Quick and Dirty Vas Measurement

[johnnyrichards]

So what is the standard for determining MMS? Sd? Are you allowed to divulge any information provided by manufacturers? I read a convincing article years ago that the "1/2 surround" rule of thumb is optimistic - but have not seen much since.

I ask because we test drivers using a method intended to bypass manufacturer claims on the premise that they may or may not be bs.

[Jeff B.]

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...

The variation we see in calculated Mms is due to inaccuracies in other parameters we are measuring. Most of the manufacturers I have worked with know the weight of their moving parts to within +/- a few grams. This value is likely more accurate than our measurement of effective diameter. And small variations in Mms will not change Vas very much. Many people who measure using delta-mass use too much mass making those results wrong too, so there are a lot of compromises all around.

[Jeff B.]

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?)

Yes. I enter the data as you described and then click "Measure Vas"

[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

[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.

[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?

[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.

[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]

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...

[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?)

[martin]

Double post deleted.

[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.

[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.

[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.]

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.