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

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

    First and foremost, thanks to all that responded to my BSC question.

    My next question is regarding measuring driver offset. The speaker I'm currently practicing with is Bamberg Audio 2TM (SS-Kit), ,which allows for bi-wiring.

    My setup = Room EQ Wizard - out to Focusrite 2i2 - out to Trends 10.1 T-amp - Left Channel connected to Tweeter-side crossover terminals, and Right Channel connected to Woofer-side crossover terminals
    Input = MiniDSP UMIK

    These are the procedures I followed:

    1) Took a full-range measurement of the speaker - On tweeter axis - Distance of 20.25" - 5.25ms Gating

    2) Took Woofer only measurements - Still on Tweeter Axis - Distance of 20.25" from speakers baffle - 5.25ms Gating

    3) Took Tweeter only measurements - On Tweeter Axis - Distance of 20.25" from speaker baffle - 5.25ms Gating

    4) Took all measurements and converted in "FRD" files (which contained frequency, amplitude, and phase data)

    5) Uploaded the files into "Acoustic Offset tab of WinPCD

    6) Set measurements distance to 0.514 meters (because 20.25" = 0.514 meters)

    7) Set "Driver 1" (aka the Tweeter) - X-offset = 0, and Y offset = 0 (because tweeter was used as the reference point)

    8) Set "Driver 2" (aka the Woofer) - X offset = 0, and Y offset = -0.1524 (because the woofer is mounted 6" = 0.1524 meters below tweeter)

    9) I believe the next step is to adjust Driver 2 (the Woofer) Z-offset until the mathematical summed response of Step 2 and Step 3 above match the measurement in Step 1 above.

    Based on these steps, I calculated the Woofer Z-offset to equal -.020m or negative 20mm?

    I'm not sure I'm confident in that result. I believe the measurements should have been closer to 40mm +/-.

    The tweeter is ScanSpeak D2604/8300 and the woofer is ScanSpeak 18W/8434

    Any thoughts on what I might be doing wrong? Some of my questions:

    a) Should gating not be used?

    b) Do the tweeter/woofer measurements represent minimum phase measurements? Or do I need to use one of Jeff Bagby's spreadsheet to derive minimum phase?

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  • #2
    20mm isn't that unrealistic an offset for a 7" woofer and 1" dome tweeter flush mounted on a baffle.

    But to use the WinPCD offset calculator, the FRD files need to be minimum phase. You can generate the minimum phase versions of the files by using Jeff's Response Modeler spreadsheet.
    R = h/(2*pi*m*c) and don't you forget it! || Periodic Table as redrawn by Marshall Freerks and Ignatius Schumacher || King Crimson Radio

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    • #3
      Thanks for responding Pete. Just to be clear, is it okay for me to take tweeter and woofer measurements with the crossover components in place, or do there measurement need to take place without any crossover components in place? Sent from my iPhone using Tapatalk

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      • #4
        Also, when using Jeff B's program to generate minimum phase, is it okay that measurements were taken with 5.25ms gating - that would result in the response below 200hz being useless. Still way below my 1800/1900 crossover point. Sent from my iPhone using Tapatalk

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        • #5
          There can't be any crossover parts in play when taking measurements. So you need three basic frequency response measurements (never moving the mic position or changing amp volume):

          - raw tweeter FR
          - raw woofer FR
          - raw tweeter and raw woofer electrically connected in parallel FR

          Then you need to extract minimum phase from the raw tweeter FR and extract minimum phase from the raw woofer FR. You leave the raw Tweeter + Woofer FR raw (no extraction). I hope that makes sense.
          Craig

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          • #6
            Your gating and measurement distance are fine.
            Craig

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            • #7
              Originally posted by PWR RYD View Post
              There can't be any crossover parts in play when taking measurements. So you need three basic frequency response measurements (never moving the mic position or changing amp volume):

              - raw tweeter FR
              - raw woofer FR
              - raw tweeter and raw woofer electrically connected in parallel FR

              Then you need to extract minimum phase from the raw tweeter FR and extract minimum phase from the raw woofer FR. You leave the raw Tweeter + Woofer FR raw (no extraction). I hope that makes sense.

              This is very helpful. Right now, the speaker has the full crossover designed and built by Mr. Bamberg himself. I will try to take measurements without any crossovers. What threw me off was the following excerpt from Vance Dickanson's Loudspeaker Recepies - Book 1- See below:
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              Quick question(s): At the very least, do I need to install a series capacitor for the tweeter (to protect it from lower frequencies) - or do you suggest taking measurements from say 500hz up instead? Or can I really take a full range measurement of the tweeter without any protection?

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              • #8
                It really doesn't matter if you have a filter in place or not for determining acoustic offset.

                It should be standard practice to use a cap when measuring tweeters, even though most will be fine for a low volume frequency response measurement. For high volume >1W such as harmonic distortion testing, a cap is a must.
                Don't waste your money on a new set of speakers, you get more mileage from a cheap pair of sneakers. Next phase, new wave, dance craze, anyways it's still rock and roll to me!

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                • #9
                  Also: REW can generate minimum phase. Click on the "Controls" icon when in the "SPL & Phase" window. Then click on the "Generate minimum phase" button.
                  "Everything is nothing without a high sound quality." (Sure Electronics)

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                  • #10
                    I just used Jeff B's Response Modeler to generate minimum phase. What is interesting is that now I can essentially reduce the z-axis offset to zero and get the mathematical summed response to align with measured full range response. I'm clearly doing something wrong. See below. The measurement from the front of the woofer basket to the middle point woofer's mangnet front plate was approximately 50mm. Same measurement for the tweeter is approximately 7mm or 8mm. Based on that, the z-offset should be approximately 42mm or 43mm.
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                    • #11
                      Please post your 3 measurement FRD files.

                      Btw, if you're trying to approximate acoustic offset, you should use the point where the voice coil meets the cone.
                      Don't waste your money on a new set of speakers, you get more mileage from a cheap pair of sneakers. Next phase, new wave, dance craze, anyways it's still rock and roll to me!

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                      • #12
                        Thanks for your help dcibel. Attached please find the three FRD files as generated by REW. If you open up the files utilizing Microsoft Notepade, you will find REW notes included at the very top. Some software requires these extra lines to be removed and the file re-saved.

                        Please note these were FRD files as generated by REW without any extra steps by me to derive minimum phase. If need be, you can PM me your e-mail and I can forward you the REW file.

                        Thanks again.
                        Attached Files

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                        • #13
                          With a DSP in use and for the tweeter, I use an inverse null measurement to determine time delay needed. If you can adjust delay per channel (using a miniDSP), I take inverse measurements to achieve the deepest null at the correct frequency. This works best with symmetrical crossovers and LR's since the lobe is forward. This measurement is the last one I take, to assure everything is good with all settings applied.

                          So setup DSP with crossovers configured with estimated delay. Invert one channel. Take gated measurement (between speakers more correct). Adjust delay up and down to achieve best null. Attached is a recent measurement showing inverted and not.

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                          • #14
                            Originally posted by dcibel View Post
                            Please post your 3 measurement FRD files.

                            Btw, if you're trying to approximate acoustic offset, you should use the point where the voice coil meets the cone.
                            Really? I always guesstimated to the woofer dust cap, figuring that as frequency goes up the radiation becomes more prominent there. I could have completely made that up in my own head though.
                            Electronics engineer, woofer enthusiast, and musician.
                            Wogg Music

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                            • #15
                              Originally posted by dkalsi View Post
                              Thanks for your help dcibel. Attached please find the three FRD files as generated by REW. If you open up the files utilizing Microsoft Notepade, you will find REW notes included at the very top. Some software requires these extra lines to be removed and the file re-saved.

                              Please note these were FRD files as generated by REW without any extra steps by me to derive minimum phase. If need be, you can PM me your e-mail and I can forward you the REW file.

                              Thanks again.
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                              Attached Files
                              Last edited by dcibel; 09-29-2017, 02:02 PM.
                              Don't waste your money on a new set of speakers, you get more mileage from a cheap pair of sneakers. Next phase, new wave, dance craze, anyways it's still rock and roll to me!

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