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Crazy Port Design Tech Question

Discussion in 'Amps and Cabs [BG]' started by Roundwound, Dec 13, 2005.

  1. Roundwound


    May 13, 2004
    Peoria, IL
    For the technicians out there...I'll be building my first bass cab in the coming weeks although I'm still tinkering with the design. I plan to use a normal tube to vent the cab. However, I did a little thinking about how tubes can influence the behavior of the air moving through them to enhance a cab's performance.

    My question is, would a concave-shaped port tube (concave inside and straight outside) enhance a cab's performance by throwing out low frequencies a greater distance? Has anyone ever "tried this at home" and come up with these or other results of any significance? The picture I have in mind is of a tube that looks exactly staight from the outside, but if you look inside of it, the inner walls decrease in diameter until the middle part and then increase for the remainder of the distance. Both ends would have the same diameter. So there's a slightly concave shape inside but the tube is straight outside.

    Here's how I arrived at this question. I have some books about aviation that I enjoy reading. These books describe how air interacts with a wing in flight. Wings are typically flat on the bottom surface and curved on the top surface, and air does moves faster along the top surface to meet the air coming from beneath the wing. There are a lot of factors that cause this, including static and kinetic energy, velocity, etc.

    I would think the same theory would apply when air and sound waves travel through such a tube design since the surface is similar to an airfoil. The only difference is the air pushed along the outside of the tube would meet the baffle towards the end of its journey, and not the outside. But I would think the air inside the tube would have a speed increase, resulting in a better throw?

    It might be far fetched, and I'm sure there are other factors to consider (possible tube noise, etc.), but am wondering if anyone has attempted that design in a production cab or on their own.

  2. seamonkey


    Aug 6, 2004
  3. The ends of ports in hifi speakerboxes are often flared to reduce port noise (chuffing).
    It doesn't improve low end 'throw' as far as i am aware.. or maybe it does: http://www.speakerplans.com/page94.html
    It does complicate calculating port dimensions for sure.
  4. I can't comment on your particular design but I can tell you that the primary goal is to keep air speed low, not high. Air moving too quickly produces noise as it begins to act like a pipe organ. Not good stuff! :meh:
  5. There are (at least) two different things going on here:

    [1] Sound waves propagate from the port without any net movement of the surrounding air.

    [2] Air is blown from & sucked into the port due to the movement of the speaker cone.

    i.e. the air you feel blowing out of the ports != the 'sound' coming out...
  6. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    There's a basic assumption being made here, that the purpose of the duct is to serve as a conduit for sound. That's incorrect. It does do so, but that's not it's primary function, which is to tune the resonant frequency of the system. The range of frequencies which do pass through the port is limited, and going to great lengths to improve their transmission isn't worthwhile. If anything a concave duct would restrict their passage via the constrictions at either end. As Link mentioned ducts with flares at either or both ends are often employed to ease airflow transition and that's something you should investigate.
  7. Read the Loudspeaker Design Cookbook, by Vance Dickason. He devotes a few pages to this very topic, and goes through the permutations of what works, and does not.

    The flared port ends reduce turbulence. I suspect using a larger port area would do the same, albeit with the associated longer duct and design headaches. IMO, flared ends are nit picking, and solved by using a larger port area.

    Port noise is only a problem in the narrow frequency range where it operates. If yours is tuned below your lowest note, no worries. If yours is within your basses note range, it won't be a problem at low power. If you design the port area large enough, port air speed <= 0.045 MACH, it won't be a problem under full power. Bigger is better, up to a point.

    A big port is easy to do in a larger box, and a pain to implement in a small box. In a really small box, a pair of passive radiators is a better idea.
  8. Roundwound


    May 13, 2004
    Peoria, IL
    Thank you all for the good information here. I'll look into that book, too. I know some of you do this for a living and I can't imagine the amount of "what if" ideas that have come to mind while thinking over design questions. I'm discovering this is a fun part of building cabs - just stewing over the "what if" scenarios. I'll be starting on the cab in January and plan to journal the process and put it all up in a thread when it's done. If I can produce a flared tube I'll test it out for kicks, but I still plan to throw a standard tube in the cab I'm building.