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Port Resonance - what should I be looking for?

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  • Port Resonance - what should I be looking for?

    I am trying to model the ND64-4 and ND64-16 in very small ported boxes, 0.015 and 0.02 cu ft. in WinISD.
    (I realize I will likely have to do a small slot port, as a round port isn't going to fit in the box.)
    I am new to modeling and am looking at SPL, cone excursion, and port velocity (which I'm keeping below 25m/s).
    However, I have never looked at 1st port resonance, because I don't know what I would be looking for. I found a little about subwoofers, but these are full-range and will probably play up to 10khz or so.

    So, what should I be targeting and worried about?

  • #2
    Port resonances can be dealt with by partially filling the enclosure and/or port. The more fill, the better the port resonances are dealt with but the more the bass tends to just become that of a sealed enclosure. The shorter the port, the higher the port resonance and the more effectively fill will deal with the resonance. Therefore there becomes a trade off between the size of the port to minimise port velocity and maximise the port resonance.

    http://sound.whsites.net/articles/boxstuff.htm

    Comment


    • #3
      Originally posted by a4eaudio View Post
      I am trying to model the ND64-4 and ND64-16 in very small ported boxes, 0.015 and 0.02 cu ft. in WinISD.
      (I realize I will likely have to do a small slot port, as a round port isn't going to fit in the box.)
      I am new to modeling and am looking at SPL, cone excursion, and port velocity (which I'm keeping below 25m/s).
      However, I have never looked at 1st port resonance, because I don't know what I would be looking for. I found a little about subwoofers, but these are full-range and will probably play up to 10khz or so.

      So, what should I be targeting and worried about?
      The first pipe resonance will occur at F1=c/(2*L+0.8*d)

      Where

      c=speed of sound (m/s)
      L = vent length (m)
      d = vend diameter (m)

      From this, you can work out at what frequency you can expect that pipe resonance to happen, based on the dimensions of your vent.

      As TMM indicated, you can lessen an impact of pipe resonance by using lining/stuffing in the box. While this method will reduce the pipe resonance a bit, it will also reduce the output at the vent by a little bit though.

      There is another "possible method" - utilize the resonances that naturally occur in a rectangular box, i.e. size the box and vent and position the vent so that its internal opening is placed at a "null" for a standing wave inside the box that's sized to have that standing wave be at the first pipe resonance frequency. This way the vent won't be "excited" at that particular frequency. I ran into this quite by accident with one of my builds. I'd shortened the ports a bit to increase Fb, and I noticed that the massive 1st pipe resonance, which was at almost the same SPL level as the main output from the vent, reduced significantly in magnitude, and based on the measured FR, I think that was because it was now a lot closer to a "null" of one of the standing waves inside the box. The first image shows the measured response of the unstuffed box with the original vents (taken years ago), and the next one shows the same box with shortened vents. Notice the significant reduction in the pipe resonance effect, just by shifting it up a few Hz (by shortening the vents), as it's getting closer to the null at 500 Hz. Again, it's an effect I ran into quite by accident, but I think it can be exploited.
      Brian Steele
      www.diysubwoofers.org

      Comment


      • #4
        Originally posted by Brian Steele View Post

        The first pipe resonance will occur at F1=c/(2*L+0.8*d)

        Where

        c=speed of sound (m/s)
        L = vent length (m)
        d = vend diameter (m)

        From this, you can work out at what frequency you can expect that pipe resonance to happen, based on the dimensions of your vent.

        As TMM indicated, you can lessen an impact of pipe resonance by using lining/stuffing in the box. While this method will reduce the pipe resonance a bit, it will also reduce the output at the vent by a little bit though.

        There is another "possible method" - utilize the resonances that naturally occur in a rectangular box, i.e. size the box and vent and position the vent so that its internal opening is placed at a "null" for a standing wave inside the box that's sized to have that standing wave be at the first pipe resonance frequency. This way the vent won't be "excited" at that particular frequency. I ran into this quite by accident with one of my builds. I'd shortened the ports a bit to increase Fb, and I noticed that the massive 1st pipe resonance, which was at almost the same SPL level as the main output from the vent, reduced significantly in magnitude, and based on the measured FR, I think that was because it was now a lot closer to a "null" of one of the standing waves inside the box. The first image shows the measured response of the unstuffed box with the original vents (taken years ago), and the next one shows the same box with shortened vents. Notice the significant reduction in the pipe resonance effect, just by shifting it up a few Hz (by shortening the vents), as it's getting closer to the null at 500 Hz. Again, it's an effect I ran into quite by accident, but I think it can be exploited.
        I think I had the same experience. I had designed the port resonance to be above the woofer passband, but when I looked for it, it was almost unmeasurable. I think the end of the port was in a null spot in the cabinet.
        Francis

        Comment


        • #5
          Originally posted by fpitas View Post

          I think I had the same experience. I had designed the port resonance to be above the woofer passband, but when I looked for it, it was almost unmeasurable. I think the end of the port was in a null spot in the cabinet.
          Yup, we may be on to something here. If you have a closer look at the second image I posted, you can see the peaks at the pipe resonance harmonics and the dips caused by the harmonics of the internal standing waves that result in a "null" that's close to the vent's entrance. At the moment the out of band resonance peaks are low enough in amplitude for that 4th order BP build to be used with a simple LP filter way out of the bandpass region, or no filter at all. Getting those dips and peaks to line up with one another would make it basically as perfect as it can get, IMO.
          Brian Steele
          www.diysubwoofers.org

          Comment


          • #6
            This gives some insight into things, but may be a bit much.....

            https://patents.google.com/patent/US7162049

            (I think using 2 different port diameters, and therefore different lengths,to maintain the same box resonance, it sort of nulls or reduces the resonance....)

            Comment


            • #7
              Originally posted by kevintomb View Post
              This gives some insight into things, but may be a bit much.....

              https://patents.google.com/patent/US7162049

              (I think using 2 different port diameters, and therefore different lengths,to maintain the same box resonance, it sort of nulls or reduces the resonance....)
              interesting patent. The image at the link is prior art though. The description of the invention suggests that one port will be at the front of the enclosure and the other at the back, and in line with the one at the front. Separate them by the right amount and you can cancel out the first pipe resonance. However I think the enclosure would have to be pretty deep if this type of arrangement is used, to accommodate the two vents and the gap between them.
              Brian Steele
              www.diysubwoofers.org

              Comment


              • #8
                So, for a woofer/subwoofer is the rule of thumb to get the resonance as high as possible within other constraints such as port velocity? (Ignoring the interesting aspect of the nulls within the box.)

                What about a full-range driver where the resonance isn't going to be above the pass band? Just keep the resonance as high as possible and use lining/stuffing to tame it as much as possible?

                Comment


                • #9
                  Originally posted by a4eaudio View Post
                  So, for a woofer/subwoofer is the rule of thumb to get the resonance as high as possible within other constraints such as port velocity? (Ignoring the interesting aspect of the nulls within the box.)

                  What about a full-range driver where the resonance isn't going to be above the pass band? Just keep the resonance as high as possible and use lining/stuffing to tame it as much as possible?
                  Yes for both. Keep the port(s) as short as practical is the rule. Generous flares help minimize port noise. For a subwoofer though, they can be pretty long and the resonance is still above the passband.
                  Francis

                  Comment


                  • #10
                    I had a 1st and 2nd port resonance problem on my "Plumber's Delight" speakers. Tang Band W5-2053 five inch subwoofer in a 3 liter box with a 26 inch long external port. I put a quarter wave "trap" on the external port to solve the problem. See attached:

                    Plumber's Delight: http://techtalk.parts-express.com/fo...notech-winners
                    Linehopper: http://techtalk.parts-express.com/sh...Esoteric-build
                    Plan-Tanic: http://techtalk.parts-express.com/sh...ght=Plan-Tanic

                    Comment


                    • #11
                      Originally posted by 4thtry View Post
                      I had a 1st and 2nd port resonance problem on my "Plumber's Delight" speakers. Tang Band W5-2053 five inch subwoofer in a 3 liter box with a 26 inch long external port. I put a quarter wave "trap" on the external port to solve the problem. See attached:
                      I remember that. Cool solution!
                      Francis

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