Measuring T/S parameters

[moldyoldy]

I'd like to hear from others about the methods used when measuring a driver's Fs and for calculation of Vas, specifically the mounting and orientation of the driver being tested. In the past, I've hung the driver from the ceiling for free air measurement, and a test box for Vas. Although my results are often fairly near published data, I've never been confident that I'm getting useful data. Below is some text from Speaker Workshop's help file on the subject, which doesn't seem entirely logical either;

"Impedance in Free Air
To measure driver impedance in free air, the driver should be suspended in air far from any acoustically interfering surfaces. Hanging it from the ceiling will NOT work - the driver must be kept physically stable or the shaking from the sound will affect the impedance measurements. The recommended approach is to take two heavy tables and two clamps then put the tables near each other and clamp the driver between them. "

This seems to imply the DUT is situated with the axis vertical, which brings gravity's effect into the equation (only there's no allowance in the real equation). Also, isn't being clamped between 2 heavy tables a little contrary to the "free air" stipulation?

"Impedance in Sealed Box- The Delta Compliance method
When measuring the impedance in a sealed box (for estimating T/S parameters) the critical value is the volume of the box. This measurement is only used for estimation of Vas."

Straightforward except no mention of axis orientation. Also, for best results, the test box needs to be fairly near the DUT's Vas in volume, which could mean having a variety of test boxes on hand.

"Add extra mass- The Delta Mass method
Add additional mass to the cone in such a way that the additional mass is known exactly and the mass won’t flex when the cone moves."

Again, implied vertical axis but no factor for gravity in the equation.

How do you guys do it, and how close to published data do you get?

[John W]

I've tried to follow the steps outlined in this article, but the speakers I’ve measured don’t match the specs too closely.

http://sound.westhost.com/tsp.htm

[moldyoldy]

Thanks, John, that's what I'd been using until recently too. It doesn't get very specific about the driver orientation either, though the illustrations show a driver "face-down" in the test box. I've checked the formulas in the spreadsheet, and there's no factoring of gravity there either.

I can't help but feel the driver should test horizontally, unless the calculations factored gravity into the picture. Some drivers would see little variance, but some would see a lot.

I've never used the delta mass method, but if the driver is positioned face-up on the bench, you'd have the bench as an interfering boundary as well.:dont-know

[Zilch]

I measure Fs horizontal, face up on the concrete floor, with the driver sitting on stainless steel blocks so the vent is clear. I believe I get good results that way, and have been meaning to fabricate some stainless rings for better support.

WT2 website shows a "Bunje" suspension method; I've been wanting to try that. There's also ap notes about methodology there. WTPRO is a member here; check his prior posts for some discussion of these matters.

Thus far, Vas has been a crap shoot for me. The nickels don't seem to be reliably "one" with the cone, and I'm not about to stick stuff there....

[moldyoldy]

Thanks, Zilch, good stuff on the WT2 site, including the first reference I've seen to justify my suspicions. Even a conversion from "free air" to "free space", and acknowledgement of sag-related issues too. Guess I just need to develop confidence in a method and go with it.

Had I never read Clark's paper ( http://jipihorn.free.fr/Projets%20en...s/dlcpaper.pdf ) on the ultimate measurement setup, stuff like this wouldn't bother me so much.

I've never done Delta Mass, but always thought a handful of tiny, flat magnets would be the ticket. Just symmetrically place pairs (one on each side of cone) to get the needed mass in a distributed area that'll handle the ride.

[moldyoldy]

'Wonder if there's a standard for this procedure, or if it's as user-definable as the old 1W/1M. I have to admit having serious doubts about some of the published T/S specs (non-JBL), after doing a little testing of my own.