Quick and Dirty Vas Measurement

[Jeff B.]

Let me toss out a quick alternative method for calculating Vas than the Delta-Mass or Delta Compliance methods that are usually discussed. I have used this as a test, and sometimes just do this when i want a quick measurement. In many cases it may be closer to reality anyway.

Suspension compliance can vary quite a bit from driver to driver, or even for a single driver that hasn't been broken in yet. But if the manufacturer lists the Mms of the driver, this is likely to be a pretty accurate number, and one that remains constant. With this being the case, simply enter it as the moving mass, along with the effective diameter and let DATS calculate the Vas from this data. Since the manufacturer probably knows the real moving mass, the result may be pretty reliable.

Jeff B.

[mzisserson]

Brilliant! Thanks Jeff. That is a great idea!

[ani_101]

Thanks... so i don't have to build entire enclosures to determine the SPL for domes... how do i enter MMS in DATS? is it the "Specified M(md)" field in DATS?

[Jeff B.]

Thanks... so i don't have to build entire enclosures to determine the SPL for domes... how do i enter MMS in DATS? is it the "Specified M(md)" field in DATS?

Yes, that is the correct field.

[Wolf]

You can also ratchet the box modelers using the Mms calculated to derive Vas in the opposite direction. When you get the Mms as spec'd by adjusting Vas, you should have a close Vas spec.
I did this on the Nephila build to get a better box sim, and it works well.

Later,
Wolf

[Jeff B.]

You can also ratchet the box modelers using the Mms calculated to derive Vas in the opposite direction. When you get the Mms as spec'd by adjusting Vas, you should have a close Vas spec.
I did this on the Nephila build to get a better box sim, and it works well.

Later,
Wolf

No, you can't. At least, not with any accuracy because you have to assume too much. If I am following you correctly, anyway. You need to know either Vas to calculate the Mms or the other way around, but you can't calculate them both. If you don't know Vas already then the calculated Mms won't be any good it will just be number based on some assumptions. Now, if you actually know Mms, like I mentioned above, then you can back into Vas using a box modeler.

[martin]

Or, if you have the electrical impedance curve (which gives you Re, the Q's, and fs), Sd, and the SPL/W/m in the pass band you can then quickly calculate the BL and Vas values. You can use the SPL from the manufacturer's data sheet (if it is reliable) or an independent measurement on a baffle. No added mass or special test box required. This has worked well for me when I wanted to simulate tweeters in a design program to work out a potential crossover.

[martin]

Double post deleted.

[Chris Roemer]

So . . . I measure F-APs, then what?
Enter "Piston Diameter", then click "Spec M(md)" button and enter mfr. Mms value in Mmd box?
Then what? Do I have to click "Measure V(as)"? Does DATS have to do another sweep?
Can't seem to get it to work (using some old data). Please hold my hand on this one?
(maybe my DATS actually needs to be plugged in to function?)

[johnnyrichards]

Is MMS the manufacturer spec we can trust? We see a lot of disparity in calculated MMS sometimes, so I wonder how reliable using this supplied specification over any other supplied specification will be. I understand it is considered a "quick and dirty" method, but...

[mzisserson]

Is MMS the manufacturer spec we can trust? We see a lot of disparity in calculated MMS sometimes, so I wonder how reliable using this supplied specification over any other supplied specification will be. I understand it is considered a "quick and dirty" method, but...

Don't know for sure, but when I thought about the amount of "stuff" that goes into the Mms will not change much. The cones are all about the same, bobbins the same size, windings are consistant and I wonder if minor variability in any of these would simply offset.

For my use, it will be nice when tinkering with drivers to have something as a Vas lithmus test before measuring. Don't think that this is a replacement or was alluded to as such.

[johnnyrichards]

I have experience applying glues using robots, and there will always be variability in that regards. The smaller the driver, the greater the effect of differences.

I have seen some pretty significant differences in calculated vs advertised MMS, which is why I question using this "quick and dirty" method.

It only takes a few seconds with a scale and some weights to do a Delta Mass test.

[johnnyrichards]

Not sure how manufacturers specify moving mass, as it also has to include X of the surround and Y of the spider, does it not?

[mzisserson]

Not sure how manufacturers specify moving mass, as it also has to include X of the surround and Y of the spider, does it not?

Good point.

[Wolf]

Now, if you actually know Mms, like I mentioned above, then you can back into Vas using a box modeler.

This is what I was referring to. In Unibox, for example, you can't enter Mms as it's a calculated parameter. You have to ratchet Vas to result in the Mms spec.
Wolf