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  • #61
    Originally posted by gregrueff View Post
    I spent a lot of time today trying to diagnose the dip/peak combo around 7 kHz seen in my tweeter's response.
    I tried applying felt wool under the grille to fill gaps, in between the tweeter and the midrange, on the grille waveguide portion, etc. All over the darn place wherever I could think.
    No matter where I put the wool felt, I was unable to affect the anomaly.

    So I took another set of measurements with the bare baffle, with grille attached but the cloth removed, and with the cloth in place.

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    It almost looks to me like there exists an inherent 1 dB dip/peak at 6-7 kHz that is shifted in frequency and amplified with both the application of the grille and cloth.
    With the application of the grille, it boosts to about a 2 dB anomaly, spread out a bit more and little bit higher in frequency.
    Then the cloth furthers this trend making it a full 4 dB dip/peak.

    I'm struggling to understand this. I'm thinking maybe this is just a product of using grille cloth.

    I'm also wondering if this has to do with the total recessed depth of the tweeter relative to the apparent front of the speaker.
    In OEM condition, the tweeter dome sits recessed to the faceplate with a round-over acting as a mild horn/waveguide. Applying the grille makes this more exaggerated.
    Assuming I haven't made a silly math error, 7 kHz is close to 1.5" which is also pretty close to the chord length from the dome tip to the front facet of the speaker.
    I would be tempted to try a different shape contour on the grill. Something less symmetrical. Small dimensions affect higher frequencies.Maybe shift one side of the pattern an inch to the side, or rethink the pattern entirely. Your grill is reflecting sound waves. My efforts would be to absorb them. Dense fiberglass, like that used in ceiling tiles might be a good option. You can cut it on an angle if desired too.

    Comment


    • #62
      Originally posted by Chris Roemer View Post
      I get closer to 1.9"?
      Haha, yes of course. Thanks Chris.
      Too many numbers on the brain I guess.

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      • #63
        Originally posted by rpb View Post

        I would be tempted to try a different shape contour on the grill. Something less symmetrical. Small dimensions affect higher frequencies.Maybe shift one side of the pattern an inch to the side, or rethink the pattern entirely. Your grill is reflecting sound waves. My efforts would be to absorb them. Dense fiberglass, like that used in ceiling tiles might be a good option. You can cut it on an angle if desired too.
        Not a bad idea.
        I have some old ceiling tiles lying around actually.

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        • #64
          Originally posted by gregrueff View Post

          Not a bad idea.
          I have some old ceiling tiles lying around actually.
          Maybe alter that contour shown in the picture above, that just happens to be at a suspect distance.

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          • #65
            I think I've also come to a good place on the crossover where I can start listening to music and adjusting baffle step as necessary, etc.
            I was having great difficulty getting my PCD models to match my measurements, but it turns out I was just being sloppy.

            The discrepancy I was having with the woofer is because I was measuring nearfield and forgetting that I then wasn't taking into account baffle step.

            So here is Rev11 below:
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            The part I really wanted to get right was the phase integration between the midrange and the tweeter.
            The acoustic center difference between them is 0.138" which tilts the tweeter lobe down.
            To align them you either have to shift the tweeter's crossover down in frequency, or shift the midrange's up.
            I was concerned with shifting the tweeter's down since it really boosts the impedance hump and has the potential to strain the tweeter, so I relaxed the midrange slope up to compensate.
            The PCD model is now predicting that the lobe is darn near perfectly straight, and my measurements are close enough to simulation that I believe them:

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            • #66
              A quick HD sweep seems to confirm that the transition from midrange to tweeter is not overly strained, at least at these levels taken at approximately 1m.

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              • #67
                You aren't showing the phase plots, Greg. Reverse Null is good, but phase plots would be better.
                Wolf
                "Wolf, you shall now be known as "King of the Zip ties." -Pete00t
                "Wolf and speakers equivalent to Picasso and 'Blue'" -dantheman
                "He is a true ambassador for this forum and speaker DIY in general." -Ed Froste
                "We're all in this together, so keep your stick on the ice!" - Red Green aka Steve Smith

                *InDIYana event website*

                Photobucket pages:
                https://app.photobucket.com/u/wolf_teeth_speaker

                My blog/writeups/thoughts here at PE:
                http://techtalk.parts-express.com/blog.php?u=4102

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                • #68
                  Originally posted by Wolf View Post
                  You aren't showing the phase plots, Greg. Reverse Null is good, but phase plots would be better.
                  Wolf
                  I'm being dense, but do you mean this one:
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                  Or this one:
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                  • #69
                    I think Ben means phase on the drivers.

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                    • #70
                      Oh okay.
                      This is a new analysis technique to me, so sorry if I'm not understanding.

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                      I couldn't figure out a way to overlay the two measurements in the OmniMic program and show the phase of both, so I guess we just have to compare the two side-by-side like this?

                      Am I understanding you guys correctly?

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                      • #71
                        If you are still set up to measure, I'd check off axis to see if anything odd shows up. Also, you can turn the speaker sideways for measurements, and measure above and below (ie vertical stuff), and you should see some reverse nulls there too at xo frequencies. They usually show up around 45 degrees or so. No need to post them, but it's a good confirmation that things are as expected. Then, if you hear something you question, you will have more measurements to look back at for possible causes.

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                        • #72
                          I meant as in #2. Thanks.
                          Wolf
                          "Wolf, you shall now be known as "King of the Zip ties." -Pete00t
                          "Wolf and speakers equivalent to Picasso and 'Blue'" -dantheman
                          "He is a true ambassador for this forum and speaker DIY in general." -Ed Froste
                          "We're all in this together, so keep your stick on the ice!" - Red Green aka Steve Smith

                          *InDIYana event website*

                          Photobucket pages:
                          https://app.photobucket.com/u/wolf_teeth_speaker

                          My blog/writeups/thoughts here at PE:
                          http://techtalk.parts-express.com/blog.php?u=4102

                          Comment


                          • #73
                            Originally posted by rpb View Post
                            If you are still set up to measure, I'd check off axis to see if anything odd shows up. Also, you can turn the speaker sideways for measurements, and measure above and below (ie vertical stuff), and you should see some reverse nulls there too at xo frequencies.
                            Excellent suggestion on the measurement technique!

                            It's way easier to reposition the cabinet on the stand than move the mic.

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                            This is a comparison of 45 deg off-axis down versus 45 deg. off-axis up.
                            These off-axis angles were chosen because a pretty deep null manifests in both directions that is easy to identify and track.
                            The fact that these nulls are approximately at the same magnitude, but also shifted in frequency by 500 Hz is consistent with a model that has the acoustic origin of the drivers off by about 3.5mm. (i.e. I can replicate this exact behavior in PCD by playing with the Z Offset)
                            This sounds like a lot, but that only equates to a 3.16 deg. downward tilt on the vertical lobe (which is what is actually happening).

                            This is very interesting!
                            Because while I still consider the phase tracking excellent, it can be improved.
                            If I modify the tweeter cap from 3.9 uF to 3.3 uF, both the reverse null deepens and the +45 deg & - 45 deg nulls converge in frequency.

                            I feel like I'm close to splitting hairs at this point though. I can almost shift an entire 1.0 degree just by playing with component tolerance.

                            Good stuff.

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                            • #74
                              Just found this post by accident and it looks quite fascinating. I've always had an interest in the exotic and was looking around for something to inspire me on my latest project. I have Zaph's Bargain Mini that I was considering turning into a 3-way using this same DA215 woofer. With the Bargain Mini, it has a 4" woofer and the tiny Aura NT1 tweeter. I am really curious about the baffle/waveguide.....wondering if I should build it out of wood like you did or cut it out of felt or even foam. You did this really cool design, but I see companies like Wilson using felt with a star pattern specifically to breakup cancellations and help with off-axis behavior.

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                              • #75
                                Originally posted by shudson105 View Post
                                Just found this post by accident and it looks quite fascinating. I've always had an interest in the exotic and was looking around for something to inspire me on my latest project. I have Zaph's Bargain Mini that I was considering turning into a 3-way using this same DA215 woofer. With the Bargain Mini, it has a 4" woofer and the tiny Aura NT1 tweeter. I am really curious about the baffle/waveguide.....wondering if I should build it out of wood like you did or cut it out of felt or even foam. You did this really cool design, but I see companies like Wilson using felt with a star pattern specifically to breakup cancellations and help with off-axis behavior.
                                Hey cheers, and welcome to the party!
                                The Duelund concept looks exotic, and it really thrives for math geeks who like perfectly solved systems (and I'm one of those guys who couldn't for the life of me replicate or produce the polynomials, but that's a different issue..) but in practice just resembles a second-order-acoustic-slope speaker with a lot of overlap between drivers.

                                Zaph's work taught me a lot and the way he was willing to post to teach others was admirable (and appreciated), but he really trended towards 4th order slopes as in that Bargain Mini. The driver in that system has some decent break-up going on starting at 7 kHz, so you can see why. You might be hard pressed to mold that driver into this concept, but you can still add the bottom end of the DA215 to it if you wanted. Inevitably, someone will advise you that's a risky proposition without a decent plan though.

                                I will say though, I'm a bit conflicted on the ideal choice for the midrange driver in this type of system.
                                On the one hand, a 3" with a smaller diameter plays well with the upper integration with the tweeter and the vertical lobe.
                                But a 4" plays so much better down low and allows for a lower crossover point for approximately equal HD. The MCM 55-1853 in your Bargain Mini system likely has the leg up there.

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