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Thread: Scan Speak D3004/602000 test

  1. #11
    Devil's Advocate Adam_MSS's Avatar
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    Other thing to think about is the comparisons or overlays like Zaph does at Zaph|Audio

    He uses a full bandwidth chart and just keeps it the same for all plots of a given type. Don't know if you'd plan to do anything like that in the future, but you could at least put yourself in a position to be able to do it later if you desire.
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  2. #12
    Tester Extraordinaire ErinH's Avatar
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    I'm talking with the web guy about a way to do comparisons. I have an idea of exactly what I want to do but it seems like it would be pretty tough to pull off for a few reasons.

    Tweeter data will be band limited down to 500hz for all. Everything else will be 20/20k.
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  3. #13
    Senior Member cvjoint's Avatar
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    Erin, are you sure these intermod plots are comparable? The Scan Speak performs the worst in 2nd order intermod. Could it be that the .1*FS rule is putting it at a disadvantage. Why are all the other tweeters fed 100hz+ tones while the Scan has to do with a more demanding 75hz? It seems to get penalized for having a low FS. .1*FS is probably just a rule of thumb. I'd vote for the same bass tone to be used on all tweeters to equal the playing field.

    Things get even more interesting if you want to test the Neo3, or some AMTs. For all practical purposes they don't have a resonant frequency.

    How about the deep chamber Scan? Feeding a 45hz bass tone is not really indicative of what it would see in any real world application is there?
    Last edited by cvjoint; 08-07-2012 at 07:46 PM.
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  4. #14
    Tester Extraordinaire ErinH's Avatar
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    Klippel AN calls for 0.1*Fs for microspeaker and tweeter IMD/HD testing. I go by that. It seems a bit more safe compared to making up numbers. Then people will flip your comment and possibly suggest I'm tailoring a test toward a specific driver rather than following a set of guidelines. Hard to argue with what Klippel's papers tell me to do.

    as far as worst in 2nd order... that may very well be a design goal to trade off high 2nd order distortion for lower 3rd order distortion. just a thought.
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  5. #15
    Senior Member cvjoint's Avatar
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    Why does Klippel recommend .1*FS? Maybe the .1 is made up, why not .2? At least I have a reason why the bass frequency should be fixed, it makes comparisons possible for a given application. .1 may be better geared for tweaking a driver, where absolutes are not important. We're not going to manufacture drivers so...

    I made the the argument of 2nd vs. 3rd in the PE thread. We don't really know if that tradeoff exists.
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  6. #16
    Tester Extraordinaire ErinH's Avatar
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    Edit: I sent an email. I'll let you know what they say.

    I don't disagree with your logic. I just would rather go by some set standard and I'm sure they chose that set of values for a reason.
    Last edited by ErinH; 08-08-2012 at 07:14 AM.
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  7. #17
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    I can sort of understand .1Fs since the 4th and 5th harmonic of 100Hz is already showing up at 400Hz and 500Hz. It's not like your graph starts right at Fs. You need some graph/scale below FS to get a good picture.

  8. #18
    Senior Member cvjoint's Avatar
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    Got a response back, Wolfgang himself with a very straightforward explanation!

    To reiterate, my question was in regards to the test tone used to derive the IMD plots Erin generates. In particular, I claimed that the bass tone should be the same for every tweeter to even out the playing field. Tweeters with low resonant frequencies had to make do with lower frequency test signals under the existing rule, placing more stress on their motors and suspensions.

    There are 3 parameters of interest for the discussion here:
    f1 = bass tone in question
    fs = resonant frequency of the speaker
    f2 = second tone, part of a voice sweep


    The test then reproduces distortion artifacts for a speaker that plays intended frequecies f2, while a bass tone f1 puts a strain on the motor to mimic complex music passages. The existing rule used: f1 = .1*Fs. Since Fs changes from tweeter to tweeter (400hz to over 2khz for 1" tweeters) the bass tone is different under many of Erin's tests.

    In answer to our dilemma Wolfgang said the following. The rule of .1*Fs has two purposes, both of which make the interpretation of the IMD tests easier. The first, it makes sure that f1<Fs. This is important because if f1 is the same as f2 the magnitude of the bass signal excitation is amplified. We generally used speakers above their Fs, for example there is a rule of thumb out there to use them at 2*Fs. Therefore were are interested in a sweep, or f2>Fs. By making f1<fs we guarantee that the bass tone is never the same as F2. That ensures the bass done generates the same displacement of the diaphragm for every f2 in the sweep. Call this condition 1: f1<fs. .1*Fs is then a tenth of the Fs, and that is why the current rule satisfies the condition, it is a fraction of it.

    Condition 2 is that f1 should be "small enough" such that distortion artifacts are close enough to f2 but far enough from f1 to not include harmonics of f1. That's sort of what durwood mentioned above, at least the first part.

    Wolfgang concluded that the rule can be modified as long as it satisfies the two limitation above.

    With that in mind I propose we set f1 for tweeter testing such that it achieves all the requirements that the .1*Fs rule satisfies while being fixed from speaker to speaker to ensure comparability across speakers. New rule: 80hz should do the trick.
    -It is guaranteed to be under the Fs of any tweeter out there. Any speaker that has an Fs under 80hz is arguably a very poor tweeter.
    -It is far enough from f2 to not include any harmonics of it in IMD distortion measurements. Speakers commonly generate 2nd and 3rd order harmonics, 4th and 5th are generally very low under common use but are usually the highest you will ever see in a magazine review, industry test (VC mag), in common testing units (like the Omnimic) and so on. For reference the 5th harmonic of 80hz is 400hz, well below the range we would run a tweeter down too, and in fact even lower than a deep chamber Illuminator resonant frequency. So even if you did want to run a very capable tweeter as midrange the tests would still be useful for that.
    -It is constant across speakers by definition, the rule is f1=80hz, not a function of Fs which changes.

    Everybody is happy. I would just hate to see Erin get to 100 tests and not have the results be comparable.
    Last edited by cvjoint; 08-08-2012 at 04:58 PM.
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  9. #19
    Tester Extraordinaire ErinH's Avatar
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    That's very similar to the reply I got from their lead tech.

    I'm not opposed to changing it. Whatever makes sense is what should be done.

    Makes me wonder if the midrange value of 1/2 Fs should be reconsidered.

    For midwoofers that rule is probably going to be close enough since most all ofthem have an Fs within a small range. Midranges sometimes can be below 100 or as high as 200hz

    LMK your thoughts. And thanks for pursuing this George.


    What sucks is I'll have to retest some tweeters.
    Last edited by ErinH; 08-08-2012 at 05:11 PM.
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  10. #20
    Senior Member cvjoint's Avatar
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    With mids and bass drivers it gets tight because the FS is so low. 20hz? What's the voice sweep range on something like a 7"?
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