Passive Radiator Question

riglehart
riglehart Posts: 276
edited October 2006 in Vintage Speakers
I have never understood something about the passive design of polk monitor woofers. Since they actually are driven by the air pressure inside the cabinet, why don't the external pressure waves radiated from the drivers cancel out the pressure wave from the sub.

Take a monitor 5 for example. When the 6.5 inch driver moves OUT, it sucks the passive IN (and vice-versa). Why doesn't this create a situation where one cancels out the other, since they are out of phase and very close to each other? Intuitively I would think this would create worse base, instead of better. Obviously it works, but I don't understand why.
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Post edited by riglehart on
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  • Ricardo
    Ricardo Posts: 10,636
    edited October 2006
    The speakers are a sealed enclosure, thus the direct in/out relation between drivers and passive; the air "moved out" by the drivers go into a much greater, unsealed volume, so there's no way that pressure would cancel the movement of the passive.....
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  • obxdiver
    obxdiver Posts: 60
    edited October 2006
    I always wondered what it would sound like to replace the passive radiator with a real 15" woofer, and connect it through a crossover so that only 80hz and below are played by this woofer.
    Seems like it would be easy to do.
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  • PolkThug
    PolkThug Posts: 7,532
    edited October 2006
    obxdiver wrote:
    I always wondered what it would sound like to replace the passive radiator with a real 15" woofer, and connect it through a crossover so that only 80hz and below are played by this woofer.
    Seems like it would be easy to do.

    The woofer would have to be sealed from the mids or you would have muddy midrange.
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  • hoosier21
    hoosier21 Posts: 4,413
    edited October 2006
    http://www.polkaudio.com/forums/showthread.php?t=25540&highlight=PASSIVE

    same thoughts I had back when
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    ADS 1230 - Polk SDA 2B
    DIY Stereo Subwoofer towers w/(4) 12 drivers each
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    Outlaw ICBM - crossover
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  • madmax
    madmax Posts: 12,434
    edited October 2006
    There is a time lag as well.
    madmax
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  • PolkThug
    PolkThug Posts: 7,532
    edited October 2006
    hoosier21 wrote:
    http://www.polkaudio.com/forums/showthread.php?t=25540&highlight=PASSIVE

    same thoughts I had back when

    That's a great thread, proof that learning can actually take place here. I learned the basic principal that things which move, make sound.
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  • heiney9
    heiney9 Posts: 25,203
    edited October 2006
    Passive Radiator
    In speaker design a passive radiator is an element that is designed to move sympathetically with the energy in the cabinet. They generally resemble a low frequency driver or woofer, but have no voice coil or any element to actively generate sound. Often they are employed in speakers instead of a port to create a bass reflex type of design. The extra mass of the passive radiator actually lowers the resonant frequency thereby allowing smaller cabinets to reproduce lower frequencies than they would otherwise be capable of. However this extra mass also causes the radiator's movement to be hard to dampen, which in turn can lead to bass hangover and a more "boomy" sound.

    I don't nec agree w/ the last statement 100%, but there is some truth to it. I much prefer a PR design over a ported design and Polk is one of the best companies at producing speakers (older designs) with PR's. That's why I feel the original Monitor series and the SDA's sound so good top to bottom. You do need an amplifier with a high damping factor to get ultra-accurate bass though.

    Give me a true D'Apollito array and a PR and I'm very happy or make me even happier and give me SDA with a PR and I'm just giddy :D

    H9
    "Appreciation of audio is a completely subjective human experience. Measurements can provide a measure of insight, but are no substitute for human judgment. Why are we looking to reduce a subjective experience to objective criteria anyway? The subtleties of music and audio reproduction are for those who appreciate it. Differentiation by numbers is for those who do not".--Nelson Pass Pass Labs XA25 | EE Avant Pre | EE Mini Max Supreme DAC | MIT Shotgun S1 | Pangea AC14SE MKII | Legend L600 | BlueSound Node 3 - Tubes add soul!
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