Measuring VAS

[Ryan_M]

I just received a WT3 - haven't taken it out of the box yet. I know it can measure VAS by the added mass method, is this method accurate so I don't have to build testing boxes for each speaker? Also can I use something like poster putty for the added mass? if so ~ how much of it should I stock up on?

[djarchow]

Here is one perspective from John Janowitz at AE speakers:

http://www.aespeakers.eu/knowledge-b...arameters.html

Regards,

Dennis

[lasse]

I just received a WT3 - haven't taken it out of the box yet. I know it can measure VAS by the added mass method, is this method accurate so I don't have to build testing boxes for each speaker? Also can I use something like poster putty for the added mass? if so ~ how much of it should I stock up on?

Hello,
I don´t know about WT3 but the method described in e. g. "Testing Loudspeakers" by dÁppolito suggests that the added mass should make the Fs go down with a certain percentage. 25-30% could be a guideline.

Yes, poster putty works fine. I have made a few mistakes in this field and one of them was to use a weight that didn´t hold together internally so I had a system of two coupled masses. You can beleive that I mobilized the DIY-community when I had TWO impedance-maximas. Had to apologize afterwards.

If you are measuring a heavy woofer, make sure that the driver is firmly attached to the base, e.g. the table. otherwise, the results will suffer.

Regards//lasse
Stockholm, Sweden

(And Tina Turner performed at the Globe Arena i Stockholm yesterday-Awsome!!)

[johnnyrichards]

I wonder if we should take it to the real extreme and test the woofer oriented in the manner to which it will be installed - if you have a slanted baffle in mind should we test our woofer on a slanted baffle? Upfiring, downfiring, front firing? If the woofer is intended to be used downfiring, than what good are T/s params which are obtained from the front firing method? The woofer changes depending on orientation (enough to give false results according to the article) so if the woofer is oriented in any way different from the testing, it stands to reason we will have an unpredictable system?

Now I am going to have to do some testing and compare how different forward firing and up firing is when testing Vas.

And compare the results to what to determine true accuracy? The manufacturer? They never lie.

[lasse]

I wonder if we should take it to the real extreme and test the woofer oriented in the manner to which it will be installed - if you have a slanted baffle in mind should we test our woofer on a slanted baffle? Upfiring, downfiring, front firing? If the woofer is intended to be used downfiring, than what good are T/s params which are obtained from the front firing method? The woofer changes depending on orientation (enough to give false results according to the article) so if the woofer is oriented in any way different from the testing, it stands to reason we will have an unpredictable system?

Now I am going to have to do some testing and compare how different forward firing and up firing is when testing Vas.

And compare the results to what to determine true accuracy? The manufacturer? They never lie.

Hello,
I have different input to the orientation issue: My own investigations, as amateurish as they may be, indicate that there is a difference in T/S values depending on the orientation of the DUT (Driver Under Test ) . I have built a "fixture" of a v-shaped piece of wood that I can place the driver in with two bolts and that in the "right" vertical position. When making measurements on a 10" woofer, I got stange results and found out that I had to put my hand on the topside of the driver to "steady" it enough for reasonable results. (perhaps this also is an input in the discussion on baffle strenght?)
On th other hand was the normal position of the DUT in school (Loudspeaker design class at Royal inst. of technology) was facing upwards.

Who can tell what´s right and wrong?

//Lasse

[curt_c]

I think Janowitz’s comments represent some sage advice, and in his production environment, the delta compliance method makes the most sense. I will add that using the compliance method requires an accurate volume measurement of the driver under test, as this volume must be added / subtracted from the test enclosure volume for accurate results.

Delta mass, on the other hand is simple, easy and repeatable. John’s remarks on Bl changes with cone displacement do bear consideration when measurements are done with the driver face up. However, the motor design of each tested driver is going to determine how much of a concern cone sag will be, and how much it will effect the results.

Other thoughts / tips for maximizing the accuracy of your measurements:

Blu-tack works well, but it takes a lot of it to provide sufficient mass. At 5 grams each, Nickels work well and can be affixed with a known weight of blu-tack. An accurate gram scale of some type will be necessary to measure the blu-tack.

If measuring delta mass with the driver facing up, insure you do not impede the airflow through the pole piece vent, if it is utilized.

Do not set your driver on any ferrous metal surface during measurements. –Your computer case may look like a dandy spot to stick the driver, but the measured Qe, and other parameters will be adversely affected.

Insure the driver is well coupled (attached) to a surface having exhibiting a high mass. For free air measurements, a clamp around the motor may provide the best coupling, but the delta compliance method requires you mount via the flange. Speaking of which:

In delta compliance, the driver must be very well sealed to the test enclosure, and the enclosure itself must have no air leaks, as obvious (or perhaps not so obvious) measurement errors may result.

Mounting the driver facing inward makes it easier to measure or calculate the additional volume in delta compliance measurements.

Soooo…. Both methods need to carefully controlled, to minimize measurement inaccuracies, and insure consistency.

C

[curt_c]

And compare the results to what to determine true accuracy? The manufacturer? They never lie.

Good point. For most of us, there really isn't any way to verify our measurements. I for one, won't lose any sleep over it though. Just make the enclosure a tad oversized, and go on to more important issues.

C

[johnnyrichards]

Do not set your driver on any ferrous metal surface during measurements. –Your computer case may look like a dandy spot to stick the driver, but the measured Qe, and other parameters will be adversely affected.

I can attest to that as I suspect I was having some issues using my floor to rest my driver on. I live in a trailer and I am pretty sure the steel beam right underneath where my driver was resting was causing a lot of issues.

Good point. For most of us, there really isn't any way to verify our measurements. I for one, won't lose any sleep over it though. Just make the enclosure a tad oversized, and go on to more important issues.

No sleep time lost for me either. With all the other things which cause unpredictability in a system, being 2 or 3% off on the T/s params isn't enough to worry about. They change under high power as it is. Theoretically, we would need to test our drivers parameters at the intended power levels etc. to compensate for the "Q" changing as voice coil temp rises, for example.

Using the woofer tester and their recommendations is good enough to get us ball park. What's a few hertz among friends?

[Brian Steele]

I just received a WT3 - haven't taken it out of the box yet. I know it can measure VAS by the added mass method, is this method accurate so I don't have to build testing boxes for each speaker? Also can I use something like poster putty for the added mass? if so ~ how much of it should I stock up on?

I'd avoid using the delta mass method with the WT3. There seems to less sample points at lower frequencies, so using a method that actually lowers the driver's resonance frequency probably isn't a good idea.

OTOH, small variations in Vas don't affed the overall response too much, so you don't have to be as anal measuring that parameter as you should be with the others.

Delta compliance is easy, and you don't have to build a box. I've used buckets, pots, existing speaker boxes, etc. to act as the "enclosure" - a piece of scrap wood with a mounting hole cut in it for the DUT, some weatherstripping to act as a seal between the baffle and the rest of the "enclosure" and perhaps a clamp or two to keep everything in place is usually more than enough. All that's important is to ensure that the enclosure is sufficiently sealed so Fb isn't adversely affected.

[Ryan_M]

Thanks for all the info,

I can see the merits of the arguments from both sides of the fence. The added mass has obvious benefits for a hobbyist like me. I'll pickup some poster putty and give it a go, if I get repeatable results with none of the 'funny stuff' as noted in Janowitz's article (thanks for that link BTW) then I'll consider the results accurate... or at least good enough for me (for now?).

I will however mount the speakers vertically on a solid mount. I have a small stand that I use for grinding optics on that it extremely stiff and weighted with 150lbs of patio stone so I'm half way there for a good mount. I'll let you know how it goes.