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Convert round port to rectangle

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  • Convert round port to rectangle

    Hi everyone,

    I put a circular 140mm long, 44mm ID port on the back of my box and am happy with the sound. I know this is a small port for two Faital 5fe120's sharing the same 9 litre enclosure but I didn't notice any major chuffing. Then again... I don't think I have a very good ear! The pipe has a 90 degree bend in it to fit into the box. I'd like to move it to the front but for that it needs to be rectangular/slotted. I've worked out an equivalent rectangular port to be 2cm X 7.5cm. I plan to play with adjustable rectangular ports in that size or slightly larger (depending on fit) so I can get as large a port as possible to fit into the box.
    I've read that by using slotted ports that utilize the bottom of the speaker as one side of the port the required length is much shorter than using a rectangular port. Is this right and is there a formula that can be used to work it out? My thinking is that if I could use slotted I could increase the area of the port and therefore decrease the chance of chuffing and get better bass response.
    Is it possible that the xmax of those drivers is just not big enough to have created major chuffing in the port? Winisd gives me 60m/s!
    Any help would be greatly appreciated.

  • #2
    If it isn't broken don't fix it. WinISD port velocity charting assumes a sine wave signal at the input signal voltage, which would be the worst case scenario. If you're not inputting the required combination of low frequencies and sufficient drive voltage to cause chuffing leave well enough alone.
    www.billfitzmaurice.com
    www.billfitzmaurice.info/forum

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    • #3
      What Bill said. Also, if it is quietly chuffing, you may start to hear it if you move the port to the front. Rear or bottom porting has that advantage.
      Francis

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      • #4
        With this one particular enclosure I made about 15 years ago with a JBL W10GTi, using a shelf port as you described, I only needed about ~65% of the length for the area I ended up using, compared to what that area would typically require. Verified with a WT3. I still have that subwoofer box actually, I use it outdoors for a mini-music system for when I cook out.

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        • #5
          I doubt you'll hear ANY chuffing @ 110dB !
          You can get "better" bass by doubling the size of your box (high 40s, instead of 60Hz).
          Put a pair of your round ports on the back. 150mm long (each) will be good. (You'll still get 108dB.)

          "required length is much shorter"
          No. SOMEwhat shorter, possibly. Definitely not MUCH shorter.

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          • #6
            Originally posted by Chris Roemer View Post
            SOMEwhat shorter, possibly. Definitely not MUCH shorter.
            It depends on the port dimensions. If it's very short and wide, with the short dimension an inch and a half or less, the port in effect has smaller area, which means it will be shorter for the same tuning as a taller port of the same area. The effective reduction in area when a port is very short also can result in more port velocity, which can result in chuffing. I've seen hundreds of pro-sound speakers where the port was made the entire width of the cabinet, resulting in a very short port. It's not the best way to do it.

            www.billfitzmaurice.com
            www.billfitzmaurice.info/forum

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            • #7
              I thought port surface area and volume were fairly proportional. No?

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              • #8
                if it is quietly chuffing, you may start to hear it if you move the port to the front. Rear or bottom porting has that advantage.
                - interesting, I had not thought of that. Thanks fpitas.

                Chris I don't want to double the size of the box because it will negatively impact the "portable" aspect of the speaker. I have read your response to my OTHER post and will try the tweeter with a 'proper' set of XO's on BOTH (sets of) drivers.

                bill Thanks for your help, I hear you on the it-it-aint-broke-don't-fix-it but I wanted to move it to the front because I wanted to make a system that could be sat with it's back close to a wall. Which is what often happens with portable speakers... people don't always take placement into consideration. I will make a rectangular port on the front to test, taking into consideration your comments on port width/height and see how it goes. I'm going to try a buy a proper tweeter instead of using the 4" so that should free up some room. Might be able to just do the circular front port, I will have to see.

                @maestro Being new to this I may make a second box with a larger shelf port, make it adjustable and take it along to my mates when I test them. I'll let you know if anything of interest pops up. If anything it will be good for my learning curve.

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