# Vas testing?



## speedie (Aug 19, 2009)

Question 
I have recently purchased a WT3 from PE 
It is my understanding that if one wants to use the weight method to test for the Vas of a driver you are to use a weight that is equal to Mms or greater?

At my workshop I used varying known weights from equal to and above the recommend and had varying test figures 
Is this an abnormal occurrence or is it the norm for this software

I sort of thought that the speaker motor would overcome the static resistance of the mass and give even results regardless of the weight used (not kilos of course)

As an indicator the heaviest mass used was 200 grams on a 12 inch earthquake element then the lightest was 55 grams

The Vas readings varied from high 80s to mid 90s over many sweeps

Any light on this matter would be appreciated
Cheers speedie


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## Sonnie (Apr 11, 2006)

I certainly cannot shed any light, but I recently purchased this kit and tested a driver. I tested the Vas using the sealed box method, but could not get the same two readings... and most of the time could not get a reading period. I have not attempted the mass method yet, but plan to at some point in time. Just been too busy to get to it.

Maybe someone can enlighten us both. :huh:


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## tshifrin (Nov 24, 2011)

My WT3 is still under the tree, so I'm no expert, but it was posted somewhere that the WT3 isn't so great for sub-drivers; doesn't measure accurately since it has such a low drive power itself. The poster said other than subwoofers, it was useful and accurate, so I decided to keep it (well, when I actually get it; been practicing my "surprised and it's just what I want" face for the givers). It was recommended to me that I just take the manufacturer's data, if it is a reputable company. As I understand, Vas depends on air density, humidity, and other tweaky variables too, which make it really hard to standardize.

Tom


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## speedie (Aug 19, 2009)

thanks sonnie and tom
sort of thought that no one was goin to reply to this simple question
i read some where that 20% viable in vas was exceptable but to me this seems to be a big error area
isnt one of the big T/S knows Vas part of what is needed for box calculations

i will try some smaller woofers and mid drivers to see if there smaller motors has any being as tom indicated

sonnie it sort of suprises me that you didnt have one in your kit?
cheers speedie


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## tshifrin (Nov 24, 2011)

From Ray Alden's "Speaker Buliding 201"

"The Fs, Qts, and Vas are the absolute minimum three values you need in order to design a closed or vented speaker enclosure. These parameters are normally given on driver specification sheets. However, variations in a batch of drivers can occur during different production runs, particularly in the suspension system, which leads to as much as 50% change in Vas and different Qts values.
There is general accord that a 20% variation in Vas will not have a serious effect on the final system response."

So measuring is excellent, since there could be as much as 50% difference between the driver and its specs. But the formula given for Vas depends on air pressure and speed of sound while the measurements are taken. Alden goes on to recommend using a WT from Parts Express to measure Vas, using the change in mass method.

I guess I'd just take the range of results you got, throw out the 2 extremes, and take the average of the rest. If they fall within a 20% range of each other, so much the better; design using that value. At least, so far as I now understand, that's what I plan to do. As a wiser-than-I forum poster says, your mileage may vary.

G'luck-
Tom


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## GranteedEV (Aug 8, 2010)

> It is my understanding that if one wants to use the weight method to test for the Vas of a driver you are to use a weight that is equal to Mms or greater?


The easiest way would be to build a sealed test box the driver will fit in. The only goal of the test box is to determine vas so it's okay even if it's high Q. Or you could build a box to the Q your current measurements "guess" is appropriate, and add maybe 15% volume. Good thing about sealed boxes is that we can modify their response with EQ so if it's off, it can still be okay!. Vented is a bit trickier to design and in such a case you would want to know your T/S Parameters fully. cheap, even greatly undersized sealed test box could be used solely for the goal of finding out vas.

Vad = Vbox x [(qtc/qtd)^2- 1]


So by measuring outdoors, the shape of the frequency response curve difference between an infinite baffle at fs (qtd) and the measured shape of the curve at the test box's resonance frequency(qtc); you can empirically determine vad. I would think that even if the WTC isn't measuring it properly, it would be "not measuring it properly" _consistently_, so you can determine vas with the difference in qtc / qtd... i'd assume the _difference_ is still accurate; the absolute value less so.


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## tshifrin (Nov 24, 2011)

I hadn't paid attention to your added "mass that is equal to Mms or greater" point. Alden recommends using a nickel, since that weight is known from the mint. Parts Express sells the WT3 with a scale, and I've seen it recommended that you use the blue sticky stuff (Blu-Tack) in a mass of about 30 grams. I haven't seen anywhere that it has to be greater than the Mms, just a significant addition to create a significant difference. Again, mine is still under the tree, so I have no hands-on experience; but all this talk is really wetting my appetite to dig in.

Tom


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## speedie (Aug 19, 2009)

hay EV 
what volume for a closed box would be the starting point for lets say a 6.5" woofer 
then lets go to say 15" does it stay the same?
it sort of smacks that there would be some differance in volume, thats only my lodgic thinking though

tom 
hope that your gadgit keeps you fueled for more speaker research
dont run out of drivers to test 
all the best for the coming season 
speedie:T


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## GranteedEV (Aug 8, 2010)

speedie said:


> what volume for a closed box would be the starting point for lets say a 6.5" woofer
> then lets go to say 15" does it stay the same?


As long as the box is sufficiently far from being critically damped or further (because that will throw off the difference between free air and test box Q ).

For example, a 15" driver with a very high Vad is the Acoustic Elegance TD15S. Its specified VAS = 467L

In about 130L it _should_ measure to a Qtc = .784 (which is also generally the qtc that will maximize power handling but not get the best extension without EQ). In free air its Qtd is 0.371

So using the formula above

Vad = 130L x [(.784/.371)^2- 1] = 450L 

3.7% is well within room for error and shouldn't audibly alter the sound if you were to use this value as your VAS.

Now using the same test box for a smaller driver might get tricky as that's a LOT of room to be operating in. Let's go with the Seas CA18RLY

Specified Vad = 20.46L 
Qtd = .447

In the same 130L box...
qtc = .504

so Vad = 130L x (.504/.447)^2 - 1) = 35.2682312L

So now all of a sudden our calculated vas ended up being 50% off. Obviously this test box was too big and so _the effect of the box to regulate_ Q became too small... this Vad value at almost 75% off actual value would not be correct. Given that, we probably want to use a box size that is "far too small" rather than a box size that is "too big"

IE 1L box
qtc = 2.021
qtd = .447

calculated Vad = 20.44 L which is a lot closer to our real value. if you can get that box qtc value even higher it'll probably help even more... while a Q = 2 for example might be unlistenable, it might be perfect for this sort of measuring procedure because the difference between qtc and qts is LARGE!

So how small is too small? I dunno... you probably don't want to put the TD15S into 1 L!


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## tshifrin (Nov 24, 2011)

Thanks EV; I'm just a tag-along here, but I learned from your example. Thanks for the lesson!
Tom


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