Ascendant Audio Atlas 12 Variable Qts Specs, Strange?

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I was checking out the specs for the Ascendant Audio Atlas 12 woofer which uses a second smaller voice coil to provide adjustable Qts:
http://www.ascendantaudio.com/Atlas 12.htm
I'm going to put more thought into these specs and how to model the driver but my initial impression is that something is wrong with the specs. The second coil is not connected to the main coil, yes it is inductively coupled and does act as a generator since speakers are reciprocal networks but if we assume constant B, then Bl for the main coil should not change as Qts is varied, yet it does in the specs.
If there's any change in B I'd expect it to go down with the second coil shorted due to increased losses in the magnetic circuit with increased loading, yet it goes up. I'd expect the second coil to act as mechanical damping on the voice coil and therefore the effective Qms should go down with it shorted, however Qms remains constant. I believe they, incorrectly, forced a constant Qms which caused the losses to show up in Qes and as lower Bl. I believe this is also causing an error in the predicted passband efficiency. I believe it should remain nearly constant as Qts is varied. The change in Qts should only alter the shape of the low end rolloff since it alters the closed box Qtc. I'd like to measure one of these drivers.

It is good that the second coil is small since any gap volume not used to produce force is wasted.

Anyone else notice this? Thoughts? Comments?

Are the Atlas designers on the forum?

Pete B.
 
Hi,

I'll try to explain it as much as I can.

The Qms doesn't change, because the suspension stays the same and the moving mass stays the same. Nothing "mechanical" is changing when playing with the second smaller coil.

The Qes is changing because you get electromotive braking, it's something electrical, so it's Qes.

When you close the second coil, the resistance is low because the circuit is a short circuit. So the Qes is small. So Qts is now smaller and BL is higher. If you put a resistor, then the inductive current must travel it, it's harder to travel it than a short circuit. The Qes is now higher. So Qts is now higher and BL is smaller.

I hope I helped you a bit on this. Maybe someone with a better explanation will step up.

BTW, you can power both coils if you want to get an even smaller Qts. You need to power them in series. You'll get higher powerhandling too. I don't know how to calculate the new parameters on the 6 ohms settings... I think BL is now 1.5 times the original value, so Qes is changing according to that, and then Qts is lowered.

Good luck!
 
I believe the specs are in error and that RDO changes Qms, not Qes as Ascendant states. Most likely they worked backwards from a measured Qts value but assumed Qms was constant. Net result for most box models is the same, except with regards to Bl and thus reference efficiency, as noted.

Oh, and I'd say the braking is definitely (electro-)mechanical, not electrical. If the braking is induced by the second coil moving in the permanent magnet's field, so no mechanical motion, no braking. Or for another example, Bl is the force in newtons exerted by the driver at 1 amp of current, right? Then how could the braking of the second voice coil reduce this force when the cone is at rest, which is how Bl is measured?
 
bwbass said:
I believe the specs are in error and that RDO changes Qms, not Qes as Ascendant states. Most likely they worked backwards from a measured Qts value but assumed Qms was constant. Net result for most box models is the same, except with regards to Bl and thus reference efficiency, as noted.

Thanks for your input, seems you've restated what I said and confirmed my observations. Yes, as I also stated I think a measurement error was made.

Oh, and I'd say the braking is definitely (electro-)mechanical, not electrical. If the braking is induced by the second coil moving in the permanent magnet's field, so no mechanical motion, no braking. Or for another example, Bl is the force in newtons exerted by the driver at 1 amp of current, right? Then how could the braking of the second voice coil reduce this force when the cone is at rest, which is how Bl is measured? [/B]


Again thanks for confirming my view. Your speaking of a static Bl measurement, essentially a DC measurement, where the braking coil is ineffective since it requires AC motion to be effective. There may be small changes in the dynamic Bl when Qts is varied by loading the braking coil.

Pete B.
 
simon5 said:
Yes I agree with both of you. The T/S parameters are wrong.

Chad is one of the guys at Ascendant Audio.
You can talk to him on SoundIllusions...

http://forum.soundillusions.net/forumdisplay.php?f=50

No, I'm not with Ascendant Audio.

Thanks for the link.
I notice that you posted over there, would have been appropriate if you had referenced back to this original thread so that he could come here, see the details, and comment.

Pete B.
 
simon5 said:
Yes I agree with both of you. The T/S parameters are wrong.

Chad is one of the guys at Ascendant Audio.
You can talk to him on SoundIllusions...

PB2 said:
Thanks for the link.
I notice that you posted over there, ...


Actually, Simon, I think everyone here (especially me) would appreciate it greatly if you posted his response in this thread. No point if a bunch of us asking him the same questions...

Thanks a bunch!

Mark
 
I called him to talk about this a while back. He said his specs were measured and correct and didn't know why his figures came out different than the work published by adire and others indicate, and wasn't too concerned about the discrepancy.

To be fair, the net result in a box in low Qts situations is probably exactly the same, with the higher settings just being a bit more efficient than expected. Looking at it another way, this is just rating your driver conservatively, which is a good thing.

I don't expect anybody has complained about their driver being too efficient.
 
bwbass said:
I called him to talk about this a while back. He said his specs were measured and correct and didn't know why his figures came out different than the work published by adire and others indicate, and wasn't too concerned about the discrepancy.

To be fair, the net result in a box in low Qts situations is probably exactly the same, with the higher settings just being a bit more efficient than expected. Looking at it another way, this is just rating your driver conservatively, which is a good thing.

I don't expect anybody has complained about their driver being too efficient.
makes sense to me, i couldn't be happier with my atlas 12, 240 watt amp, and shiva 142 L ebs box design tuned to 20hz. crippledchicken
:D
 
bwbass said:
I called him to talk about this a while back. He said his specs were measured and correct and didn't know why his figures came out different than the work published by adire and others indicate, and wasn't too concerned about the discrepancy.

To be fair, the net result in a box in low Qts situations is probably exactly the same, with the higher settings just being a bit more efficient than expected. Looking at it another way, this is just rating your driver conservatively, which is a good thing.

I don't expect anybody has complained about their driver being too efficient.

I brought this up only because I'd like correct T&S parameters so that I can accurately model some applications for this driver. It is unfortunate that Chad seems to dismiss the issue. I hope he will join in here or send me a few (wink, ha ha) to measure.
It seems to me that if they were going to get any measurement right it would be the high Q situation since the "gold" winding is open circuited. But this configuration showed the lowest Bl and I think therefore the low Qts data with the higher Bl is optimistic rather than conservative as you stated. It suggests that the driver is less efficient than predicted which is what happens when gap volume is wasted with the second coil. Still it's a nice driver, I think they're going for $100, that's an excellent price.

Another way to lower the Q is to drive the gold winding through a resistor (to lower the power to it since it is a lower power VC) in parallel with the other winding. It makes a stronger motor and will result in higher Bl and higher pass band efficiency. The parallel impedance might be too low for some applications.

The web page is under construction here's a link to the archived page: http://www.ascendantaudio.com/archive/Atlas 12 high Qts T-S specs.htm

Pete B.
 
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