Woofer Tester WT3 - Tips and Problems

[yvessimon]

To add mass, I didn't know how to do it properly: i don't want to mess up my drivers with glue, and place an unattached weight on a cone is not valid.
I found THE solution: put magnets above and under the cone.

Now, a big problem: I don't believe Qts (calculated by WT3 without weight added). My Seas W26 are supposed to have a Qts of 0.35.
I measured 0.45. But, if the measured Fs of 23.5 Hz (specs 20 Hz), the Qts of 0.45 (specs 0.35) and the VAS of 192 l (specs 162) are correct, I should have a Fs in the 60 liters enclosure of 48 Hz. but WT3 measures 36 Hz.
This is not consistent.
Please help, thanks,

Yves

P.S. i was told that 2 volts are required to make proper measurements of Fs. Does WT3 always delivers 2 volts? If not, what can I do ?

[Jeff B.]

> To add mass, I didn't know how to do it
> properly: i don't want to mess up my drivers
> with glue, and place an unattached weight on
> a cone is not valid.
> I found THE solution: put magnets above and
> under the cone.

> Now, a big problem: I don't believe Qts
> (calculated by WT3 without weight added). My
> Seas W26 are supposed to have a Qts of 0.35.
> I measured 0.45. But, if the measured Fs of
> 23.5 Hz (specs 20 Hz), the Qts of 0.45
> (specs 0.35) and the VAS of 192 l (specs
> 162) are correct, I should have a Fs in the
> 60 liters enclosure of 48 Hz. but WT3
> measures 36 Hz.
> This is not consistent.
> Please help, thanks,

> Yves

> P.S. i was told that 2 volts are required to
> make proper measurements of Fs. Does WT3
> always delivers 2 volts? If not, what can I
> do ?

As one of the beta testers for the WT3 I put it through the paces, as did the other testers, then we exchanged lots of emails about what we were finding. I believe the results you are seeing are very accurate, much more so, in fact, than the advertised values for the driver.

I am not sure who said you had to have 2 volts to measure Fs though, it does not need to be that high. In fact, it needs to be a current value in amps and not voltage anyway to measure impedance correctly across the bridge.

Now, to do the added mass use US nickels, which are almost exactly 5.0 gms each, and just sit them on the cone.

Jeff B.

[curt_c]

Since its hard to balance all those nickels on the point of a phase plug: (and I never get a good reading that way anyhow...)

This frugal Scot found that the plastic lid off of a can of CaiLube contact cleaner weighed almost exactly 7 grams, and fits nicely over (doesn't touch) the phase plug on all the drivers I've measured recently. Simply stack the nickels on top of the cap.

I'll bet if you go to your local grocery store and smile at the nice lady at customer service, she would weigh the cap of your choosing on their postal scale...

C

[curt_c]

Don't set the DUT (Device Under Test) on a ferrous metal surface. (Like your computer.) As handy as it is to just stick it right on there, I found this can raise the measured Qes by a significant amount...

C

[dlr]

Jeff's idea of using nickels is a good one. I use them, but I put a tiny piece of Blu-Tac on the nickel to attach it to the cone. I press them on gently with a finger on the back side of the cone. The Blu-Tac comes off very easily, though some paper cones might have a small hint of it having been attached.

I have an electronic scale that measures in grams. The added weight of the Blu-Tac is pretty close to 0.1 grams (6 nickels weighed together with Blu-Tac was 30.6 grams), so if you use a small dab of Blu-Tac on a nickel, use 5.1 grams per nickel. The Blu-Tac keeps the nickel in place long enough to get a good measurement. Be sure to distribute them evenly around the cone as well, roughly half-way from former to surround would be my recommendation.

The Blu-Tac with nickels works for 4" midranges up to the largest woofer. Typically you add enough weight to shift the Fs down somewhere in the 25-50% range. The WT3 may have a different recommendation on added weight though, that I don't know.

dlr

[mike1234]

[tomdaugherty]

Nickle is nonmagnetic. Tom Daugherty

[mike1234]

...he says he places magnets on both sides of the speaker cone. Don't get me wrong. I like the idea. I just have concerns about measurement accuracy. It seems that they could interact with the voice coil when it's charged. Also, if the frame is steel and the magnets are anywhere near the frame, couldn't this create issues too.

> Nickle is nonmagnetic. Tom Daugherty

[bill]

> ...he says he places magnets on both sides
> of the speaker cone. Don't get me wrong. I
> like the idea. I just have concerns about
> measurement accuracy. It seems that they
> could interact with the voice coil when it's
> charged. Also, if the frame is steel and the
> magnets are anywhere near the frame,
> couldn't this create issues too.

I don't think magnets on the cone are going to affect the voice coil significantly. For one thing, it's pretty well isolated by the narrowness of the gap between the pole pieces and the size of the driver's magnet. For another, the magnets on the cone are going to move with the cone and hence the voice coil to a very large extent, so they won't induce any current into the voice coil.

HTH,
Bill

[djarchow]

Curt,

Good tip. Another thing I have found to make a difference is the way in which you mount/hold the driver for the impedance measurement. Most companies clamp the driver in a vice by the magnet. This is usually how I do it, making sure to minimize any surfaces that would interfere with the airflow from the driver.

Also if the driver has a vented pole piece, it is critical to not set the driver down on the magnet, blocking the pole piece during the measurement.

regards,

Dennis

[Wolf]

3 pieces of rubber sport floor-tile, sandwiching 2 pieces of 3/4" MDF. 8" diameter will most likely be sufficient for most magnet sizes. I used 6x9 cutouts. One outer piece of tile has a large 2" hole in it, and the opposed tile piece has a .5" hole in it. Adhesive or screwed together. There are flared notches to the sides of the "block" to eliminate any back pressure on the pole vent. Depending on the driver's pole diameter determines which side to use. I set this on the floor and the driver on top of it, radiating up/down. A carpeted surface is better, but I believe this is mechanically isolated from the floor enough to take proper measurements, and not enact another resonance into the end result.

Please- do not use magnetic or ferrous materials anywhere near the driver in test-mode. This will alter the results, and damping values. A little coin-jitter is negligible. A medicine bottle will also work for phase-plug drivers. A clay-ring will work, but mass needs to be measured to be accurate.

Later,
Wolf

[Wolf]

Unless the frame is cast aluminum or the like, they will attract to the steel frame, and the top-plate. It depends on the magnets' strength I suppose, but I would think it'll change the mechanical damping properties in measurements.
Later,
Wolf

[mike1234]

Well then... I'll have to give this idea a try. Pretty cool idea, Yves.

> Unless the frame is cast aluminum or the
> like, they will attract to the steel frame,
> and the top-plate. It depends on the
> magnets' strength I suppose, but I would
> think it'll change the mechanical damping
> properties in measurements.
> Later,
> Wolf

[mike1234]

[yvessimon]

> Don't set the DUT (Device Under Test) on a
> ferrous metal surface. (Like your computer.)
> As handy as it is to just stick it right on
> there, I found this can raise the measured
> Qes by a significant amount...

> C
Thanks, it could be a part of the problem.

Yves

[yvessimon]

The effect of these small magnets is negligeable
on the coil.

Yves

[yvessimon]

> ...he says he places magnets on both sides
> of the speaker cone. Don't get me wrong. I
> like the idea. I just have concerns about
> measurement accuracy. It seems that they
> could interact with the voice coil when it's
> charged. Also, if the frame is steel and the
> magnets are anywhere near the frame,
> couldn't this create issues too.

Yep, but my drivers have non-magnetic cast-alloy frames.

[yvessimon]

Thanks, very impressive... but for support, I do have better: my hand.
I hold the driver by its magnet.
Very good for health too (with a large woofer, and if you take many measurements).

Yves

[jonpike]

> Jeff's idea of using nickels is a good one.
> I use them, but I put a tiny piece of
> Blu-Tac on the nickel to attach it to the
> cone. I press them on gently with a finger
> on the back side of the cone. The Blu-Tac
> comes off very easily, though some paper
> cones might have a small hint of it having
> been attached.

There's a product called "Quake Hold" (hmmmm, maybe only in our CA market?) which is a type of "museum putty". It's a white putty that is very sticky, and leaves an undetectable amount of residue. You really can't tell it was there.

> I have an electronic scale that measures in
> grams. The added weight of the Blu-Tac is

I wonder if those little electronic scales that were mentioned a few years back are still available on that ebay store? I got one, good for this kind of thing...

> The Blu-Tac with nickels works for 4"
> midranges up to the largest woofer.
> Typically you add enough weight to shift the
> Fs down somewhere in the 25-50% range. The
> WT3 may have a different recommendation on
> added weight though, that I don't know.

Yeah, I saw hongrn's WT2 in action, they use a very low signal level... it surprises me when folks talk of laying the nickels down... in my old Speaker Workshop and the newer SoundEasy setups, them coins would dance! Normal test levels are at a higher level. Then, if the weight is loose, your readings will suck. Different approaches...

[dlr]

> There's a product called "Quake
> Hold" (hmmmm, maybe only in our CA
> market?) which is a type of "museum
> putty". It's a white putty that is very
> sticky, and leaves an undetectable amount of
> residue. You really can't tell it was there.

I've got white putty versions of Blu-Tac, not quite as sticky though, haven't seen the museum putty stuff. I suspect it's similar in compound, but it would be good to use if it left no residue. The Blu-Tac only does for stuff somewhat porous in general, as long as it's not left on for any length of time. Paper cones are the ones most affected.

> I wonder if those little electronic scales
> that were mentioned a few years back are
> still available on that ebay store? I got
> one, good for this kind of thing...

I got mine in a group buy some years ago.

> Yeah, I saw hongrn's WT2 in action, they use
> a very low signal level... it surprises me
> when folks talk of laying the nickels
> down... in my old Speaker Workshop and the
> newer SoundEasy setups, them coins would
> dance! Normal test levels are at a higher
> level. Then, if the weight is loose, your
> readings will suck. Different approaches...

I've had coins come loose when the Blu-Tac didn't hold because it was re-used, tackiness was almost gone or I just didn't press it on well. They buzz like crazy at the right frequency. I always do sine sweeps starting at 5 or 10Hz for T/S though, never MLS.

dlr