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mhtplsh 11th November 2007 04:22 AM

Thiel small testing
 
Woofer tester of TENMA manual says:

"Vertical Orientation (Cone is facing upward)
Simply laying a driver down on a bare concrete surface and performing tests is definitely easy to do."

Is this the simplest & best?

planet10 11th November 2007 05:13 AM

I believe that the driver under test wants to be away from boundaries.

dave

Ron E 11th November 2007 02:21 PM

Re: Thiel small testing
 
Quote:

Originally posted by mhtplsh
Is this the simplest & best?
Try it both ways and judge for yourself. I don't usually notice much difference with the drivers I have tested.

FYI, T/S theory is somewhat predicated on the driver being in a "similar acoustic environment", and free air measurements are nothing like being in a box. ....So the true test would be if the predicted response in a box is better having measured free air, laying on a surface, or mounted on a test baffle.

John_E_Janowitz 12th November 2007 04:10 AM

I have a lot of experience measuring driver parameters. I've unfortunately had to delegate hours and hours into getting a system setup to accurately measure parameters on all of our woofers. We finally have the system functioning the way it needs to for repeatability. We needed to measure the same driver on multiple passes and get the same results. A tolerance of 10% doing the same test was not an option. In reality, with running multiple passes, we need to be within no more than 1% tolerance. Nothing is changed, so the measurement shouldn't change. I've also walked several customers through things Here are some things that are very important.

The first thing I want to note is that you CANNOT fire a driver upwards. No matter what the woofer tester or any other system says, you just can't if you want accurate measurements. Several reasons. Your Bl measurement will be low. The driver instantly sags to some extent, putting it away from it's magnetic center, where Bl is low. Depending on the driver this can be up to a 10% decrease in Bl right away as you are away from the magnetic center. Firing upwards means not only is the motor working against the suspension but against gravity. Extra force to fight against makes the Bl seem even lower. Then when you add mass you are fighting even more against gravity and further sagged from center. The further you sag from center, the stiffer the compliance seems also. Now this begins to effect your measurements of Fs, Vas, Mms, etc.

The next thing is do not do the delta mass method. Use delta compliance. Adding mass to the cone means you probably have to fire it upwards, or have something sticky on the cone so you can fire vertically. With the cone firing up with added mass, you further stress the suspension into a place it isn't supposed to be and futher put the coil away from center. Bl is even more decreases, suspension seems stiffer that it is, and you can't get an accurate measurement of the shift with the added mass anyway which means your electro mechanical parameters are all off.

Then, ANY movement of the driver, mass added, or baffle holding the driver will throw off the impedance curve drastically. A door slamming, a fan blowing at the driver, truck driving by, etc can all throw it off. This doesn't mean you need a perfectly silent room, but make sure you minimize the noise. It also won't throw it off the same every time. This will show up as an irregularity in the impedance curve. Blips, bumps, dips, etc that you will see in both the magnitude and phase at and around the driver Fs will drastically throw off every possible parameter. The following are a series of measurements done by a customer. When measuring the driver he got Mms measurements that varried from 130grams up to 250grams. The mass was physically verified on a postage scale of 210grams.

Here are the images:

http://www.aespeakers.com/pics/AE%20...urements-5.gif

http://www.aespeakers.com/pics/AE%20...urements-7.gif

http://www.aespeakers.com/pics/AE%20...urements-6.gif

You can see all the bumps in both the magnitude and phase around the peak at Fs and in the one driver a double peak even. When calculating parameters a curve fit is done of the measured impedance curve. With these irregularities, the entire curve fit is thrown off.

So how do we measure accurately? First start out with the Delta Compliance method. This means you need a free air baffle, and an enclosure of known volume. We use 50L for 15" and 25L for 12" right now. Make sure to fire the driver horizontally, not up or down. You want both the coil to be centered. Make sure nothing moves. This is very critical as you have seen in the measurements above. The woofer creates and equal and opposite force on the enclosure, table, etc. We recently built our "shark cage" which is a steel structure over 250lbs bolted with 6 threaded rods into the concrete floor to be the bench for clamping our measurement enclosures down to. While you may not need to go this extreme, make sure it is well mounted. Make sure you can repeat the same impedance curve nearly identically. Praxis allows you to hold a curve, then remeasure. If the two are nearly perfectly overlaid, we realize the test is valid. If it looks different, something is wrong. If there are weird peaks, bumps, a flat top on the impedance curve, rounded top, etc, something is most likely moving.

When measuring the second step in the sealed enclosure, make sure it is perfectly sealed. If it is not, the enclosure will seem larger than it actually is, showing less shift in the given volume than it should. This throws off all your compliance related measurements. Your in box peak should be taller and narrower than the original. If it isn't, something is wrong.

The following shows two good impedance curves that you can then withdraw accurate parameters from:

http://www.aespeakers.com/pics/IB-15...-imp-curve.png

The first red peak shows the free air and the yellow shows the in box measurement. The small blips at 600hz and above are due to surround resonance, etc and are far out of the range we are concerned about for T/S parameters.

Also one final note, all the equations for Vas, Mms, Bl, etc are based on using 1/3 of the surround diameter and not 1/2. I see many software packages incorrectly stating to use 1/2 for your Vas measurements. This just doesn't work as it inflates Vas and Bl to accommodate for it. Praxis is the same way. We tried doing it, but we would get an Mms and Bl value both inflated by about 20% when doing so. Properly measuring 1/3 of the surround gave correct values. We always check our measurements based on known Mms values physically weighed on a postage scale. We then fine tune the diameter measurement until it shows the correct Mms value being measured. Then we know the rest will all fall in line as well.

John

mhtplsh 12th November 2007 09:08 AM

Thanks John_E_Janowitz,
I appreciate ur nice & informatery explanation.
Just check the following.

what i understand :
We must do the testing with Delta Compliance method.
Driver must be mouted horizontal.
Needs free airbaffle & SEALED enclosure.
Free air baffle means a sheet of plywood/mdf with driver mouted in the center.

Is this ok?

mhtplsh 12th November 2007 02:32 PM

Quote:

The vented box method is even better as you only need one measurement, not two! -Andrews
In one of the thread i saw the above comment. Andrews hv not furnished any further info about it even after asked for.
Anybody can give details about vented box testing method?

Ron E 13th November 2007 12:10 AM

Quote:

Originally posted by John_E_Janowitz
The first thing I want to note is that you CANNOT fire a driver upwards. No matter what the woofer tester or any other system says, you just can't if you want accurate measurements.
Depends on the driver. The higher the Vas or Mms, the more sag there is. I measure no difference with recent 4" and 6" woofers. Have not built a sub in a while....

Quote:

Originally posted by John_E_Janowitz
Firing upwards means not only is the motor working against the suspension but against gravity.
You described the change in operating point, which is due to "gravity", there is no additional effect. Also the effect is only significant for drivers with appreciable sag.

Quote:

Originally posted by John_E_Janowitz
The next thing is do not do the delta mass method.
Good arguments.

Quote:

Originally posted by John_E_Janowitz
Then, ANY movement of the driver, mass added, or baffle holding the driver will throw off the impedance curve drastically. We recently built our "shark cage" which is a steel structure over 250lbs bolted with 6 threaded rods into the concrete floor to be the bench for clamping our measurement enclosures down to.

Good info. I reserve comment on the "shark cage". If you are testing cabinets with mounted PR's, I can see the utility.

Quote:

Originally posted by John_E_Janowitz
We always check our measurements based on known Mms values physically weighed on a postage scale. We then fine tune the diameter measurement until it shows the correct Mms value being measured. Then we know the rest will all fall in line as well.
What you measure on the postage scale is Mmd, not Mms. Mms includes air loads which cannot be measured on a postal scale.
Geometrically, 1/2 of the surround makes more sense. Because notation varies, I cannot really argue your results without seeing them in full.

Delta compliance doesn't require diameter anyway right ;)

Ron E 13th November 2007 12:24 AM

Quote:

Originally posted by mhtplsh
In one of the thread i saw the above comment. Andrews hv not furnished any further info about it even after asked for.
Anybody can give details about vented box testing method?

You put the woofer in a vented box and measure Fl, Fh and Fb.
From this you can get Vas, Fs.

John_E_Janowitz 13th November 2007 02:34 AM

Quote:

Originally posted by Ron E
You described the change in operating point, which is due to "gravity", there is no additional effect. Also the effect is only significant for drivers with appreciable sag.
The additional effect is that as the cone is moving upwards it is fighting against gravity. The acceleration of gravity is 9.80665 m/s (32.17 ft/s). This becomes a vibes equation that gets complicated due to non-linear BL and Cms curves, but can be approximated. I'll work on that soon. The main point is that on the upward stroke the motor is fighting against gravity and on the downward stroke it is working with gravity. All in all this means your inward and outward stroke is not the same.

Quote:

Good info. I reserve comment on the "shark cage". If you are testing cabinets with mounted PR's, I can see the utility.
This is just simply for T/S parameter measurements on drivers. Not full cabinets. I can show you easily how a simple chirp tone on a 12" or 15" driver can cause enough movement that even this structure cannot stop. All mounting of the driver to cabinet and cabinet to the table needs to be completely rigid. We use toggle clamps that can apply a large amount of force. Rubber ends had to be removed. Soft material behind the driver to seal the enclosure also needed to be removed. The Chirp tone is essentially a very fast sweep. Doing it quickly creates a large amount of force. Doing it slower is much less bound to movement, but then is more bound to issues due to room interaction.

Quote:

What you measure on the postage scale is Mmd, not Mms. Mms includes air loads which cannot be measured on a postal scale.
Geometrically, 1/2 of the surround makes more sense. Because notation varies, I cannot really argue your results without seeing them in full.
Yes, Mms will be slightly more than the physical Mmd, but typically not by too much. It's usually no more than 1% or so. This is why you need a very open freeair baffle. As you add in more of a baffle the loading gets larger. If you look at all of the original formulas from Theil and Small, they specify that the effective moving area is 1/3 of the surround. If you were to actually be able to see a cross section of what is moved as the surround goes in and out, you'd see that it is equal to about 1/3 of the surround that is moving air.

Quote:

Delta compliance doesn't require diameter anyway right ;)
If that were the case it would make my life a lot easier. Calculating Vas, Mms and Bl does require diameter unfortunately. I had someone training on how to measure parameters. Couldn't figure out why Mms measured 100grams more than it should have been and BL was 25% too high. :) oops.

John

John_E_Janowitz 13th November 2007 02:36 AM

BTW, are you the same Ron E that I would have known from on the basslist back 10 yrs ago or so that had the chocolate cheesecake recipe?

John


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