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Thread: Measuring T/S parameters

  1. #1
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    Measuring T/S parameters

    I'd like to hear from others about the methods used when measuring a driver's Fs and for calculation of Vas, specifically the mounting and orientation of the driver being tested. In the past, I've hung the driver from the ceiling for free air measurement, and a test box for Vas. Although my results are often fairly near published data, I've never been confident that I'm getting useful data. Below is some text from Speaker Workshop's help file on the subject, which doesn't seem entirely logical either;

    "Impedance in Free Air
    To measure driver impedance in free air, the driver should be suspended in air far from any acoustically interfering surfaces. Hanging it from the ceiling will NOT work - the driver must be kept physically stable or the shaking from the sound will affect the impedance measurements. The recommended approach is to take two heavy tables and two clamps then put the tables near each other and clamp the driver between them. "

    This seems to imply the DUT is situated with the axis vertical, which brings gravity's effect into the equation (only there's no allowance in the real equation). Also, isn't being clamped between 2 heavy tables a little contrary to the "free air" stipulation?

    "Impedance in Sealed Box- The Delta Compliance method
    When measuring the impedance in a sealed box (for estimating T/S parameters) the critical value is the volume of the box. This measurement is only used for estimation of Vas."

    Straightforward except no mention of axis orientation. Also, for best results, the test box needs to be fairly near the DUT's Vas in volume, which could mean having a variety of test boxes on hand.

    "Add extra mass- The Delta Mass method
    Add additional mass to the cone in such a way that the additional mass is known exactly and the mass won’t flex when the cone moves."

    Again, implied vertical axis but no factor for gravity in the equation.

    How do you guys do it, and how close to published data do you get?

  2. #2
    Senior Member John W's Avatar
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    I've tried to follow the steps outlined in this article, but the speakers I’ve measured don’t match the specs too closely.

    http://sound.westhost.com/tsp.htm

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    Thanks, John, that's what I'd been using until recently too. It doesn't get very specific about the driver orientation either, though the illustrations show a driver "face-down" in the test box. I've checked the formulas in the spreadsheet, and there's no factoring of gravity there either.

    I can't help but feel the driver should test horizontally, unless the calculations factored gravity into the picture. Some drivers would see little variance, but some would see a lot.

    I've never used the delta mass method, but if the driver is positioned face-up on the bench, you'd have the bench as an interfering boundary as well.:dont-know

  4. #4
    RIP 2011 Zilch's Avatar
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    I measure Fs horizontal, face up on the concrete floor, with the driver sitting on stainless steel blocks so the vent is clear. I believe I get good results that way, and have been meaning to fabricate some stainless rings for better support.

    WT2 website shows a "Bunje" suspension method; I've been wanting to try that. There's also ap notes about methodology there. WTPRO is a member here; check his prior posts for some discussion of these matters.

    Thus far, Vas has been a crap shoot for me. The nickels don't seem to be reliably "one" with the cone, and I'm not about to stick stuff there....

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    Thanks, Zilch, good stuff on the WT2 site, including the first reference I've seen to justify my suspicions. Even a conversion from "free air" to "free space", and acknowledgement of sag-related issues too. Guess I just need to develop confidence in a method and go with it.

    Had I never read Clark's paper ( http://jipihorn.free.fr/Projets%20en...s/dlcpaper.pdf ) on the ultimate measurement setup, stuff like this wouldn't bother me so much.

    I've never done Delta Mass, but always thought a handful of tiny, flat magnets would be the ticket. Just symmetrically place pairs (one on each side of cone) to get the needed mass in a distributed area that'll handle the ride.

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    Hmmm....

    'Wonder if there's a standard for this procedure, or if it's as user-definable as the old 1W/1M. I have to admit having serious doubts about some of the published T/S specs (non-JBL), after doing a little testing of my own.

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    Senior Member Hoerninger's Avatar
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    Quote Originally Posted by moldyoldy View Post
    I have to admit having serious doubts about some of the published T/S specs (non-JBL), after doing a little testing of my own.
    Moin (*) Moldyoldy,

    you are absolutely right, there are great discrepancies. On the other hand it must be considered that two speakers of the same model behave similar although their measured spec are (a bit?) different, there seems to be a sort of compensation. Many years ago I read a serie about TSP measuring and they stated this. And they made clear that the results depend on the method of measuring.
    My concern has been more that pupils can handle it, acuracy was not at first place.
    ___________
    Peter

    *) "Moin" means in northern Germany "hello", "how are you" and the like. First of all it is short and it can be said all day long, morning, midday, evening and at night. It is said that people in northern Germany do not talk so much, and in the country side it is a real fact. So, with "moin" all is said.
    Sometimes it is said "Moin moin"
    This mean that the one is in a good mood, the person seems to be talkative.

  8. #8
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    Moin, Peter! Always good to learn a new word, especially one that can replace a lengthy dialogue. Danke.

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    Senior Member duaneage's Avatar
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    I use the original woofer tester and get good results. I tested three different drivers with it and then used manual methods. All three came within 1% of what the woofer tester produced. I use modeling clay wrapped in plastic for weight.

    For the Vas computations I have a 1 cu ft test box with different cutouts. After I am done with a project I test the driver in the box for Vcb and Vfb and the results are alwys what I am expecting.

    One thing I have noticed is that if two drivers are the same model but have different T/S specs, they seem to work the same when built into boxes. Differences in one parameter seem to be compensated with another. Doesn't seem to make sense but it does.

    I also notice the FS drops 10 - 20% after warmup and after a breakin period. Run a new driver for a day with 25 Hz at 1 watt to loosen up the cone before testing

  10. #10
    Member WTPRO's Avatar
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    If you are wondering about Cms-vs-Drive level, the WT has a built in test to extract that information.

    http://www.woofertester.com/cms_non_linearity.htm

    Regarding how much of an effect Cms variation will have on response, consider how much stiffness the box air spring will provide. If the ratio favors the box, Cms variation wont matter much.

    And how close can you get with good measurement and modeling? Here is another link. These kind of results are quite typical using nothing more than the delta mass test. And... I did not even have to enter the box volume or tuning. The software finds this automatically and aligns the simulator!

    http://www.woofertester.com/wt2boxbuild.htm

    Regarding the bouncing nickel problem.

    1) Lower Idrive. The drivers being used on this forum have a lot of oomph and it is not uncommon for the cone acceleration to be higher than gravity can keep the mass firmly in place.

    2) Glue them together in groups of 5 or so. This will add a teeny bit of mass, so you will want to measure the stack before using them.

    Hope this helps,
    Best regards,
    WTPro

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    Thanks Keith!

    I usually use Turner's Two Way Tape to attach the nickel stack to the center of the dust dome.

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    Senior Señor boputnam's Avatar
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    Quote Originally Posted by Giskard View Post
    Thanks Keith!

    I usually use Turner's Two Way Tape to attach the nickel stack to the center of the dust dome.
    Yea, thanks Keith. Very cool.

    Sorry again to have missed your stand at CES. We were late; it was busy. Another time...
    bo

    "Indeed, not!!"

  13. #13
    Administrator Mr. Widget's Avatar
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    Quote Originally Posted by Zilch View Post
    I measure Fs horizontal, face up on the concrete floor, with the driver sitting on stainless steel blocks so the vent is clear.
    I have measured Fs on a carpeted concrete floor raised on blocks and also with the woofer suspended and firing vertically in open air... here is a fairly significant difference... even hanging the driver near other objects will affect your results.
    Quote Originally Posted by Giskard View Post
    I usually use Turner's Two Way Tape to attach the nickel stack to the center of the dust dome.
    I use double faced tape too... I use one that releases easily when you put a drop of denatured alcohol on it... I made an aluminum donut that I stick on the dustcap.


    Widget

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    Thanks for all the input, guys! Since several of you are apparently getting satisfactory results with the driver in a face-up position while resting on a substantially solid surface, measurement error from a vertical axis orientation must be far less than I expected. The error does exist though, and will directly influence the resultant data as postulated by Clark;

    "At and below a direct radiator's primary resonance frequency, suspension stiffness becomes equal to and surpasses the mass reaction to the driving force. While this is below the "flat" frequency range, it is important to downward response extension and the transition from fiat to roll-off. Thus, suspension stiffness variations with position become important below the "fiat" range. Diaphragm excursion is highest in this range making the importance of suspension linearity second only to force factor linearity.
    However, in the Thiele-Small model suspension linearity becomes critically important because four of the five T-S parameters contain suspension stiffness as a factor in their definition."

    He goes on to point out that compliance and Bl are the main parameters that are position-dependant in regards to T/S parameter measurement. An ideal test platform should therefore rigidly fix the driver in a position of horizontal axis with no nearby solids or boundaries, for both the Fs and Vas tests. I have yet to devise a practical and convenient way to satisfy these conditions at home.

    From a hobbyist POV, it appears that the position-induced error of a face-up driver rigidly fixed on a solid surface is likely not as bad as the error obtained from a less-rigid suspended mount with a horizontal driver axis and no nearby boundaries.

    Hmmm.....think I'll try one mounted to my fireplace mantel.....

  15. #15
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    Quote Originally Posted by moldyoldy View Post
    From a hobbyist POV, it appears that the position-induced error of a face-up driver rigidly fixed on a solid surface is likely not as bad as the error obtained from a less-rigid suspended mount with a horizontal driver axis and no nearby boundaries.
    I believe that is true. Just buy yourself a punch down panel frame and bolt it to your floor. It is rigid enough.

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