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Speaker Design Question #2 - Measuring Driver Offset (z-axis) - WinPCP

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  • #46
    Originally posted by dkalsi View Post
    All that being said, "z-offset" couldn't possibly have multiple values could it?
    Yes. The offset is a function of delay to the "apparent" acoustic center. That "apparent" center changes if the minimum-phase changes, which it will if a tail is changed in the SPL. SPL and phase are mutually dependent.

    Theoretically, there should be only one exact value for z-offset right?
    Yes and no. For any given phase response of two files, there will be one. The no is because offset, as mentioned above, is dependent on the phase of both files. If one changes, the offset will change, a little or a lot. That's why I so often say to use the same files for design or at least use the same tail if you post-process the files additionally, such as adding a different box from software (such as Unibox) or if you were to average tweeter measurements for a windowed response. Few do the latter AFAIK.

    If that is the case, then I know I just need to learn more about setting "tails".
    The key is to be consistent. You can set tails in a random manner if you wish as long as you use the same tails in any later post-processing. Keep in mind that the phase change for a box spliced into a woofer or possibly even a midrange won't have a huge affect on the offset. It's the lowpass (driver top end tail) that has the biggest effect.

    I was also hoping you (or others) can chime in and possibly explain why my simulated response doesn't match the measured response. If I have measured the value of each individual crossover component, if I have specified the measurement axis to be the same exact point where the measured response (inclusive of crossover) was taken, if I have specified the correct crossover layout, shouldn't simulated response = measured response? I only seem to get my simulated response to be close to the measured reference response (@ 0.775meters) when I adjust the measurement distance to 2 meters.
    If the procedure for measurements and the offset are done correctly and the measurement system provides consistent results, the correlation should be very close.

    dlr
    WinPCD - Windows .NET Passive Crossover Designer

    Dave's Speaker Pages

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    • #47
      Thanks Dlr - your above post helps tremendously.

      In essence, the acoustic offset isn't necessarily the measured (like with a ruler) the delta between the radiating point of the two drivers mounted on a vertical plane, it is instead the acoustic delay embedded in the frd files created. Is that correct?

      So long as I then use the same files are utilized in my crossover design (while specifying the corresponding z-offset related to those files), then the simulated response should be very close?

      __________________________________________________ ____________________
      EDIT:


      Dlr - I apologize. I just went back to your earlier post and noted that you have already covered this - I feel like such an idiot :-( Didn't mean to exhaust your effort explaining this to me over and over. For those that are curious, I am alluding to the following explanation previously provided by dlr in an earlier post:

      "Remember that offset is relative, that is, it's a value required so that the two minimum-phase files sum to the measured sum. If you change either file, the relative offset needs to be updated to account for the change.

      The absolute value of the offset isn't really important. All it amounts to is the difference between the two measurements that "aligns" the time delays between the measurements to correspond to the difference required for the generated phase of each driver file. There is no "correct" offset, only the offset needed for the measurement phase as generated for the SPL of the files."
      __________________________________________________ _____________________

      I think I'm now past asking the same question over and over about minimum phase :-)

      My next step is to figure out why my simulated response only closely matches the measured response, which was taken at .775meter, when I set the "Measurement Distance" in System tab to 2meters. I figured that if I have specified the baffle layout (i..e, the x,y,z coordinates of the driver) as well as provided the radiating diameter of the drivers, the program should be able to approximate how the speaker sound perform at 0.775 meters.

      It will be helpful to understand what may be causing that variance.

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      • #48
        A good example under time alignment.

        http://www.sbacoustics.com/index.php/technical-notes/

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