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What are ports?

Discussion in 'Amps and Cabs [BG]' started by theshadow2001, Jul 11, 2005.

  1. theshadow2001


    Jun 17, 2004

    Was trying to look up some info on JBL speakers (btw those guys really dont know how to make a site) and came across the notion of ports.

    I've always seen them on speakers but I really don't know what they are or how they work what pruprose they serve or how they affect sound or....well I think you get the idea

    So could someone explain ports to me?

  2. Ports extend the low frequency range of a speaker cabinet by channelling the backwave outward into the listening area. There is much much more to it than that but that is the short answer. Hopefully some of the cab gurus will chime in and explain it further.
  3. Trevorus


    Oct 18, 2002
    Urbana, IL
    Ports are tuned little tubes oropenings in a cabinet that are a certain volume (length, width, and height) that allow a speaker to work to it's peak performance.
  4. theshadow2001


    Jun 17, 2004
    Yeah I thought it might have something to do with the back wave as I looked in the ports on my stereo speakers and they just seemed to be holes leading into the back of the speaker cone.

    I saw something about them being tuned as well to me this indicates that they only work or, possibly more accuratley, work at their optimum at a certain frequency. Is this right?

    Because they are tuned does it have something to do with resononance?

    Also because it allows the back wave out is there a possibility of causing deconstructive interference in the output wave?
  5. Ports do allow only certain frequencies to pass. The diameter, length, and number of ports affect what frequencies will pass through. The box size determines what size ports are needed to pass the desired frequencies through.

    I don't know how resonance interacts with port tuning.

    I might be wrong but I don't think the back wave passes through the ports out of phase with the front wave. This involves some tech mumbo jumbo I don't understand. :D
  6. Was doing a search when I came across this post. It seemed close enough to ask my question.
    I have a ported 4*10 Peavey TVX cab. The port(s) are along the bottom of the cab. If you are not familiar with the cab they are elongated ports instead of single 'hole' ports.
    This cab is used as a monitor only with low volumes being pushed through it.
    Anyways... my question is what would happen if I were to block the port(s)? Could I damage the speakers in some way? (doesn't seem likely). Would it tighten up the sound at close range or become muddy and blah...bblah? :eyebrow:
    I have never played on a sealed cab so I don't know what they sound like in person.

    I'm just in an experimenting mood these days, thats why I ask.

    Thanks in advance.
  7. It is ok to try this out. It should tighten up the sound and lose low frequency extension. I have done this many times with my bass cabs without a problem.
  8. theshadow2001


    Jun 17, 2004
    I can see how the overall size and shape could let only certain frequencies through (a bit like making a note in a tube like a flute or something?)

    Is there like a standard frequency range that you should be letting through a port or is it like anything else in music just a matter of opinion.
  9. The desired frequency extension is dependant on the speakers you are using and the size of the box they are in. There is a lot of info on this subject here and elsewhere on the internet. Try a search using "vented cabinets" or "ported cabinets" or "bass cabinet design". You get the idea
  10. IvanMike

    IvanMike Player Characters fear me... Supporting Member

    Nov 10, 2002
    Middletown CT, USA
    places for ships to hang out when they're not sailing. :smug:
  11. cirwin


    May 2, 2005
    It's all about low freq., of course. If you put a speaker in a sealed box, you lose half the power - that is the air moved by the back of the cone. The problem is that the "backwash" is out of phase with the "frontal wave." This will cause cancellation problems. The impedance of the port (its size and the mass of the air in it) phase shifts the backwash so it reinforces the low freq. In the 1970's, two engineers - Thiele & Small - worked out some formulas that relate the characteristics of a given speaker, the size of a box, and the port size to the usabble low freq. There are programs on the 'net that will allow you to plug values in and design your own box.

    Since the box is designed to the parameters of the speaker, swapping a brand x speaker for a brand z speaker in a box may not work particularly well. Most speaker companies now supply Thiele-Small parameters for their speakers, so you can calculate all this stuff. (Electro-Voice used to have plans for building boxes for their speakers that included a port-cover. You could change the tuning of the box by removing or replacing the cover.)

    As far as I know, closing up the ports won't harm the speakers. But it will change the box tuning AND the efficiency of the cabinet.

    Charlie Irwin
  12. I'll post my results in a few days. THanks for the info....didn't mean to throw the post off track.