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  • Impedance in and out of box

    Hi,



    A question on the impedance plot of a driver in-box compared to the raw driver



    Attached is a picture of the impedance plots (DATS, 18h break in) of a new Dayton RS270-8 woofer in a 21 L CB and the same driver ”speciment” raw out of box.



    I noticed that the plot of the raw driver, laying flat on the table is very smooth and totally according to the textbook but the plot of the same driver mounted in a 21L closed box, quite heavily damped, shows a more irregular pattern. The same phenomena can be noted for the ”other woofer”



    What is your guess? Can this be explained by the fact that the driver is turned 90 degrees? Is this a ”box-issue” that can be fixed with internal bracing? Should this be ignored?

    Regards//lasse


    Click image for larger version

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    Perry Mason talking to his dentist:

    "Do you swear to take the tooth, the whole tooth and nothing but the tooth, so help you God?"

  • #2
    I thought wigglies like that, especially when absent in the free air measurements, meant box resonances. That's my uneducated guess.
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    • #3
      A leak, or box resonances are my guesses.
      Francis

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      • #4
        Those don't look like any DATS plots I'VE ever seen? Shouldn't see those things w/a change in strictly driver orientation. Could anything be loose, or "buzzing"? 30Hz is pretty low for something like that. Loose phase plug possibly? Did the free-air T/S parms measure okay? Nothing else loose: spider, surround, flapping v.c. leads perhaps?

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        • #5
          Maybe your box stuffing is hitting the back of the cone?
          Francis

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          • #6
            Have you tried using SpeakerWorkshop to do the impedance sweep to compare to the DATS sweep?

            I can't remember the sample rate / size settings, should be in the unofficial manual. From memory, a lower sample rate (~ 48K) and higher sample size (needed for low frequency accuracy) of ~ 131k will work best. you can increase the sample size to 262k - will take longer. Don't forget to set for at least 5 samples for accuracy.

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            • #7
              Can you post pictures of the box internals? What size woofer? Have you chamfered the back if a smaller (< 6") woofer to allow it to breath? What is the nearest surface / obstruction in the box? (a brace?)

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              • #8
                Hi again,



                OK, this is DATS-info in Speaker Workshop so that two curves could be shown at the same time.



                My guess is that this is related to the box or the not-glued back of the bow. The raw drivers behave normally, so I don´t suspect any malfunction in the actual drivers. The baffles are glued in place over a large portion of the surface (see the pictures) so they should be rigidly mounted but there is no bracing in the boxes apart from corner rods as can be seen.



                Do you think that this will be a real problem in terms of sound Quality issues??



                Best regards//lasse


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                Perry Mason talking to his dentist:

                "Do you swear to take the tooth, the whole tooth and nothing but the tooth, so help you God?"

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                • #9
                  Could a clue be that the "disturbance" is on the same frequency as the free-air resonant fq of the driver??

                  //lasse
                  Perry Mason talking to his dentist:

                  "Do you swear to take the tooth, the whole tooth and nothing but the tooth, so help you God?"

                  Comment


                  • #10
                    Since you have a removable back, re-do the measurement "open back" and see if it changes. Will help determine if a back wave or cabinet resonance issue. If the problem is still apparent, then I'm guessing it will be to do with driver mounting on the baffle (vibration?) if the problem disappears with the back off - try putting it back on with first an additional side brace (easy to install) then maybe a rib vertically on the removable back - to break up any resonance

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                    • #11
                      Actually - a better option might be just to use some clamps from the outside to damp any resonance (side to side, and front to back.

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                      • #12
                        Have you tried measuring the impedance with the stuffing removed?

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                        • #13
                          Originally posted by lasse View Post
                          Could a clue be that the "disturbance" is on the same frequency as the free-air resonant fq of the driver??

                          //lasse
                          Hi,

                          my "guts" tell me that no panel in my boxes has a resonant frequency as low as of abt 30 Hz in any vibration mode.

                          I have searched the Internet for a paper och textbook that can shed som light on this but I haven´t found any that I understand (to be honest).

                          Has anyone of you any knowlede in theory for natural vibration modes of plates with different support of the edges??

                          regards//lasse
                          Perry Mason talking to his dentist:

                          "Do you swear to take the tooth, the whole tooth and nothing but the tooth, so help you God?"

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                          • #14
                            Hi,

                            Problem (or at least reason for it) solved.




                            The “truth” sometimes comes to me in my sleep and I think that it did this time. It struck me that when I mounted the three-layer baffle and the heavy driver in it, the speaker tended to be a little “front-heavy”. Being a naval architect, used to worry about stability and center-of-gravity, I wondered about this and considered the possibility of this meaning trouble in the future. I settled this line of thought by placing (on the drawing) the three pounds 8,2 mH woofer inductor in the back of the cabinet.


                            Now, this morning I decided to repeat the test conditions for the driver from the driver-only measurement described earlier in this thread. I put the speaker on its back on the kitchen workbench, Italian marble, 200 lbs, and measured the impedance. The result can be seen in the attached picture.


                            Now the real question remains. Is this 30 Hz “solid-body-vibration” a real problem and if so, what to do about it?


                            I will NOT drive spikes down into my wooden floor!



                            Regards//lasse


                            Click image for larger version

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                            Perry Mason talking to his dentist:

                            "Do you swear to take the tooth, the whole tooth and nothing but the tooth, so help you God?"

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                            • #15
                              Originally posted by lasse View Post
                              ... what to do about it?...
                              Decoupling?

                              "Not a Speaker Designer - Not even on the Internet"
                              “Pride is your greatest enemy, humility is your greatest friend.”
                              "If the freedom of speech is taken away, then dumb and silent we may be led, like sheep to the slaughter."

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