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Port output - in or out of phase?

Discussion in 'Amps and Cabs [BG]' started by Gabu, Feb 16, 2002.

  1. Gabu


    Jan 2, 2001
    Lake Elsinore, CA
    When you have a ported enclosure sound eminates from the ports as well as from the speaker, right? If that is the case, is this in phase or out of phase with the speaker?
  2. FalsehoodBass


    Jul 22, 2001
    Denver, CO
    i don't know the answer to this but i'm gona bump it up so that maybe someone will.

    I would assume that the port radiates in phase with the speakers, since if it were out of phase it would cancel your sound so the resuld would be nothing.... like the port would push air out while the speaker sucked it in, so you get nothing.

    if this were true... that would mean there would be a lag in the wave you recieved from the speaker and the port of at least one cycle... it would also mean that port placement is REALLY important, because the time it takes the wave to get from the speaker to the port inside the cabinet would be a really big deal..

    it also seems that the effect would be frequency dependant, so that it woud amplify some frequencies but cancel others... but maybe that's what port tuning frequency means.... i duno... its fun to think about but it would be nice to hear from someone who really knows what's going on
  3. There are three important resonant points in a ported system. The lowest is the lower resonance (Fol), next is the tuning frequency (Fob), and the upper resonance (Foh).

    The port and speaker are 180 degree anti-phase at Fol. The cabinet acts like a solid connecting rod between the driver and the port. The driver moves inward, air in the port moves outward. Phase cancellation is maximum here. This, along with acoustic unloading below Fob is why you don't run vented boxes below Fob.

    Next is Fob where the port is tuned. This is the system resonance. Radiation from the port is 90 degrees anti-phase with the driver but this is still additive when combined with the driver. The port generates nearly all of the sound at Fob. The driver has maximum damping at Fob, and will be almost motionless.

    Higher up is Foh, the upper resonance. The port and driver are 100% in phase which contributes to the "boom" present in some poorly designed (read: too small) vented boxes.

    Vented boxes have the worst group delay numbers at Fob. A large delay (> 24 msec) makes the sound sloppy and muddy. The opposite of this is a sealed enclosure which has group delay timings around 4 msec or slightly higher. Sealed boxes provide tighter, more punchy response but they don't go down nearly as low.

    IMO, port location is irrrelevent as long as it is not blocked. This usually starts a heated discussion, so please note "IMO".
  4. FalsehoodBass


    Jul 22, 2001
    Denver, CO
    what are the typical numbers for Fol, Fob, & Foh. and what happens above Foh?
  5. Each of the three numbers is dependent on the volume of the box, the driver T/S characteristics, and the tuning frequency. Plug these numbers into any of the design programs and vary the box volume and tuning frequency.
  6. Gabu


    Jan 2, 2001
    Lake Elsinore, CA
    hmmm.... so in a vented enclosure, using speakers that go down to 60hz, I should tune the box to 60hz. If I tuned it lower, then as a general rule I could expect distortion below 60hz? (ala speaker farts)
  7. cassanova


    Sep 4, 2000
    i would have to say no to that, mainly because i see 4x10 enclosures tuned to a rating of 30-16.
  8. No. The tuning is determined by the driver's Thiele-Small characteristics.

    The drivers in my spread sheet use the DB Keane algorithm for achieving the lowest F3, but with slight ripple in the response. The WinISD algorithm uses a different formula for the flattest response, but a higher F3.

    The designer takes the basic "optimum" for a given driver, then experiments with the various combinations of cabinet volume and tuning to achieve the desired results.

    With drivers that roll off at 100 Hz, there ain't nothin' you can do about them. Somebody said you can't polish a turd, but you can stick a flag in one.

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