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How important is a roundover on the top and bottom?

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  • #16
    Re: How important is a roundover on the top and bottom?

    Originally posted by spasticteapot View Post
    You won't mind my asking, then, if you can back this up with some hard facts?
    Not really needed to, diffraction is a well-documented phenomenon and creates dips and peaks of several dB in many cases.

    Comment


    • #17
      Re: How important is a roundover on the top and bottom?

      Originally posted by charlielaub View Post
      Sure, bite on this. The following statement:
      "the diffraction caused by cabinet edges and baffle protrusions is probably at least as hearable as the diffraction caused by the vase your wife or girlfriend put on top of your speaker, which is to say, not at all."
      Is a load of crap and involves excessive generalization.
      -Charlie
      Hey, cut the guy some slack. What he is saying is, don't obsess over it.

      Everything is audible, but how much does it distract from the musical experience? How significant are some diffraction artifacts? It kind of depends.
      You can never completely eliminate all of it, you learn to live (listen) to it.
      If the vase on top of your loudspeaker is rattling, you can easily remove the vase, but dealing with every iota of diffraction is virtually impossible.

      Until you have absolute point sources?

      Comment


      • #18
        Re: How important is a roundover on the top and bottom?

        Dunno, but it seems like Linkwitz agrees with Dickason, actually:

        Originally posted by Linkwitz
        While I try to minimize visible diffraction ripples in the frequency response for good measure, I have no evidence that even strong diffraction effects have significant audible consequences, except for the transition region, the "baffle step", where radiation goes from omni-directional to forward firing.
        He also suggests (as I did) that a waveguide controlling directivity at lower frequencies would reduce "illumination" of the cabinet edges and thus minimize diffraction....
        Last edited by Zilch; 12-16-2009, 08:06 PM.

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        • #19
          Re: How important is a roundover on the top and bottom?

          Hey, cut the guy some slack. What he is saying is, don't obsess over it.

          Everything is audible, but how much does it distract from the musical experience? How significant are some diffraction artifacts? It kind of depends.
          You can never completely eliminate all of it, you learn to live (listen) to it.
          If the vase on top of your loudspeaker is rattling, you can easily remove the vase, but dealing with every iota of diffraction is virtually impossible.

          Until you have absolute point sources?
          Sure, why obsess over it?? Who cares about that exaggerated upper midrange, anyway? BTW, point sources would be the worst in terms of diffraction effect, because the smaller the driver the worse the diffraction, 2-inch drivers and smaller, especially.

          Actually, this is one of the main reasons why you are supposed to take actual measurements of your drivers in the cabinet before designing the crossover - because you will then be able to compensate for the diffraction peaks in the crossover network. This is what all good designers do.

          Diffraction peaks are definitely audible. Most every cabinet has them except a perfect "sphere".

          -Charlie
          Charlie's Audio Pages: http://audio.claub.net

          Comment


          • #20
            Re: How important is a roundover on the top and bottom?

            Originally posted by badman View Post
            Not really needed to, diffraction is a well-documented phenomenon and creates dips and peaks of several dB in many cases.
            I'm aware that it's a well-documented phenomenon, but nobody seems to agree on what to do about it. Depending on who you ask, the best solution is a spherical baffle, a heavily padded baffle, a baffle with big roundovers, or just ignoring the issue because there's nothing you can do about it.

            I guess the question is: How do I apply this data in my projects?

            Originally posted by Zilch View Post
            He also suggests (as I did) that a waveguide controlling directivity at lower frequencies would reduce "illumination" of the cabinet edges and thus minimize diffraction....
            I've wondered if this was true. Perhaps Zaph has some data on the subject?

            Comment


            • #21
              Re: How important is a roundover on the top and bottom?

              Originally posted by spasticteapot View Post
              I guess the question is: How do I apply this data in my projects?
              Minimize it.

              This, from a renowned designer, in a personal communication:

              I can't count the number of products I've worked on that had CD flares on dome tweeters. Its a nice approach for getting the treble directivity to match the top of the woofer's directivity, smoother response on the box, etc.
              http://techtalk.parts-express.com/sh...4&postcount=11

              Comment


              • #22
                Re: How important is a roundover on the top and bottom?

                Originally posted by Zilch View Post
                This, from a renowned designer, in a personal communication:
                I was thinking pretty much the same thing. I've got a pair of ScanSpeak 6600s I'd like to use with a pair of W18E001s (assuming I can trade my W18EX001s....I got the wrong woofers!). From what I understand, a waveguide has the following benefits:

                1. Regulated dispersion. The very low crossover point I intend to use (~1.5khz) combined with steep slopes should help a bit with this, but my understanding is that a waveguide is a very good "brute force" method to make off-axis dispersion even with that of the woofer at the crossover point.

                2. Increased efficiency at lower frequencies. Greater efficiency = less dispersion, especially since the Excels like low crossover points.

                3. Reduced diffraction. While giant felt-covered spheres might be very good acoustically, they're not very nice to look at. While the use of a waveguide can introduce its' own diffraction issues, it should be possible (based on Zaph's empirical research) to keep these above 17khz where they're quite difficult to hear, and in the process reduce diffraction below 10khz where it's a lot more noticeable.

                Do I understand this correctly?

                EDIT:

                Okay, why can't I say "B.A.L.L.S"? I can see why this might get filtered, but it's going to make any discussion of the Great American Pastime extremely awkward.

                Comment


                • #23
                  Re: How important is a roundover on the top and bottom?

                  Originally posted by spasticteapot View Post
                  I'm aware that it's a well-documented phenomenon, but nobody seems to agree on what to do about it. Depending on who you ask, the best solution is a spherical baffle, a heavily padded baffle, a baffle with big roundovers, or just ignoring the issue because there's nothing you can do about it.

                  I guess the question is: How do I apply this data in my projects?



                  I've wondered if this was true. Perhaps Zaph has some data on the subject?
                  From my experience (with direct radiator drivers and not horns) there are two main effects of diffraction:
                  1. the baffle step
                  2. a hump near where the baffle step starts to drop off, e.g. around 1 kHz

                  There is lots of material out there about the baffle step. What I see is a lack of much mention of the hump that remains after the baffle step transition has been compensated for. Let me illustrate:
                  response of driver in box, no baffle step compensation:


                  response of driver in box, with baffle step compensation (blue line):


                  As you can see from the lower plot, even after you use baffle step compensation, you are left with a 2dB peak that is more than an octave wide. You can bet that this will be audible. This is typical behavior for direct radiator drivers that are not large compared to the baffle width.


                  Here is another example. This time the driver diameter is small (2 inches, e.g. RS52 mid dome):
                  response of driver in box, no baffle step compensation:


                  response of driver in box, with baffle step compensation (blue line):


                  The smaller diameter driver has accentuated the diffraction "oscillations" that were more damped in the first example that used a larger diameter driver. If this 2-inch diameter driver has strong off axis energy (and the RS52 does) then this effect WILL occur and the response will have similar peaks and valleys. This will be audible.

                  So, I hope I have illustrated my point. I am not saying that this occurs for every combination of baffle width, shape, driver diameter and so on, but for the majority of cases involving a driver mounted in the center of the baffle of a rectangular box, you really need to take these effects in to account if you want to end up with a good sounding speaker. IMHO. So I really must disagree with the statement from Dickason posted by Zilch.

                  You may feel differently. That's fine.

                  -Charlie
                  Charlie's Audio Pages: http://audio.claub.net

                  Comment


                  • #24
                    Re: How important is a roundover on the top and bottom?

                    Originally posted by charlielaub View Post
                    From my experience (with direct radiator drivers and not horns) there are two main effects of diffraction:
                    1. the baffle step
                    2. a hump near where the baffle step starts to drop off, e.g. around 1 kHz
                    I'm not arguing that these are an issue - far from it. However, I thought that roundovers were more of an issue for tweeter response - wouldn't a roundover be too small to make much of a difference below 2khz? (I'm a bit confused.)

                    Incidentally, while I'm new to speaker design, I have taken a look at the EDGE, and it gives me pretty promising results. This is what I got when I put a 180mm driver on a baffle 8" x 12":



                    Considering that the net result will be used with an active crossover, it seems like it's a pretty usable result - everything is nice and smooth well past the planned ~1.5khz crossover point. Or am I looking at it wrong?

                    Comment


                    • #25
                      Re: How important is a roundover on the top and bottom?

                      RE roundovers and tweeter response - I think that someone posted earlier in this thread that the roundover has to be pretty large to be effective, so even a 3/4" roundover might not have much effect at 2kHz. Make sure that you flush mount the tweeter, as the reflection from the edge of the faceplate will cause ripples in the response.

                      RE your design and Edge: looks good. You are correct, it is pretty smooth up to 1.5kHz so that should work well. You might want to model the tweeter location as well, and try to minimize the ripple amplitude above 1.5 kHz by adjusting the location until you get the best looking response. Also, Edge does not take in to account the fact that as some frequency the off-axis response starts to become less intense than the on axis response (e.g. the driver starts to "beam"). You can look at the tweeter's off axis response plot (if available) and see if you can discount the Edge's prediction of response irregularities, e.g. if the response is down 6dB at 45degrees off axis then any predicted irregularities at that frequency are probably not really going to happen because the cabinet edge is not sufficiently "illuminated" compared to the on-axis response.

                      -Charlie
                      Charlie's Audio Pages: http://audio.claub.net

                      Comment


                      • #26
                        Re: How important is a roundover on the top and bottom?

                        Originally posted by spasticteapot View Post
                        Do I understand this correctly?
                        I believe so, yes.

                        I build with waveguides almost exclusively, and consider baffle step a separate issue, as Linkwitz suggests....

                        Comment


                        • #27
                          Re: How important is a roundover on the top and bottom?

                          I wasn't talking about one big noticeable midrange hump. What I was referring to, was all the little squiggly stuff in the treble region. You'll have a real tough time crossovering all of that out.


                          Originally posted by charlielaub View Post
                          Sure, why obsess over it?? Who cares about that exaggerated upper midrange, anyway? BTW, point sources would be the worst in terms of diffraction effect, because the smaller the driver the worse the diffraction, 2-inch drivers and smaller, especially.

                          Actually, this is one of the main reasons why you are supposed to take actual measurements of your drivers in the cabinet before designing the crossover - because you will then be able to compensate for the diffraction peaks in the crossover network. This is what all good designers do.

                          Diffraction peaks are definitely audible. Most every cabinet has them except a perfect "sphere".

                          -Charlie

                          Comment


                          • #28
                            Re: How important is a roundover on the top and bottom?

                            ...What I was referring to, was all the little squiggly stuff in the treble region. You'll have a real tough time crossovering all of that out.
                            That's right, your component count would quickly climb. But there are other ways to better the situation, namely via some diffraction modeling that is done before you cut holes for the drivers. By moving the drivers around on the baffle (as space permits) you can sometimes find a location that has significantly smoother response, e.g. the higher frequency squiggles are minimized. You are then left to deal with one main hump, which can be notched out or perhaps integrated in to the crossover overlap region, depending on its frequency and breadth.

                            -Charlie
                            Charlie's Audio Pages: http://audio.claub.net

                            Comment


                            • #29
                              Re: How important is a roundover on the top and bottom?

                              Originally posted by charlielaub View Post
                              That's right, your component count would quickly climb. But there are other ways to better the situation, namely via some diffraction modeling that is done before you cut holes for the drivers. By moving the drivers around on the baffle (as space permits) you can sometimes find a location that has significantly smoother response, e.g. the higher frequency squiggles are minimized. You are then left to deal with one main hump, which can be notched out or perhaps integrated in to the crossover overlap region, depending on its frequency and breadth.

                              -Charlie
                              Are there any guidelines or rules of thumb that you can give for positioning the tweeter, other than just moving it around somewhat randomly? In his book, Ray Alden suggests making the distance to one edge an imaginary number, but of course he was a math teacher by profession.

                              Comment


                              • #30
                                Re: How important is a roundover on the top and bottom?

                                First rule of thumb is that the distances should be unequal.

                                With waveguides, the edges are less of an issue than their own mouth contours, and perfect axisymmetric round ones generate an on-axis notch, hence the Geddes "I don't listen on axis" syndrome.... ;)

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