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Help! Impedance Measurement Issue

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  • Help! Impedance Measurement Issue

    Hi everyone,

    I'm fairly new to this all and have run into a bump in the road. I'm attempting to measure the impedance response of the ND16FA-6 to be used in my first crossover. I followed the excellent stickied how-to guide and am able to get measurements with both ARTA and REW. On multiple tweeers the impedance looks reasonable, with the hump close enough to the Dayton specs that Fs seems to be correct. The thing is, the phase seems to be off by about +30 degrees. I'm using a Behringer UCA202 and two non-inductive 47-ohm resistors in series in my jig and ran the calibration in ARTA successfully.

    Since I see similar results with multiple tweeters in both REW and ARTA, I'm guessing this is either an issue with my jig or an issue with the UCA202. I've attached a screenshot of ARTA showing what I mean. The shape of the phase plot is what I'd generally expect, but it seems that the whole thing is somehow shifted upwards. Anyone have any ideas? Any help would be greatly appreciated. Thanks!

  • #2
    This is for an XT25tg30. As far as the rising impedance starting around ~100hz, I also got that using the Behringer uca222 with ARTA and REW. I used a 100ohm in the jig. AFAIK it's due to too the uca222 being too noisy.

    Click image for larger version

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    Dats looked like this.

    Click image for larger version

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    • #3
      I just ran it on the nd105-4 and the graphs almost matched the graphs from the spec sheet exactly. For some reason the phase still runs way high above 2k but below that both phase and magnitude look good. Maybe it is just the Berhinger having issues with phase in higher frequencies...


      • #4
        Click image for larger version

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ID:	1384635 Here's an ND105-4 from 8 yrs. ago. Note the Fs was 15Hz lower back then.
        Your Z should not start to rise dropping below Fs (although that region won't play into XO design).
        Your phase should cross the "0" right right at resonance - as yours appears to.
        Your measured phase seems OK around Fs, but too high on both ends of the FR, for some reason.

        Note: Impedance phase (errors) won't affect your XO design as far as FR and driver summing goes, THAT is determined by the phase data in your .frd files.
        About all Z-phase does is affects how your combined (filtered) driver load presents itself to an amp.
        If your measured data looks "reasonable" an octave (or 2) on either side of your XO point, I'd still use it for XO design.
        There'd be nothing wrong with just using your jig to just obtain T/S parms though, and going w/mfr. Z-plots for your .zma design files.


        • #5
          Thanks for the explanation, Chris! Good to know that the self-measured .zma files aren't really crucial to the overall design.

          I'm considering to experiment by measuring some non-inductive resistors and comparing impedance phase results. Maybe it'll be possible to build a script that offsets the inherent error of the Berhinger for driver measurements I take.