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Small Speaker Box Design

Discussion in 'Electronic Basics' started by Bill Bowden, Apr 16, 2007.

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  1. Bill Bowden

    Bill Bowden Guest

    Trying to optimize a small wooden speaker box for voice (speech)
    frequency range. It's a 5 X 3.5 X 1.5 box made out of 1/8 inch
    basswood with a 2.5 inch speaker inside. I drilled thirteen 1/4 holes
    in front of the speaker which sounds reasonable with the backside
    open, but the bass response is too high with a solid back on the box.

    I figure a few air holes drilled in the backside of the box are needed
    to improve the midrange response. Should the holes be placed directly
    behind the speaker, or offset toward the side for best speech
    response? Are a few large holes better than many more smaller holes?

    -Bill
     
  2. Bob Masta

    Bob Masta Guest

    I'm not sure I can help solve your problem, but I'll take a stab at
    why it *is* a problem:

    The bass response of a conventional speaker system falls off
    rapidly below its resonant frequency. The resonant
    frequency is controlled by the moving mass and spring stiffness.
    If the speaker is mounted so the back side faces a sealed
    box, then the air in the box adds to the spring stiffness and
    raises the resonant frequency. A speaker designed for
    such a box typically has a very soft suspension spring, to
    allow for the extra spring effect of the air. I also may have
    more moving mass, which also lower the resonance.
    However, more mass reduces efficiency.

    If you put holes in the back of the box, you relieve the
    spring pressure, but you also allow the sound from the
    back of the driver to wrap around to the front, where it
    can cancel the front sound (since the back of the speaker
    is moving the opposite direction). This phenomenon gets
    worse at lower frequencies, where the sound travel time
    is a smaller fraction of the wavelength. That's why you
    never see open-back speaker enclosures in hi-fi applications.

    One popular alternative to a sealed box is a vented
    or "bass reflex" enclosure. The vent or "port" is typically
    on the front of the box, so it's output can reinforce the
    output from the speaker cone. Their is still the same
    issue with cancellation, since the port output is still
    coming from the back of the speaker. The difference is
    that now the port acts as a second resonant mass
    in the system. (There is usually a tube behind the port,
    which holds air that acts as the mass, plus the enclosure
    air.) By careful tuning of the port length and diameter,
    and box dimensions, the combined system has a lower
    resonance that is lower than for the sealed box. (Their
    is also an upper resonance that is higher, but is not a
    problem.)

    There are still more exotic approaches, such as horns
    or transmission lines, but, as with the bass reflex, they tend to be
    larger than the sealed box.

    So the simple thing for you is to keep the box sealed
    but use a speaker with a lower resonant frequency,
    or a bigger box, or both. Note that the box should
    typically be "dead": The sides should not flex or
    have any resonance of their own. 1/8 basswood
    would thus not be the best choice; speaker builders
    typically use particle board (Medium Density Fiberboard
    or MDF) for woofer enclosures, often times with
    internal cross-bracing.

    If you want to mess around with the system, you
    can try adding mass to the speaker cone to lower
    the resonance (and efficiency). Try carefully pressing
    some "Mortite" or similar always-tacky window caulk
    around the center of the cone. If you find a mass that
    seems to improve things, and you can tolerate the
    loss of output, you can replace the added mass with
    something more permanent like silicone sealer. Just
    remember that it needs to be confined to the little
    ring around the center that has the voice coil
    behind it. Personally, I think you won't find this to
    be worth the effort on such a small speaker, but it
    might be educational.

    One last note: All else being equal, a more powerful
    magnet is what gives more efficiency. Efficiency
    is given as dB output at 1 meter for 1 watt input...
    more is better.

    Best regards,


    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
    www.daqarta.com
    Scope, Spectrum, Spectrogram, Signal Generator
    Science with your sound card!
     
  3. Bill Bowden

    Bill Bowden Guest

    Yes, those are good ideas, but there isn't room in the box for a front
    air hole, so I'm limited to holes, or slots in the backside. I played
    around with a solid back and found I can tune the sound somewhat by
    slowly opening the back to some small angle allowing a regulated
    amount of air to escape. Or, I can slide the back so a small slot is
    exposed, but the sound quality seems best with half the backside or
    more uncovered. So, it appears to need a lot of holes. There is a
    circuit board on the backside of the speaker which forces most of the
    air to travel to the edges of the box and around the board before
    escaping to the rear.

    I'm going to do more experimenting with a cardboard back with holes
    that can be moved around to see what happens.

    -Bill
     
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