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  • MTM difraction effects - what is going on?

    This is a graph of the in box response of my OWII tweeter fronted by a 15uf cap to protect it. The speaker is an MTM with the tweeter 8.75" from the top of the baffle, and in the middle of the 8" baffle. This was measured at 1 meter on axis with the tweeter using a 5ms gate due to my small room. Note the area between ~4K and 10K. The OWII should have a 1.5db bump about 4K and then be very flat on out with a slight rise in the last half octave. I'd expect it to be very nearly flat from 4k at 91db in this graph, out to about 15K, then the rise.

    This rise of 3db at 4k and subsequent wobbles are not expected, and I think it is (but not sure) too high in frequency to be a baffle edge diffraction effect.

    My OWI's are quite flat through this range in the Malbecs.

    Perhaps this is much ado about nothing, but does anyone have any ideas about cause and how best to manage.


  • #2
    Re: MTM difraction effects - what is going on?

    It could be diffraction, there's no upper frequency limit to the effect of diffraction. Have you tried to model it? It could be diffracting off the woofer too.

    Try putting some thick felt around it and see if it goes away.
    Don't even try
    to sort out the lies
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    • #3
      Re: MTM difraction effects - what is going on?

      Originally posted by diy speaker guy View Post
      It could be diffraction, there's no upper frequency limit to the effect of diffraction. Have you tried to model it? It could be diffracting off the woofer too.

      Try putting some thick felt around it and see if it goes away.
      It is not reflected in Response Modeler. As you say, it could be the woofer surrounds. I've never been able to get along with Paul Verdone's program, but perhaps I'll try that.

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      • #4
        Re: MTM difraction effects - what is going on?

        Originally posted by maynardg View Post
        This is a graph of the in box response of my OWII tweeter fronted by a 15uf cap to protect it. The speaker is an MTM with the tweeter 8.75" from the top of the baffle, and in the middle of the 8" baffle. This was measured at 1 meter on axis with the tweeter using a 5ms gate due to my small room. Note the area between ~4K and 10K. The OWII should have a 1.5db bump about 4K and then be very flat on out with a slight rise in the last half octave. I'd expect it to be very nearly flat from 4k at 91db in this graph, out to about 15K, then the rise.

        This rise of 3db at 4k and subsequent wobbles are not expected, and I think it is (but not sure) too high in frequency to be a baffle edge diffraction effect.

        My OWI's are quite flat through this range in the Malbecs.

        Perhaps this is much ado about nothing, but does anyone have any ideas about cause and how best to manage.

        The Hiqpuphons are very wide dispersion tweeters. You probably are seeing diffraction from the nearby drivers, twice as bad in an MTM of course. This happens with almost all tweeters, just more so for a Hiquphon. Regarding the higher frequencies, is the flush mount really close or is there any gap?

        Try making a measurement off-axis a bit to see how the response changes, maybe just 10 degrees.

        Here's my OW1 on a large baffle:


        Here it is on the offset baffle with extensive felt for diffraction control (you wouldn't want to see it without the felt):


        The blip on mine at 15K is probably the less-than-ideal flush mount.

        You should also look at the FFT with maybe a 3ms window or so and be sure to eliminate all reflections. You'll still see what's happening from 300Hz or so up.

        dlr
        WinPCD - Windows .NET Passive Crossover Designer

        Dave's Speaker Pages

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        • #5
          Re: MTM difraction effects - what is going on?

          I agree with Dave. At the higher frequency the issue is most likely diffraction, and at that frequency it has to be something very near the tweeter. The most obvious culprit would be the woofers, both surround and cone. As Dave said, the wide dispersion of the OWII makes it more prone to diffraction effects because more energy is being radiated along the baffle plane.
          Click here for Jeff Bagby's Loudspeaker Design Software

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          • #6
            Re: MTM difraction effects - what is going on?

            Originally posted by Jeff B. View Post
            I agree with Dave. At the higher frequency the issue is most likely diffraction, and at that frequency it has to be something very near the tweeter. The most obvious culprit would be the woofers, both surround and cone. As Dave said, the wide dispersion of the OWII makes it more prone to diffraction effects because more energy is being radiated along the baffle plane.
            Yep, same thing I saw in the Schumakubins, and it didn't need the Hiq to exhibit the issue. It was quite audible with a slow on-axis frequency sweep. It does change some as you move off axis though.

            Ah, the trade offs.

            Maynard. Try converting to a 2.5-way, with the tweeter on top.
            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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            • #7
              Re: MTM difraction effects - what is going on?

              Maynard. Try converting to a 2.5-way, with the tweeter on top.
              Not yet - maybe later

              1) That would place the woofers lower in the TL and require redesign
              2) I don't want to cut that blinking baffle again
              3) I won't like how that OWII looks on top with the double truncation
              4) Not yet happy with the bass response either.

              ?? Guidance on felt
              ?? What would happen if I recessed the woofers a la Continuum?

              Comment


              • #8
                Re: MTM difraction effects - what is going on?

                Originally posted by maynardg View Post
                Not yet - maybe later

                1) That would place the woofers lower in the TL and require redesign
                2) I don't want to cut that blinking baffle again
                3) I won't like how that OWII looks on top with the double truncation
                4) Not yet happy with the bass response either.

                ?? Guidance on felt
                ?? What would happen if I recessed the woofers a la Continuum?
                I wouldn't let it worry you too much. Most diffraction effects like this aren't real audible because they change on different axes, so they tend to average out a little. If a peak is there over a wide window, then tweak that region down a little with the crossover.
                Click here for Jeff Bagby's Loudspeaker Design Software

                Comment


                • #9
                  Re: MTM difraction effects - what is going on?

                  Originally posted by Jeff B. View Post
                  I wouldn't let it worry you too much. Most diffraction effects like this aren't real audible because they change on different axes, so they tend to average out a little. If a peak is there over a wide window, then tweak that region down a little with the crossover.
                  See what all this damnable measurement gets me. Actually, Jeff, this is audible - I noticed a bit of edge in the highs right away. A contrast with the OWI in the Malbecs makes it more easily heard. Will play with the XO a bit.

                  Generally, these speakers are really good. Need to reach for a bit more down low and address the edge in the highs.

                  Comment


                  • #10
                    Re: MTM difraction effects - what is going on?

                    Originally posted by maynardg View Post
                    Not yet - maybe later

                    1) That would place the woofers lower in the TL and require redesign
                    2) I don't want to cut that blinking baffle again
                    3) I won't like how that OWII looks on top with the double truncation
                    4) Not yet happy with the bass response either.

                    ?? Guidance on felt
                    ?? What would happen if I recessed the woofers a la Continuum?
                    Felt near the woofer surround in between the tweeter and surround. You need to make the woofer invisible to the tweeter, and the felt attempts to perform that task.

                    Recessing may help a bit, but you still have two large cavities right next to the tweeter, and that will have diffraction issues as well.
                    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
                    Byzantium Project & Build Thread || MiniByzy Build Thread || 3 x Peerless 850439 HDS 3-way || 8" 2-way - RS28A/B&C8BG51

                    95% of Climate Models Agree: The Observations Must be Wrong
                    "Gravitational systems are the ashes of prior electrical systems.". - Hannes Alfven, Nobel Laureate, Plasma physicist.

                    Comment


                    • #11
                      Re: MTM difraction effects - what is going on?

                      Not that I'm trying to get you to change, which I'm not, but making it a TMM would not likely require much if any change to the ML-TL design. Yes, the design center would shift down a couple of inches (to the midpoint between the two midwoofers) with the most likely effect being an additional small wiggle or two in the response above 200 Hz, but I seriously doubt you'd have to change either stuffing or the tuning frequency.
                      Paul

                      Originally posted by maynardg View Post
                      Not yet - maybe later

                      1) That would place the woofers lower in the TL and require redesign
                      2) I don't want to cut that blinking baffle again
                      3) I won't like how that OWII looks on top with the double truncation
                      4) Not yet happy with the bass response either.

                      ?? Guidance on felt
                      ?? What would happen if I recessed the woofers a la Continuum?

                      Comment

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