for Thiele calculations, which is better?

[Lito]

Added mass or sealed box. TIA

Lito

[richie00boy]

Added mass as it gets around the problem of driver surround and box losses. However, your weight needs to be very accurately known and it's quite hard to attach something to the cone in the right place(s) without ruining it.

[CrisTUFR]

And for the love of God, please don't try bubble gum! Unfortunately, I speak from experience here. It does work, but it leaves more than a nasty mess behind...

If you can get yourself to a sealed box of known volume, that'd work great. Unfortunately, I do not know of a good method to create a box that'll work for all sorts of driver sizes. It doesn't make much sense to have an 8" box, a 6" box, etc.

I think that blu-tac might work out, and not leave much of a mess. My past experiences with that on painted walls have not been the best as I still remember seeing blue-ish stains left behind afterwards. Perhaps silly putty would be best?

For the known mass, I think that coins work pretty well as they're a fairly fixed weight. If you can somehow get them onto the speaker in an even arrangement, you should be fine. Also, you have to try to use a different amount of weight given the speaker size since too little or too much weight on a particular speaker size won't give you accurate results.

It's a real pain, and I think that buying drivers with known parameters is easiest.

Cheers,
Chris

[richie00boy]

All you need to know from the manufacturer is the moving mass, Mms. Then you can do a few simple measurements and work all the others out, without resorting to test boxes.

Whether DIY people do as good a job as the manufacturer is another matter... I'd bet that most use relatively cheap multimeters and don't calibrate their setup properly.

[Ron E]

Added mass method requires knowedge of Sd. Whatever anyone says, this is not all that simple to measure accurately. One can guess and include 1/3-1/2 of the surround, but then one is left with an estimate. Some drivers have an Sd mismatch between front and back sides. Trusting the Mfg for this number or Mms is asking a lot.

I did an uncertainty analysis of the added mass method, and for low resonance drivers, frequency measurement accuracy is the limiting factor, and for light drivers, added mass accuracy is the limiting factor.

I think the best way to measure Vas is the Vented/Sealed box method. You measure the frequencies of the peaks in vented and sealed boxes (same box, just plug the vent), apply a few calcs and you have an accurate Vas. The frequencies of the peaks are substantially unaffected by high inductance motors, but a tight box is best.