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Old 14th November 2007, 01:06 AM   #11
Ron E is offline Ron E  United States
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Quote:
Originally posted by John_E_Janowitz
The additional effect is that as the cone is moving upwards it is fighting against gravity.
That is false, but you are correct that nonlinearities complicate things. Actually, if the cone sags appreciably it will actually be easier to move up than down Draw a free body diagram - remember those?

Quote:
Originally posted by John_E_Janowitz
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.
I know what a chirp is. The question: at what input level?
At what input level does it cease to be a "T/S" test and become a "proprietary" parameter test?

Quote:
Originally posted by John_E_Janowitz
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.
Mms-Mmd depends on the acoustic environment, off the top of my head 20-30 grams on a 15" woofer is certainly not out of the question.

Quote:
Originally posted by John_E_Janowitz
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.
Vas does not require Diameter when using delta compliance. You are using known Mmd, guesstimated Diameter and presumably "design Bl" and checking your T/S derived Bl and Mms against them, not something an average DIYer can do.

Bl and Cms can be measured directly.
Sd can be measured in several more accurate ways.
The "real" difference between Mms and Mmd can be determined by a vacuum resonance measurement.

T/S assumes no inductance, generally, and inductance complicates parameter measurement, making the resonance peak asymmetric. Since you are particular about accuracy, do you know how to compensate - or do you leave it to a black box?

Chocolate cheesecake...never touch the stuff .

I seem to remember a time having something to do with Nick and Bassbox and PR's and when you expressed a distinct preference for not eating cheesecake when served. Then Bassbox got rewritten
Good times....
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Old 14th November 2007, 04:43 PM   #12
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Quote:
Originally posted by Ron E
That is false, but you are correct that nonlinearities complicate things. Actually, if the cone sags appreciably it will actually be easier to move up than down Draw a free body diagram - remember those?
Hmm... I dropped out of school so I wouldn't have to worry about those things anymore. haha This actually goes beyond more than a simple physics problem. I never took Vibes class, but now I wish I would have. Luckily my friend did graduate and went through to get a masters in engineering mechanics. He's setting up the equations so we can work through them.

In general though, firing horizontally doesn't have the initial offset and doesn't have the nonlinear force on up and downward stroke. I never recommend up or downfiring woofers. The other problem not in measuring parameters, but in long term use, is that the driver sags and becomes permanently more biased over time due to the suspension wearing out.

Quote:
I know what a chirp is. The question: at what input level?
At what input level does it cease to be a "T/S" test and become a "proprietary" parameter test?
With praxis we measure with less than 1V input. With the very short cycle chirp you can still feel the table moving. I was amazed how much force is actually applied.

Quote:
Vas does not require Diameter when using delta compliance. You are using known Mmd, guesstimated Diameter and presumably "design Bl" and checking your T/S derived Bl and Mms against them, not something an average DIYer can do.

Yes, you are correct. Vas doesn't require diameter. The relationship to Bl and Mms does. looks like I messed up in the loading slightly also. I stated no more than 1%, but it looks like no more than 5% would be a more accurate number. In any case, our former build house would send us drivers specifying 375grams Mms. When physically weighing the parts, they would weigh 250grams. Obviously any loading will not the nearly that much. Bl was also equivalently inflated. they would specify motor strength as Bl^2/Re for comparison to other drivers which was obviously giving a false comparison. Vd comparisons were also inflated due to the larger Sd. Correcting diameter to 1/3 surround as the original T/S papers suggest instead of 1/2 solved the issues and makes for a fair comparison.

Obviously, calculating exactly how much air is being moved by the surround would be more correct but not a simple measurement. It's something that varies throughout the travel of the driver.

Quote:
The "real" difference between Mms and Mmd can be determined by a vacuum resonance measurement.
this is a more difficult measurement though. Definitely not something a DIYer is going to do. Probably something we should be able to do at some point though.

Quote:
T/S assumes no inductance, generally, and inductance complicates parameter measurement, making the resonance peak asymmetric. Since you are particular about accuracy, do you know how to compensate - or do you leave it to a black box?
Yes, the peak will be asymmetric to some extent, the higher the inductance the more it becomes an issue. I honestly don't know if our QC software takes it into account or not. I believe it will look at both sides of the peak and take an average. In general though, getting all of the other conditions out is something much more serious than compensating for the inductance as the effects are fairly minor unless you have extremely high inductance drivers.

Quote:
I seem to remember a time having something to do with Nick and Bassbox and PR's and when you expressed a distinct preference for not eating cheesecake when served. Then Bassbox got rewritten
Good times....
Yeah, those were the days when I was too busy playing with speakers to go to class. Learned a few things since then. Lot more to learn yet.

John
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Old 16th November 2007, 06:18 AM   #13
simon5 is offline simon5  Canada
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Quote:
Originally posted by Ron E
That is false, but you are correct that nonlinearities complicate things. Actually, if the cone sags appreciably it will actually be easier to move up than down Draw a free body diagram - remember those?
Hi, not wanting to argue but to learn since this is a bit counterintuitive.

Is it easier to move up because you are under the gap ? So the magnet is more powerful to move up than down ?
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Old 16th November 2007, 12:19 PM   #14
oshifis is offline oshifis  Hungary
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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. 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.
This sag can be compensated by flowing a variable DC current from a current source. It would have the additional benefit that you could calculate Bl from the cone mass and the current (provided you could measure the zero displacement position precisely).
I think the major argument against firing up/down is that you can not place the speaker far enough from the floor and close reflecting surfaces may influence the measurement.
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Old 16th November 2007, 03:18 PM   #15
mhtplsh is offline mhtplsh  India
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Yesterday & Today i tested the driver with woofer tester old model.
Yesterday i tested with sealed cabinet/free air & today i tested with free air/delta mass.
The driver was the same.
yesterday today
in a sealed box delta mass
Fo 41.65 hz 41.6445 hz
Re 3.5443 ohm 3.5470 ohm
Qms 6.60 6.4881
Qes 0.5584 0.5558
Qts 0.5113 0.5119
Rm in Ns/m 2.417 2.197
(Mechanical)
Cm in 260.88 268.231
micron/Newton
Mm in gms 56.00 56.4677
BL 9.64 9.54
spl 84.93 85.07
Vas .5572 cu. ft .5729 cu. ft.
Le 2..20 2.1932



I think i am getting almost similar results from both the testing methods.
In delta mass i did the weight of the modeling clay on a very presision scale 32.67gms.
When removed i weighted it as 32.65gms.
Waiting for remarks from fellow members.
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Old 16th November 2007, 03:26 PM   #16
Did it Himself
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Quote:
Originally posted by simon5


Hi, not wanting to argue but to learn since this is a bit counterintuitive.

Is it easier to move up because you are under the gap ? So the magnet is more powerful to move up than down ?
It's easier to move up because the suspension is trying to pull the cone back up. Think of it like pulling back a crossbow.
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Old 25th December 2007, 05:46 PM   #17
mhtplsh is offline mhtplsh  India
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with the help of all the fellow members i am able to get good results of thiel small. suggestion of John_E_Janowitz was very helpful in solving getting proper results in testing problem.
Now i want to know more about new USB software by three parties.
I hv posted new thread :
http://www.diyaudio.com/forums/showthread.php?s=&threadid=114228
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