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Multi-Way Conventional loudspeakers with crossovers

Vas Question
Vas Question
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Old 1st November 2010, 05:40 AM   #11
head_unit is offline head_unit  United States
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Originally Posted by Ron E View Post
This is wrong. Change it slightly to:
Vas represents the volume of air that when compressed [by a piston of the same diameter as the speaker] exerts the same force as the compliance (Cms) of the suspension in a particular speaker.
Good catch!

Stiffness is the real, physical parameter. But Thiele and Small (and Benson maybe?) wanted to get all the parameters into the same domain, so you could apply electric filter theory for design simulation.

Stiffness in Newtons/meter is not too helpful to designers. But mathematically transformed by the cone area, it becomes cubic meters.

Aha! That's like the volume of a box! So now we can apply standard capacitor formulas for the combined stiffness of speaker + sealed box. Since the mass stays the same* then a new resonance can be found from the mass + combined stiffness, and the new combined Q calculated as well.

(*Well, except for changes caused by the shape of the box, generally quite small. LEAP calculates that for you.)
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Old 1st November 2010, 03:06 PM   #12
kabennett is offline kabennett
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I'm still trying to digest all these wonderful responses, but just wanted to take a moment to thank you all for them. This is without a doubt a phenomenal online community, and I hope to learn enough to be able to contribute something back.
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Old 1st November 2010, 04:34 PM   #13
Cal Weldon is offline Cal Weldon  Canada
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Vas Question
You already have. You thanked those who took the time to help.
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Old 1st November 2010, 09:44 PM   #14
thetubeguy1954 is offline thetubeguy1954  United States
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David,

I also want to thank you for your excellent explanation and analogies! It's the very first time I actually got a grasp on what Vas meant. I think it's all to common when someone that's knowledgeable on a given topic responds to a newbie ---{although I've been an audiophile/music lover for over 43 years now, I'm just a newbie to DIYing}--- like myself, to respond in a way they believe is simple and easy to understand. When in reality it's ONLY simple and easy to understand "if" you already have knowledge about the topic!

That's why analogies are so helpful. For example: Before becoming disabled, I repaired computers and would need to talk to many clients that just couldn't grasp how their harddrives and memory were functioning. I had an analogy that once stated was easily understood and grasped by almost everyone immediately! I'd explan the HD is like a file cabinet and your memory is like a desktop. So if you clicked on WORD, you're telling your assistant named "windows" to go to the file cabinet called HD and access the folder in it called Word and then place it on your desktop, which your assistant "windows" refers to as memory.

In any event I hope my analogy helped you understand why your analogy was so helpful to me. Again my sincerest thanks...

Thetubeguy1954
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Last edited by thetubeguy1954; 1st November 2010 at 09:49 PM. Reason: To make myself clearer
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Old 2nd November 2010, 11:17 AM   #15
speaker dave is offline speaker dave  United States
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Originally Posted by thetubeguy1954 View Post
David,

I also want to thank you for your excellent explanation and analogies! It's the very first time I actually got a grasp on what Vas meant.

Thetubeguy1954
Happy to help out.

Most of these concepts are pretty straightforward if carefully explained. A lot of other people have helped me out along the way and I'm happy to be able to pass a little knowledge along.

I think we could put together a list of terms and definitions, perhaps in a wiki approach. Maybe we should have a thread: "Could somebody please explain...?" and let various newbies ask any question and others take a crack at explaining it. Good questions and answers could be collected in an FAQ or "terminology" area.

Thanks to everybody else for the other excellent responses!

David S.
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Old 2nd November 2010, 10:37 PM   #16
kabennett is offline kabennett
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Quote:
Originally Posted by speaker dave View Post
I think we could put together a list of terms and definitions, perhaps in a wiki approach. Maybe we should have a thread: "Could somebody please explain...?" and let various newbies ask any question and others take a crack at explaining it. Good questions and answers could be collected in an FAQ or "terminology" area.
How about a sub-forum titled "Could somebody please explain...?" (instead of a thread), and then particularly useful and lucid and evolved/edited threads could be made sticky, making for a quick reference for those of us deficient in knowledge, memory, or both....

What do you think moderators?
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Old 2nd November 2010, 11:02 PM   #17
jcandy is offline jcandy  United States
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Originally Posted by sreten View Post
Vas is simply the the equivalent box volume of the drivers suspension. If the driver is put in a sealed Vas box, Fs goes to Fbox and Qts goes to Qbox by multiply by 1.414 (root 2). In a sealed 1/3 Vas box, Fbox and Qbox are double Fs and Qts.
I think this is a great post. This analogy can be extended to see how Fc (speaker in closed box) improves only marginally for Vbox > Vas. Let a = Vbox/Vas:

a=1/3: Fc = 2 Fs
a=1/2: Fc = 1.7 Fs
a=1: Fc = 1.4 Fs
a=2: Fc = 1.22 Fs
a=3: Fc = 1.15 Fs
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Old 10th September 2014, 01:25 PM   #18
arjunm009 is offline arjunm009
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Quote:
Originally Posted by speaker dave View Post
The term Vas came into common use because it was a way for Thiele and others to generalize a woofer without worrying about its particular diameter. That is, a design with the woofer/box system defined partly by Vas would work the same whether it used 10" diameter or 6" diameter. (At least for small signal performance.)

Not sure about the Eminence definition but the first definition you gave is correct: the volume of air that has the same compliance as the driver suspension.

Picture a box with a piston on the front that you can press to compress the air inside the box. Since you are compressing the air to a smaller volume, there will be a restoring force. If the box is very large a given "distance of press" will compress the inside air a small percentage and the restoring pressure will be weak. As the box gets smaller the enclosed airspring gets stiffer and the restoring force greater. Some particular volume of air would have a stiffness equal to any given woofers stiffness.

Another way to think of it is: "Take away all of the suspension of the woofer and find a box of just the right size to give it back the same stiffness." (or return it to the same resonance) The size of that box is equal to Vas.

David
Then when designing enclosures, does it mean to have a box volume appropriate to the value of Vas?

At some places, there is a recommended enclosure volume mentioned for sealed and ported. But they are all generally different from Vas.
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Old 10th September 2014, 02:07 PM   #19
godfrey is offline godfrey  South Africa
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Originally Posted by arjunm009 View Post
Then when designing enclosures, does it mean to have a box volume appropriate to the value of Vas?
Yes, among other things. Fs and Qts in particular have to be taken into account as well.
Quote:
Originally Posted by arjunm009 View Post
At some places, there is a recommended enclosure volume mentioned for sealed and ported. But they are all generally different from Vas.
Yes, box volume is often different to VAS, depending on the other driver parameters and the desired result.
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Old 10th September 2014, 06:15 PM   #20
fastbike1 is offline fastbike1  United States
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Vas and box volume are virtually never the same.
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