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Bass Cabinet Porting

Discussion in 'Amps and Cabs [BG]' started by porcineaviator, Mar 19, 2005.

  1. porcineaviator


    Mar 18, 2005
    Hi Guys -

    This question was asked on the construction sticky, but not answered. Can I reliably design a slot port by giving WinIsd a port size of the internal width of the cabinet x 1/2/3 cm and use a slot of the length given?

    This leads on to how putting a right-angle bend in a port affects things. One design I'm looking at has a circular port length which would only fit into the height of the cab. I don't want the port outlet on the bottom or top because of obstruction by the floor/amp top. Can I put a right-angle bend into it and have it coming out of the back?

    One last thing - what is the upper limit for vent air speed?

    Many thanks.
  2. ESP-LTD


    Sep 9, 2001
    As I understand it, you can calculate the ports any way you want as long as the area and length remain constant. BUT, they will be wrong. The formulas will get you close, but unless you can accurately account for every feature effecting the volume of your cabinet you will only be 'close'; maybe even 'close enough'. Likewise, there may be formulas to account for the difference between round ports and anyother shape, BUT they are also going to be an approximation at some frequency or another.

    As fas a I know, calculations can only get you close, and round ports are a lot easier to trim than shelf ports.
  3. A slot port is effectively longer than its physical dimensions, so the answer is no. By design, a slot port uses one cabinet wall as a port wall. Length is calculated differently than a tunnel port.

    A 90-degree bend has no appreciable effect on port operation. Much more important is a port diameter that is too small, which will cause "chuffing" (port noise) at full power. The formula for port area in square inches is: (0.02032 * Sd * Xmax * Fb), with all dimensions in inches. The port area double when two drivers are used. This formula gives a MACH 0.45 value at maximum displacment, which will not make noise. You can safely go up to about MACH 0.10 before appreciable noise sets in.

    Note that port noise is directly related to cone displacment and frequency. At low loudness levels, it is a non-problem. At frequencies higher than 1/2 octave above the tuning frequency, it is always a non-problem. For example, my JBL E155 tuned at 30 Hz is never a problem for my lowest note of 41 Hz. If I played a 5-string (31 Hz), I would have to pay more attention to port diameter.

    The slot port allows for the largest port area from the smallest possible baffle board size. It is more difficult to construct, and especially more difficult to tune. Once done, it works well and provides additional stiffening to the cabinet.
  4. porcineaviator


    Mar 18, 2005
    Thanks, ESP - sure - the plastic pipe's a lot easier to trim afterwards! Any views on the other query - putting a right-angled bend in the port so as to get it out of the back rather than the top or bottom?
  5. porcineaviator


    Mar 18, 2005
    Many thanks bgavin - most informative. Could you enlighten me on one thing - when using Xmax in calculations, should I be using the one-way excursion or the total?
  6. gfab333


    Mar 22, 2000
    Honolulu, Hawaii

    Thanks for the enlightening port info for those of us who are non-engineer types.

    just curious, what does port noise / "chuffing" sound like in laymen's terms? :confused:

    Holy @#$! Your profile lists one of your hobbies as subwoofer engineering! does this mean we'll be seeing a line of Bgavin cabs coming out to compete Accugroove, Acme, Schroeders, etc.? I know that many of these boutique brands were start ups from someone's passionate hobby.
  7. Port noise is often heard as a whistling or whooshing sound.

    A more insidious problem is power compression from a too-small port, same as what happens with a driver. As more power is applied, no more apparent volume occurs (compression).

    To answer your question, there are a lot of boutique manufacturers in the market right now that offer nice products. One would have to find a niche to be competitive. My interests are bi-amp rigs, time aligned and optimally engineered. I'm a software engineer, and frankly there is a whole lot more money in this, than in making specialty speaker cabs.

    If you are seriously interested in something different from the boutique lines, check out Bill Fitzmaurice's bass horns. He has a number of authorized builders who can put one together for you. His DR250a mid-bass horn, sitting on top of a Tuba will devastate everything else, and probably for a lower cost than a new D410XST.

    As of this writing, a thorough measurement analysis and set of comparison sound clips is missing. I suspect this will be rectified by summer this year, as there are a number of folks seriously interested in the Tubas, including myself. I bought plans for several models, and will be putting them through a rigorous measurement and testing process. I need (3) Tuba36 Slims (24W x 36H x 36D") for PA reinforcement, so these will be first.

    Also understand that bass horns require size to reach way down low. Sitting on the ground, three of these will provide full horn loading down to 56 Hz, which is plenty low for PA reinforcement. The Tuba design allows for a significantly lower response below the horn cutoff, but at a reduced level.