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Isetta - port chuffing

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  • Isetta - port chuffing

    Hi guys

    I built a Paul Carmody Isetta and pretty much followed his design - same dimensions, same drivers and same filter design.

    As an amplifier, I bought the 2.1 Sound Import EU (Europe's version of Part Express I believe) amp with Bluetooth.

    For the outside, I used slate veneer, something we already use for our business, designing translucent slate veneer lamps.

    The speaker sounds great and offers amazing volume and bass for such a small thing, exactly what I was looking for.

    However, I have a problem and I don't know how to resolve it. With a few songs that have a high SPL at very low frequency but low volume for everything else (e.g. Robert Miles, Children, first 10 seconds), I get audible reflex port chuffing.

    A few words about my reflex port. I bought one of those black, flared speaker port tube that wasn't long enough for the Isetta. So I took a 40mm OD / 36mm ID PVC tube and glued it onto the end of the flared port tube which is "1.5 ID (or 38mm), so about a 1mm step on each side.

    Because I wasn't sure how sensitive reflex ports are for steps, I used a file and honed it down a bit, but there is still a step. Additionally, I added a bit of polyfill inside the speaker (not around the reflex tube) to dampen things a little.

    Still, I have can hear chuffing under the circumstances described above, that hasn't gone away.

    What could it be?
    • Is it because of the step?
    • Are 36mm simply too slim vs the 38mm Paul recommends?
    • Or is it simply something you have to accept for this kind of extreme speaker design?

    Any feedback is appreciated.

    Cheers guys

    Phil from Scotland

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  • #2
    1-1/2"id is a bit undersized for a 5" woofer that can stroke 9mm. However, a 1-3/4" tube would need to be 4" longer.
    You might be able to achieve the same tuning w/the proper passive radiator - which would be "chuff free".

    I wouldn't mind seeing a pic of your port exit.


    • #3
      Thanks for your feedback Chris. Here a few pictures of the port exit and the inside.

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      • #4
        Very nice looking build by the way. I would second the suggestion for passive radiator(s). If you do go that route, it looks like your back panel is your only option size wise, so make sure you have rubber feet to prevent the cabinet from walking around. If you can do one on each side that would be better as their forces oppose.


        • #5
          I don't ordinarily like to end a tube right behind a woofer like that, but... I don't think it's any of your problem (OR your internal "step").
          The exit has a very generous roundover (it couldn't be much better).

          It's for this reason that I try to vent out the rear, if possible (but that might not have helped in this instance).
          I think you're just up against the flow limits of a 1-1/2"id port (w/THAT woofer).

          A PR would most likely be your only recourse.
          Either turn it down, or don't listen to those tracks (HA!). Just enjoy what you've built there.


          • #6
            A flare/chamfer on the inside edge of a port is also important. You could try buying another of that same flared port and putting them back-to-back to get the length you need. it would be an easy experiment that does not change the existing format.


            • #7
              On another note, your slate veneer looks fantastic! That is a great product and you did a great job on the joints, too!
              Very nice build.


              • #8
                Originally posted by dynamo View Post
                Very nice looking build by the way. I would second the suggestion for passive radiator(s). If you do go that route, it looks like your back panel is your only option size wise, so make sure you have rubber feet to prevent the cabinet from walking around. If you can do one on each side that would be better as their forces oppose.
                I was also going to compliment this clean build. Nice work


                • #9
                  Others who know more can chime in…

                  I wonder if you could use a slot port (if you have room) or use some sort of flexible port material such as the Kef LS50?


                  • #10
                    You could experiment with lengthening the existing port. That will lower the tuning a bit and move the peak air velocity down the frequency range a tad as well, perhaps below the offending notes you hear now. A PCV slide coupler and tape will allow you to try increased lengths without much hassle. I'd start at 10" and maybe go up to 12" total port length.
                    Last edited by Millstonemike; 12-07-2019, 02:27 PM.


                    • #11
                      After you get the vent issue sorted out, I'd love to hear more about the slate veneer you used.
                      As others have said, beautiful work.
                      Zarbo Audio Projects Youtube Channel: * 320-641 Amp Review Youtube: *Veneering curves, seams, using heat-lock iron on method *Trimming veneer & tips *Curved Sides glue-up video
                      *Part 2 *Gluing multiple curved laminations of HDF


                      • #12
                        Hi guys

                        thanks so much for the great advice!! I am new here and really appreciate it.

                        Since I don't want to mess-up this first prototype too much, I am liking the idea of lengthening the port a little in a few steps and/or add a flare at the other end. It is very easy to do and if it resolves the problem for this prototype, fab. If I decide to build more of them (something I would like to do), then I need to rethink the port vs. pr advice you offered and maybe build a v2 prototype.

                        As for the slate, this is part of our design studio work. We have been experimenting and working with the material for the last two years. My background is electronics and IT (done for 27 years), but I quit because I got totally fed up with it and wanted to start something quite different. I call myself a lighting designer these days and run the studio with my wife.

                        At the moment, we are working mainly with slate veneer for our designs. See to get a better idea. We are very fond of the material, both translucent and non-translucent (as the Isetta). The slate is on a hessian/cotton backing, is about 0.6-1.0mm thick and astonishingly flexible. If you are good with your hands, spend a few days with the material and know how to use a sharp Stanley knife, this is the look you can create. I absolutely love working with slate veneer, it is a real joy.

                        If you are interested in a nice piece for a project, drop me a mail and we can organise you something good looking. We have the grey/black stuff we used for the Isetta, but quite a number of other colours, including beige/sandstony, grey/black/beige, reddish and more.