To Bill: Main differences between Bass reflex and Closed

Discussion in 'Bass' started by Alécio Costa - Brazil, Feb 14, 2003.

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  1. Alécio Costa - Brazil

    Alécio Costa - Brazil Well-Known Member

    Mar 19, 2002
    I would like to know the main advantages and disadvantages of each design. I have heard the old popular legends that closed cabinets need much more power and that rear ports help eliminate standing waves.

    I know all this might be BS. So nothing better than have the right Pro guy explain us a little bit of this.

    Bill, about that idea of closing the Monitor Ones rear ports...

  2. Rowan

    Rowan Active Member

    Jan 7, 2003
    Well I 'm prepared to make a fool out of myself!...

    Basically a reflex (ported design) can provide a more efficient and extended bass response down to around it's cutoff point (-3db).

    Below this the response drops off quite rapidly at 12db per octave.

    A sealed enclosure will provide a much more gradual rolloff below the cutoff point (usually higher)at a rate of 6db per octave.

    A sealed enclosure will generally provide a better transient response as the driver is dampened by the spring effect of the air trapped inside the enclosure.

    However it's possible to tune a bass reflex design to have a lower cutoff point than an equivalent acoustic suspension design and, for smaller sized speakers used in monitoring applications, this may be more desirable.

    A larger sealed enclosure design will allow for more bass extension than a smaller sealed box, but the trade off is they need to be quite large to reduce the effect of the trapped air inside. Once large enough for the effect to be negligable they are then generally refered to as 'infinite baffle' designs. The catch is an infinite baffle design would need to be substantially larger than a bass reflex design (all other things being equal) to produce similar bass extension.

    And that may negate them from the world of nearfield monitoring for example.

    As far as sealing the port on your speakers, this will change their response quite considerably at the low end making them at least 'different' if not difficult to mix on. Unless you can accurately plot their response before and after and are able to adapt yourself to the changes, I don't think I'd be recommending sealing the ports... especially on a smallish enclosure.
  3. audiowkstation

    audiowkstation Active Member

    Jun 29, 2001
    Rowan has the theory correct.

    One way to improve the monitor ones (mentioned before) is to make them an aperiodic design or "leaky acoustic suspention" by inseting a piece of foam on the port. Not exactly airtight, you will better align the driver to the encloser (for that speaker) and eliminate some of the boom around 60hZ.

    Dynaudio and scan-speak companys produce a "variovent" that is the aperiodic design principal for some of their enclosures. Dynaco A-25's used this principal as well.

    This does three things.

    It flattens the huge rise in impedance (50 ohms for monitor ones) at port resonance, it provides for lower f3 (lowest useable frequency, -3dB down from average) and it cleans some of the mud from 100 to 225.

    Experiment with the sponge in the port, this is a "poor mans" variovent.

    Not all ported speakers can benefit. Monitor ones can!

  4. Alécio Costa - Brazil

    Alécio Costa - Brazil Well-Known Member

    Mar 19, 2002
    Thanks Dear. But how should it be? Could it be done with that 1 inch spounge that comes insid the ADAT shipping boxes?
    Nice weekend
  • AT5047

    The New AT5047 Premier Studio Microphone Purity Transformed

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