Vas Question

[kabennett]

What does the Vas compliance measurement correlate to? Most definitions in reference materials I have simply describe Vas as the volume of air that has the same acoustic compliance as the driver suspension. While I understand that a higher Vas means a higher complicance, I've never understood what the number itself means. I just came across this on the Eminence website:

"Vas represents the volume of air that when compressed to one cubic meter exerts the same force as the compliance (Cms) of the suspension in a particular speaker. "

I've seen this version of the definition before, but can't recall where. But I don't understand. I've never seen a driver with a Vas larger than a cubic meter, so you wouldn't be able to compress the Vas volume. Is this definition accurate? If so, are they saying that the vacuum force generated by expanding the Vas (say, 20 litres) to 1000 litres is the same as the amount of force as the compliance of the cone?

[speaker dave]

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

[kabennett]

Excellent explanation, thank you! Somehow I'd always thought that Vas was relative to the speaker diameter since larger speakers tend to have a larger Vas.

Using the Scanspeak 4434G family as an example:

5.25" Vas: 12.89 litres
6.5" Vas: 20.86 litres
8" Vas: 122.5 litres

They all use the same suspension (I'm assuming), the larger sizes just have more of it. Shouldn't the larger sizes be stiffer (in terms of total restorative force) and proportionally compliant (relative to Sd)?

[speaker dave]

Originally Posted by kabennett

Excellent explanation, thank you! Somehow I'd always thought that Vas was relative to the speaker diameter since larger speakers tend to have a larger Vas.

Using the Scanspeak 4434G family as an example:

5.25" Vas: 12.89 litres
6.5" Vas: 20.86 litres
8" Vas: 122.5 litres

They all use the same suspension (I'm assuming), the larger sizes just have more of it. Shouldn't the larger sizes be stiffer (in terms of total restorative force) and proportionally compliant (relative to Sd)?

That would all make sense if you think about.

Compliance and Vas are related, in fact Vas is proportional to Cms times Sd squared (compliance times area squared). So if you think three woofers share the same suspension elements, or at least have reasonably close compliances, then Vas would grow for the larger units by area squared or diameter to the fourth power.

Think of it this way: the larger woofer, by virtue of its greater cone diameter would need a much larger box (Vas) to yield the same stiffness. Why?, because for every cm of travel it compresses the air in the box that much more. (More in volume reduction, a large box reduces the percentage reduction.)

Hope I haven't confused you again.

David

[sreten]

Hi,

Vas basically tells you about the drivers suspension stiffness and the ratio
of the cone mass to its Fs, basically higher cone mass for a given Fs means
a lower Vas, and this means lower efficiences and smaller boxes.

Note that drivers designed for sealed loading can have Vas >> Vbox, whilst
for vented loading Vas should be ~ 2 times Vbox , the final involved parameter
is Qts - free air and in box. The square root ratio of Vas to Vbox determines
the change of system total Q.

rgds, sreten.

[Pano]

Thanks for asking. So many of the Thiele Small parameters get short, dry explanations around the web. Such as your Eminence example above.

It would be great to have fuller explanations and the practical ramification of the parameters in a FAQ here on diyAudio.com - Thanks to Dave and Sreten for the nice answers.

[jamikl]

I'm with Panomaniac on this one. Thanks for the question and a big thanks for the extremely lucid replies. I agree, too, that similar lucid replies on the other TS parameters would be good to see permanently on this site.
jamikl

[Ron E]

Originally Posted by kabennett

I just came across this on the Eminence website:

"Vas represents the volume of air that when compressed to one cubic meter exerts the same force as the compliance (Cms) of the suspension in a particular speaker. "

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.

[sreten]

Hi,

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.

e.g. a driver Fs=30Hz, Vas=30L, Qts=0.35 : in a sealed 10L box Qts=0.7
(Butterworth), Fbox= 60Hz and as its Butterworth F-3dB also = 60Hz.
The above driver could also be used in a vented box, around 18L.
(Vented boxes are usually around double the volume of 0.7 sealed,
though you cannot practically vent a driver with Qts above ~ 0.5.)

rgds, sreten.

Bigger drivers need bigger boxes and they have bigger Vas than small drivers.

[Francec]

panomaniac,
"I agree, too, that similar lucid replies on the other TS parameters would be good to see permanently on this site."

I think this would be a great idea, an educational part of the forum (for the still learning like me).

Frank