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Old 26th December 2005, 03:37 PM   #1
beppe61 is offline beppe61  Italy
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Default A kind question on Qts.

Dear Sirs,

I have a kind but maybe trivial question about woofers parameteres.
I know already that the Vas is a measure of the resistance to motion of the cone, the surround, and the spider.
Something like its inertia.
The lower the Vas the higher the inertia ? (1st question)

But if I have two woofers, the first with a Qts=0.41 and the second with Qts=0.835, what does this mean in practical terms?
I mean, is the Qts a measure of how is dampened the woofer’s cone ?
For a woofer with high Qts an amp with a good damping factor is needed?
Thank you very much indeed.

Kind regards,

beppe61
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Old 26th December 2005, 09:53 PM   #2
Wizard of Kelts
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Quote:
I mean, is the Qts a measure of how is dampened the woofer’s cone ?
Pretty much. However, a speaker with a Qts of 0.83 is considered well-damped.

It should be pointed out that for decades, most sealed systems had a Qtc between 0.7 and 1.0. And they were considered well put together systems.


Just about any solid state amp, mid-fi or above, should have no problem driving a speaker of any Qts whatsoever. Damping factors in solid state amps are more than adequate for just about anything.

One thing about your speaker with a Qts = 0.83, though. If you put it in a sealed box, that Qts will rise. If you put it in a box equal in volume to it's Vas, for instance, the Qts will rise to a Qtc =1.2. Which is not horrible, just a little less tight in the bass than you might like. It will still have tighter bass than just about any ported system.
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Old 26th December 2005, 10:00 PM   #3
anatech is offline anatech  Canada
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Hi kelticwizard,
Quote:
One thing about your speaker with a Qts = 0.83, though. If you put it in a sealed box, that Qts will rise. If you put it in a box equal in volume to it's Vas, for instance, the Qts will rise to a Qtc =1.2. Which is not horrible, just a little less tight in the bass than you might like. It will still have tighter bass than just about any ported system.
I really can't agree with you on that. A proper B4 can have tight bass, and normally does if the tuning is low enough. A woofer with a Qts=0.84 should be mounted in a wall (infinet baffle) or guitar amp. It has a small magnet for it's size.

Qts=0.383 is almost perfect for a B4 alignment. 0.41 is in the range unless Fs is too high making a ported box a poor choice.

-Chris
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Old 27th December 2005, 01:36 AM   #4
Wizard of Kelts
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Chris:

Below are the step responses for selected Closed Box and Vented box alignments, from Small's papers. The colored text is added by myself.

As you can see, even in a vented alignment where the Vas/Vb ratio, (a) is 4, the bass is slightly sloppier than a Closed Box alignment where Qtc = 1.3. And that Vented alignment is a QB3 alignment, tighter than the B4 you mentioned.

A Closed Box alignment of Qtc = 1.2 will be better than Qtc = 1.3. Note how much smoother the Qtc = 1.0 alignment is.

I happen to like vented boxes, I think their advantages outweigh their disadvantages, in most cases. But the step response of most of them is not as tight as a Closed Box with a Qtc=1.2.
Attached Images
File Type: gif step resp closed-vented.gif (27.6 KB, 424 views)
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Old 27th December 2005, 03:56 AM   #5
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"I know already that the Vas is a measure of the resistance to motion of the cone, the surround, and the spider.
Something like its inertia."

No, it's related to suspension stiffness, not mass.

Also note that Qts refers to Q of the driver alone.

Qtc is Q in a closed box, and will always be higher than Qts because the air adds spring stiffness.

Well, that's true for an unfilled box; stuffing can add air friction losses that lower Q.
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Old 27th December 2005, 07:10 AM   #6
beppe61 is offline beppe61  Italy
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Quote:
Originally posted by anatech
Hi kelticwizard,
I really can't agree with you on that. A proper B4 can have tight bass, and normally does if the tuning is low enough. A woofer with a Qts=0.84 should be mounted in a wall (infinet baffle) or guitar amp. It has a small magnet for it's size.
Qts=0.383 is almost perfect for a B4 alignment. 0.41 is in the range unless Fs is too high making a ported box a poor choice.
-Chris
Dear Mr. Chris,
thank you so much for your kind and precious reply.
I am like a child on his first day at school.

Thanks a lot.
Kind regards,

beppe61
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Old 27th December 2005, 07:12 AM   #7
beppe61 is offline beppe61  Italy
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Quote:
Originally posted by kelticwizard
Chris:
Below are the step responses for selected Closed Box and Vented box alignments, from Small's papers. The colored text is added by myself.
As you can see, even in a vented alignment where the Vas/Vb ratio, (a) is 4, the bass is slightly sloppier than a Closed Box alignment where Qtc = 1.3. And that Vented alignment is a QB3 alignment, tighter than the B4 you mentioned.
A Closed Box alignment of Qtc = 1.2 will be better than Qtc = 1.3. Note how much smoother the Qtc = 1.0 alignment is.
I happen to like vented boxes, I think their advantages outweigh their disadvantages, in most cases. But the step response of most of them is not as tight as a Closed Box with a Qtc=1.2.
Dear Sir,

thank you so much for your very interesting reply.
Very interesting picture. Thank you very much.
Kind regards,

beppe61
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Old 27th December 2005, 07:18 AM   #8
beppe61 is offline beppe61  Italy
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Quote:
Originally posted by noah katz
"I know already that the Vas is a measure of the resistance to motion of the cone, the surround, and the spider.
Something like its inertia."
No, it's related to suspension stiffness, not mass.
Also note that Qts refers to Q of the driver alone.
Qtc is Q in a closed box, and will always be higher than Qts because the air adds spring stiffness.
Well, that's true for an unfilled box; stuffing can add air friction losses that lower Q.
Dear Mr. Katz,
thank you very much for your extremely kind and valuable reply.
As usual I was simplifying things too much.
Is there a parameter that can be related to the inertia of the driver?
For instance the mass of the cone?
I am asking this because at the end and IMHO it is a matter to force the cone to move and get it to stop after the impulse.
Or not?

Kind regards,

beppe61
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Old 27th December 2005, 09:08 AM   #9
e-side is offline e-side  Netherlands
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Quote:
Originally posted by beppe61

For instance the mass of the cone?
Isn't this the Mms parameter?

Mms can be calculated with this formula:

1/((2*PI*Fs)^2*Cms)

where Fs is the frequency resonance (Hz) of the driver, PI is about 3.142 and Cms is calculated as follow:

Vas/(Po * c^2 * Sd^2)

where Vas is the equivalent air volume (m^3), Po is the density of air (1.12 kg/m^3), c is the speed of sound (344 m/s) and Sd is effective piston area (m^3).

best regards

Erwin
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Old 27th December 2005, 04:18 PM   #10
anatech is offline anatech  Canada
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Hi kelticwizard,
Thanks, I stand corrected as far as impulse response is concerned. I was speaking for experience as music normally has decaying waveforms as opposed to a straight impulse. This will cover up that behaviour from the speaker unless it gets out of hand. I set my system Q around 0.7 ish for sealed or ported designs.

My decision for a ported or sealed box hinges on the low frequency cutoff and whether or not the low end is filtered or not before the amplifier. I avoid driving a speaker below it's resonant frequency if possible.

-Chris
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