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Midwest Audio Fest

It’s that time audio enthusiasts! Registration for the 2019 Speaker Design Competition is now open! Visit midwestaudiofest.com for details and to list your speaker project. We are excited to see all returning participants, and look forward to meeting some new designers this year, as well! Be sure your plans include a visit to the Parts Express Tent Sale for the lowest prices of the year, and the Audio Swap Meet where you can buy and trade with other audio fans. We hope to see you this summer! Vivian and Jill
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Dual-Woofer Measurements with Omnimic

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  • Dual-Woofer Measurements with Omnimic

    I'm starting work on a W-M/T-W center channel (woofers in parallel) and was curious as to how the woofers should be measured. I see a few potential options:
    1. Measure both woofers separately, add both responses to PCD
    2. Measure 1 woofer only (without a connection to the second) and let PCD assume the response and impedance are the same

    Should the woofers ever be wired in parallel and measured as one for the measurements? I assume this would only work farfield. Of course they have to be when its time to wire the crossover or else things just won't work as designed.

  • #2
    You'll need two Omnimics to measure two woofers there's no way around it!

    Comment


    • #3
      Just bustin' your balls Adam!

      You have an array that you are measuring.

      Are you set up for portable?

      Best way....

      Go to a Parking Lot.
      Stand your cabinet up on the ground.
      Place your mic about listening height (ear height when your sitting) and intended listening distance distance from the speaker.
      Throw some fiberglass or moving blankets on the ground between the speaker and mic covering about 7' x 7' area or so.
      Measure with both drivers connected parallel.

      Comment


      • #4
        If I only knew... That's why I can have 5 mic's in the omnimic software! Glad there's fun to be had here.

        Unfortunately, not set up for mobile, so I'll have to make do indoors. For the 2-way I just finished, I simply followed Jeff's guide for indoor measurements - but not sure how best to apply it here.

        Comment


        • #5
          Multi-woofer vertical arrays help mitigate floor/ceiling reflections and have a specific relationship with the floor.

          The above measurement method takes this into account.

          Also, very large vertical array's sum together differently at different listening distances as do other speakers except that a large speaker could still be in a transitional state at their intended listening distance and so you may want to actually perform some or all of your measurements at the intended listening distance as detailed above.

          Your mid and tweeter measurement could be done at a closer distance and referred to the distance of your woofer measurements by scaling their amplitude and baffle to mic distance appropriately.

          A large line source (not what you're building) I would want to measure entirely at the intended listening distance or the middle of it's intended range of listening distances.

          Comment


          • #6
            You can do this inside too Adam,

            Just use the biggest room you have, get in the middle, move everything out of the way (no reflective surfaces!) and set your gating for the nearest surface not including the floor (you will be including floor effects in your woofer measurements, the coverings you place on the floor between the speaker and mic should reduce floor interaction at the high end of your woofer measurements).

            For the midrange and tweeter you can lift up the speaker to get the midrange and tweeter as far as possible from ALL room surfaces and measure from as close a distance as possible for the baffle edge diffraction and drivers to still sum close enough to their infinite distance sum for your purposes.

            You know a long time ago I worked at a large hotel and took my equipment into one of the ballrooms in the middle of the night.

            Was able to use nice long gate times and measure low!

            BTW a battery and inverter will allow you to make measurements outside.

            You could even use your car for the battery.

            Run the motor and shut it off during measurements.

            Comment


            • #7
              Just assume the two woofers are one when they are equidistant from you measuring axis, usually the tweeter axis. In PCD use the single woofer, the Y (well no for a horizontal CC, X=0) offset is zero. This is the easiest when the woofers share the same cabinet.

              Wire them in parallel if that’s the final wiring and measure. Use the tweeter to set your SPL level. Measure each driver, do not move the mic. Then measure TW plus Mid and TW plus Woofer pair, don’t move the mic. With these measurements you can determine the offsets.

              I then move the mic and change the volume and take a near field measurements of the mid and one woofer. Use the Response Bender program to add the near field to the far field then extract min phase.
              Measure the woofer pair in box for your zma.

              Hope this helps. If this was a center channel and off axis was critical we would derive the impedance file for a single driver and adjust the spl of the driver pair for a single.
              Last edited by jhollander; 12-28-2018, 08:03 PM. Reason: spelling
              John H

              Synergy Horn, SLS-85, BMR-3L, Mini-TL, BR-2, Titan OB, B452, Udique, Vultus, Latus1, Seriatim, Aperivox,Pencil Tower

              Comment


              • #8
                Duh...

                Just reread this is a center channel. Here are a few more comments. I like to measure the center channel where its intended to be used. This will get the diffraction correct for the location. It's OK to use the above method with the summation point of the two woofers as a single woofer, but the off axis sim in PCD will not work correctly.

                If you want to see the comb filtering effect use the same measurements as above then in the Response Blender decrement the woofer pair measurement by 3.01 dB, then extract min phase. Now all you need is the single woofer impedance file in half the volume. I use the old Response Modeler to create the impedance file for the single woofer in half the volume with the same tuning.

                Let me know if this makes sense. I may have assumed a few items along the way...
                John H

                Synergy Horn, SLS-85, BMR-3L, Mini-TL, BR-2, Titan OB, B452, Udique, Vultus, Latus1, Seriatim, Aperivox,Pencil Tower

                Comment


                • #9
                  Originally posted by jhollander View Post
                  Duh...

                  Just reread this is a center channel.
                  Sheesh...…

                  I got the ball and took off in the wrong direction! (like Adam Sandler)

                  Disregard what I said!

                  I thought we were talking a large vertical WMTW.

                  Just measure according to Jeff or jhollander's instructions with the woofers connected in parallel.
                  Last edited by daryl; 12-28-2018, 11:12 PM.

                  Comment


                  • #10
                    Thanks for all of the information - your point about measuring in the intended location is well made, but not directly applicable in this case. I'm actually building 2 of them, for 2 separate locations with rather different surrounding furniture and video equipment - so I was planning to design with full BSC (the front baffle in both cases will be 2+ feet from the back wall) although one will have a TV to cause "baffle step" where the second will be used in a front projection room and behave normally.

                    Is it critical to model the comb filtering? The baffle is built and I've been fairly careful to design the baffle so that the woofer CTC spacing and the M-T spacing are less than 1 wavelength apart at their respective XO frequencies. I modeled with manufacturer data and off axis looked really good until the null showed up at around 45 degrees, which is plenty far enough off axis for these rooms.

                    If we're considering the woofers as a single unit, does that then change the definition for farfield measurements i.e. does the far-field measurement need to be at ~3x+ the total radiating dimension, or approximately 54" (~18" from the far radiating ends of the woofers) or would a more typical ~24" work, since each woofer independently will be radiating into the far-field at that point?

                    A photo of the baffle, with integrated grill but without the magnets and cloth is attached.

                    Comment


                    • #11
                      Originally posted by Adam_M View Post

                      Is it critical to model the comb filtering?

                      If we're considering the woofers as a single unit, does that then change the definition for farfield measurements i.e. does the far-field measurement need to be at ~3x+ the total radiating dimension, or approximately 54" (~18" from the far radiating ends of the woofers) or would a more typical ~24" work, since each woofer independently will be radiating into the far-field at that point?

                      A photo of the baffle, with integrated grill but without the magnets and cloth is attached.
                      No, you don't need to model. You can model with dummy drivers in PCD if you want to test your x-o off axis. However, I would also measure off axis with either the raw drivers and/ or completed x-o. I want to avoid correcting a FR dip that fills in off axis.

                      With Omni mic I'd start with 2x, 1 meter, and move the mic back to see if the peaking around 1,000 Hz changes. I think that 2x would be OK.
                      John H

                      Synergy Horn, SLS-85, BMR-3L, Mini-TL, BR-2, Titan OB, B452, Udique, Vultus, Latus1, Seriatim, Aperivox,Pencil Tower

                      Comment


                      • #12
                        I'm on to the crossover design of this speaker and would welcome suggestions - it seems like a lot of components. I have a couple of specific questions although general simplification suggestion would be great:

                        Woofers (2x W18E001): I can take the 2nd inductor out and adjust the other components, but it causes problems with the M-T crossover because the W-M side of the bandpass crossover needs tweaked. I guess that's a comment as much as a question, but it seems strange to have 3rd order electrical for this. I was also considering steel laminate on the 1mH for the low DCR and slight efficiency bump. Any issues with that?

                        Mid (MDM55): I have the padding resistor in front of the rest of the XO. I don't see that done often. Is there a reason to not do that, other than power handling? Do I need to double up that resistor (either 2 at .5 value in series or 2 at 2x value in parallel). A traditional l-pad ends up with strange peaking in the response.

                        Tweeter (OW2): Similar question as the mid regarding the RC in front of the XO. Another topology I don't see much. That cap lifts the top end though. Also, there's a bit of a diffraction peak at 7.5k. I'm going to put some felt on the grill frame - Its probably in the 1.75" from the tweeter range to tame it. DO you think the peak as it exists will be audible?


                        Comment


                        • #13
                          Originally posted by Adam_M View Post
                          Woofers (2x W18E001): I can take the 2nd inductor out and adjust the other components, but it causes problems with the M-T crossover because the W-M side of the bandpass crossover needs tweaked. I guess that's a comment as much as a question, but it seems strange to have 3rd order electrical for this. I was also considering steel laminate on the 1mH for the low DCR and slight efficiency bump. Any issues with that?
                          If you take out the second inductor on the woofer then lower the order on the mid to keep the phase match. I don’t mind a few lumps below 1,000 Hz as it’s harder to hear them and I like a dip in the 500 to 700 Hz. range. I also like using the steel laminates down to 1 mH, but I might trade that big cap for a larger inductor, kind of depends what your impedance looks like.

                          Originally posted by Adam_M View Post
                          Mid (MDM55): I have the padding resistor in front of the rest of the XO. I don't see that done often. Is there a reason to not do that, other than power handling? Do I need to double up that resistor (either 2 at .5 value in series or 2 at 2x value in parallel). A traditional l-pad ends up with strange peaking in the response.
                          For the mid, now that you have your x-o in PCD, I like to move over to XSim to optimize. You can see the power handling by component. I like to double up or use a higher wattage resistor. You can move the resistor to the other side of the low pass or high pass and get similar results. This is much easier in XSim.

                          Originally posted by Adam_M View Post
                          Tweeter (OW2): Similar question as the mid regarding the RC in front of the XO. Another topology I don't see much. That cap lifts the top end though. Also, there's a bit of a diffraction peak at 7.5k. I'm going to put some felt on the grill frame - Its probably in the 1.75" from the tweeter range to tame it. DO you think the peak as it exists will be audible?
                          For the tweeter, you can move the RC to the other side of the x-o and adjust (XSim). The RC could be a waste unless you get at least a 1-2 dB lift at 10K or lower. The peak at 7.5K might go away off axis, did you happen to look? This would be a something that I think would be audible when voicing.

                          If you want to post your files you might get more ideas (you would need to filter the good from the bad). IMO keep tinkering and buy enough parts to test a few ideas.
                          Last edited by jhollander; 01-04-2019, 10:58 AM. Reason: spelling
                          John H

                          Synergy Horn, SLS-85, BMR-3L, Mini-TL, BR-2, Titan OB, B452, Udique, Vultus, Latus1, Seriatim, Aperivox,Pencil Tower

                          Comment


                          • #14
                            Cool, thanks! I fired up Xsim, and was able to simplify the XO a bit. Got rid of the 3rd order inductor and shrunk the big cap in the LP and got a more conventional L-pad in the mid. The HP RC has almost exactly 2dB of lift at 10K, decreasing below that but certainly present. I had to take the measurement rig down for Christmas, so it'll have to go back up to check the peak at 7.5k - hopefully this weekend.


                            Regarding the power measurements, I'll probably be using 12W mills (I have most of the values I need laying around). When simulating power dissipation in Xsim, the series resistor in the filter mid hit 38ish watts of at 20v at about 2.4k. I'm of course not going to be feeding a 2.4k sine wave to this thing at 20v, so I'm not sure how to interpret that information. Is 1 12w mills OK in that application or should I double it up?

                            Also, how do you get the delays set correctly in Xsim? I tried the z offset from PCD directly as well as the hypotenuse of the triangle formed by the XYZ positions of the drivers as the offset, and neither summed as they do in PCD.

                            Comment


                            • #15
                              Also, is this impedance particularly nasty? I'm thinking the phase angle briefly exceeding +-30 degrees is ok, but not optimal. This is obviously a 4 ohm speaker, but is that dip to under 3 in the upper midrange problematic? I'd expect the dip to about 3 in the lower midrange would be more problematic, but I don't think there's anything to be done with that - it kind of is what it is when paralleling those 2 woofers.

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