there was talk about how it “projecting” was a feature. It was clear that far-field it would make no difference response wise.

https://www.t-linespeakers.org/FALL/dipole.html

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(2001 according to the date stamp)

dave
 
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Having the woofers facing cone to magnet still has the same force cancellation because it’s the same mass on both sides. The reason for this arrangement is to have the suspension force asymmetry self-cancel. This reduces non linear suspension induced distortion. As one driver pushes, the other pulls in the opposite direction but since it is flipped, they both push the air out. I do notice a lack of cabinet vibration though. When speaker are punching this hard (98dB) you generally can feel a lot vibration, but not here.
 
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X, your structure is smallish, it is still indirect coupling -- bracing the drivers magnet to magnet (and use ready rod as bolts for pairs of opposed drivers, you can maximize the effect. If you don’t like seeing the magnets or the baskets get intrusive sonically, it gets a bit trickier, you need 2 slots on the front (but you get even more structural integrity).

dave
 
X, your structure is smallish, it is still indirect coupling -- bracing the drivers magnet to magnet (and use ready rod as bolts for pairs of opposed drivers, you can maximize the effect. If you don’t like seeing the magnets or the baskets get intrusive sonically, it gets a bit trickier, you need 2 slots on the front (but you get even more structural integrity).

dave
This might seem like a naive question but perhaps there are factors in play that I’m not aware of, so: do you get the same level of output to the rear of the cone as at the front and does the frame / magnet structure etc not create distortions?
 

kazap

Member
2011-04-15 8:42 am
it is still indirect coupling
Force cancellation of dual opposed woofers with direct magnet to magnet bracing and rods is hard core. In a sealed sub with monster drivers and kilowatt amps that would be a beautiful thing. I can imagine a sealed sub in full cry with no box vibration while the ceiling implodes and the walls crack.

But for a SLOB? I wonder if the forces with OB drivers are high enough to warrant ideal force cancellation of opposed pair coupling? Its all about which compromise is best.
 
I think the XSD has a woofer “box” with a 5x slot on one face and drivers on two opposed faces. The box is so small that the stiffness from aspect ratio of thickness to length is very high. The weakest link is the actually the thin sheet metal used to make the basket on this budget woofer. It probably flexes more than the wood does. Even with threaded rod, which I claim is not needed in this application, the limiting component is the basket flex. A cast basket driver would be the next step but the cost difference is significant. For SPL of circa 85dB normal listening, I don’t think the solid magnet to magnet coupling is needed. Also, that would make the internal volume larger and reduce the responsiveness and articulation. Think of it as a larger air chamber or mechanical lumped “capacitor” in the system.
1652114332143.jpeg
 
This might seem like a naive question but perhaps there are factors in play that I’m not aware of, so: do you get the same level of output to the rear of the cone as at the front and does the frame / magnet structure etc not create distortions?
Hi Studley,
You bring up a good point about the asymmetries added by air flow through the back driver basket arms and spider etc. It might have an effect. Probably an experiment to measure distortion with and without would show it. But I’m too lazy to open it up and swap 4 drivers around to test it.

One thing to keep in mind if we can ignore air flow disturbances, mechanical features don’t interact with sound waves if the wavelength is significantly larger than the feature size. A rule of thumb is to make sure the feature size is smaller than 1/3rd of the wavelength.

For the top woofer frequencies of interest or 470Hz, this is 0.24m/3 or 9.5in for us Amarikans. So for slot woofer dimensional features smaller than 9.5in it’s “invisible” to the bass frequencies we are interested in.
 
Assuming distortion comes in part from suspension K(x) and motor BL(x) non linearity. By having one driver move in opposite direction but mechanically, still having both pushing air out is how the distortion is cancelled - sort of like how long tail pair (LTP) input stages cancel even order distortion.

That’s why the drivers are arranged cone to magnet and wired anti-phase electrically but in phase mechanically.

Note the extremely low 2nd harmonic distortion below 60Hz. About -55dB at 50Hz (that’s very low for open baffle 90dB).
1652209349764.png
 
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Newtonian forces yes, but why a reduction in second. One set would need to be wired acoustically out-of-phase (ie no bass) to acheive that.

dave
wired out of phase but the front slot radiation alternates between front and back side of the woofers so summing in phase acoustically (every other woofer is inverted electrically and acoustically so still in phase). All even order distortion cancelled if woofers are matching.
 
Assuming distortion comes in part from suspension K(x) and motor BL(x) non linearity. By having one driver move in opposite direction but mechanically, still having both pushing air out is how the distortion is cancelled - sort of like how long tail pair (LTP) input stages cancel even order distortion.

That’s why the drivers are arranged cone to magnet and wired anti-phase electrically but in phase mechanically.

Note the extremely low 2nd harmonic distortion below 60Hz. About -55dB at 50Hz (that’s very low for open baffle 90dB).
View attachment 1052974
yes that distortion graph is impressive. Where does the unEQ’ed woofer response roll off from? are there a resonance from the cavity on the back (hollow back)? maybe what the pyramid shaped damping is for?