TS parameters Vab vs Vb

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Kral57

Vab is defined as the volume of air having the same acoustic compliance as the enclosure.
.
Vb is defined as the Internal Volume of air in an enclosure.
.
Why is Vab different from Vb, and how is it calculated? Does it have something to do with the stuffing?

Ron E

Vab is defined as the volume of air having the same acoustic compliance as the enclosure.
.
Vb is defined as the Internal Volume of air in an enclosure.
.
Why is Vab different from Vb, and how is it calculated? Does it have something to do with the stuffing?

There's a few theories, with varying degrees of empirical support, but typically you hear that stuffing "adds" 5-20% to volume. I think it more likely that stuffing just damps the airspring and really causes a loss in bass, but I don't have measurements to back that up (yet).

djk

"stuffing just damps the airspring and really causes a loss in bass"

If you stuff a cabinet with a Qtc=1.0 until it reaches a Qtc=0.707, you have lost 3dB of output at box resonance.

sreten

Hi,

Its Vas not Vab and it is :
"the volume of air having the same acoustic compliance as the suspension".

Vas is the stiffness of drivers suspension, independent of the box volume.

e.g. a driver with Fs = 30Hz and Qts =0.35 in a box of 1/3xVas will have
Veffective = 1/4xVas, Fbox will double to 60Hz and Qbox will double to 0.7.

rgds, sreten.

soho54

Its Vas not Vab
Nah, he is correct with Vab.

Vab is the old school method of accounting for the effects from stuffing a sealed enclosure. It is based on specific heat variances, IIRC. If there is no stuffing/lining then Vb=Vab.

Look for a paper on sealed enclosures by Richard Small.

Kral57

Small's Paper

soho54 is correct. Section 7 of Small's paper "Closed-Box Loudspeaker Systems Part I: Analysis" has a complete description of the theory of the effects of stuffing on Vab, as well as a (not so easy) method of measuring the effects of stuffing. Thanks.

bjorno

soho54 is correct. Section 7 of Small's paper "Closed-Box Loudspeaker Systems Part I: Analysis" has a complete description of the theory of the effects of stuffing on Vab, as well as a (not so easy) method of measuring the effects of stuffing. Thanks.

Correct, Add this information for a more complete picture of Vab:

b

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AndrewT

I don't think it is additional damping that causes the stuffing effect.
I think it is adiabatic expansion/compression that causes the stuffing effect.

edit,
Bjorno's extract covers it in the section entitled Compliance.

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sreten

Hi,

My mistake, OK its the effective volume of the box not the actual volume.

Stuffings and linings increase Veff by slowing the speed of sound and as
I understand it the rate of contraction and expansion of the "medium",
whilst also adding some resistive damping to airflow.

I found out the hard way some foams are too dense and can and
does damp airflow too much when used in bulk instead of linings.

If you vary the amount of stuffing in a box Fb will vary, this is
easiest to model by saying the effective box volume is changing.

The effect on damping is more subtle, if Qtc does not vary the
way you would expect it to just by varying Vbox, I think you
would need to manually adjust some damping co-efficient.

I think some simulators do a better job of modelling linings
and stuffings than others, WinISD AFAIK is not good.

rgds, sreten.

buggsson

Either Small's first, or second paper on sealed boxes also show you how to actually measure the effect of stuffing, when you are done with simulations.

sreten

Hi,

Practically with whatever stuffing you have the maximum is minimum Fbox.
But for fairly light stuffing you can assume it is effective, not harmful.

rgds, sreten.

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