Question about cone excursion.

Discussion in 'Home Theater Projects' started by John t., May 2, 2005.

  1. John t.

    John t. Agent

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    I had always been under the impression that a sealed box would reduce the cone excursion as opposed to a vented one.

    Is PE correct in saying that the vented enclosure is the one that reduces cone excursion?

    If they are correct, wouldn't a driver in a ported enclosure have less distortion, be more controlled and handle more wattage than if it was in a sealed enclosure?

    (I thought the cone would move more in a vented enclosure due to some of the air coming out through the port.)

    I'm confused because from what i've read, the driver would be more controlled and handle more wattage in a sealed enclosure.

    BTW, I'm assuming that cone excursion = the cone's movement in and out, so the less or the shorter the movement of the cone, the less distortion and the more controlled the sub will sound.

    thanks.
     
  2. Mattak

    Mattak Stunt Coordinator

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    There are different kinds of "distortion," though. A ported enclosure has a larger group delay than sealed, and has phase issues between the driver and the port.
     
  3. Allen Ross

    Allen Ross Supporting Actor

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    Ah i see you post on two sites, the answers on DIYAudio pretty much cover it up,

    The resistance of the air moving in and out of the port (it can't do it as fast as the driver) tames the port movement, but anything below the tuning freq and excursion goes sky high.
     
  4. Dan Wesnor

    Dan Wesnor Second Unit

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    A ported driver might have less cone excursion at a given output level, depending on the exact design. You need to do the math to know if it does or not.

    Even if it does have less excursion (and it won't be much less), there will always be pipe resonances, which sound like crap, and midrange leakage. I'm not a big fan of ports.
     
  5. Mike Keith

    Mike Keith Second Unit

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    This is correct, the driver in a vented enclosure will have less movement until it hits it's Fb, then the cone will lose control. A sealed alignment will have a smooth roll-off and far less excursion below Fb though.

    To truly understand how a port works read this;
    http://www.hometheaterhifi.com/volum...ayporting.html
     
  6. Ryan Schnacke

    Ryan Schnacke Supporting Actor

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    "Even if it does have less excursion (and it won't be much less)"

    In a ported enclosure you will ALWAYS have MUCH less excursion near the tuning frequency. Above Fb, the vented enclosure's excursion advantage gets smaller and smaller until it eventually matches the sealed enclosure.

    The case for below Fb (driver unloading) has been covered.
     
  7. Dan Wesnor

    Dan Wesnor Second Unit

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    True, but once you've hit excursion, you've hit it, and distortion goes through the roof. If you're hitting excusion at 50Hz, it doesn't matter whether or not you're hitting it at 20Hz also.
     
  8. John t.

    John t. Agent

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    Does pipe resonance sounds boomy?
     
  9. Allen Ross

    Allen Ross Supporting Actor

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    sounds like a muddy hum that isn't anywhere need the sound it should be reproducing.

    run a sin sweep, if you find one that does it slow enough you can hear it for a short second.
     
  10. Rory Buszka

    Rory Buszka Supporting Actor

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    Vents do not work by resistance. They work as Helmholtz resonators, which essentially form a mass-spring system with the volume of air in the box. Ideally, at the tuning frequency the cone is practically standing still while the air in the ports is moving furiously and accomplishing the majority of the output. The air in the ports and the active driver cone will be moving inwards at the same time. As a result, the air in the box is pressurized by the motion of the air in the vent, which then increases the spring-y force on the active driver cone and brings the cone to a stop. The energy absorbed from the cone by the air in the box pressurizes the box even more and forces the air in the port to change direction. Then the same thing happens when the cone and air mass are traveling outward from the box: the air in the box is decompressed and wants to become compressed again, so it will suck the driver cone and the port's air mass back into the box. That is how both bass-reflex vents and passive radiators work, except that in the case of the passive radiator, the air mass in the port is replaced by a mechanical mass on the diaphragm.
     
  11. Ryan Schnacke

    Ryan Schnacke Supporting Actor

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    "True, but once you've hit excursion, you've hit it, and distortion goes through the roof."

    What does it mean to "hit excursion"? Do you mean hitting the excursion limits?

    "If you're hitting excusion at 50Hz, it doesn't matter whether or not you're hitting it at 20Hz also."

    Not sure what you're getting at here. A well-designed ported enclosure will be tuned so that Fb is near or below the lowest frequency that will be reproduced. In this way the woofer will not be unloaded in the intended frequency range. And in this ported enclosure the woofer will have lower excursion than a sealed box of the same size or of the same tuning frequency. So, with respect to cone excursion, the ported enclosure has the advantage for all frequencies at or above Fb. If you want to discuss group delay then I'll be the first to admit that the sealed sub will have the advantage.

    And on the pipe resonance topic the general consensus has been that because the subwoofer's bandwidth is limited to low freqs, pipe resonance is not concern unless you're dealing with a really long port. I know its a non-issue for ports under 20 inches, but I forget the ROT limit.
     
  12. Dan Wesnor

    Dan Wesnor Second Unit

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    Say you're using a 8" mid-woof with 6mm of excursion. If you play it loud enough that you're hitting 7mm of excursion at 50Hz, then you've hit excursion. If the port is tuned to 20Hz, and a simultaneous 20Hz tone would cause 0mm of excursion, you would be using 7mm of excursion. So it doesn't matter that your 20Hz tone isn't causing the woofer to move, you're still exceeding excursion because of the 50Hz tone.

    Pipe resonance sounds like you're blowing across the top of a coke bottle, only at a different frequency depending on the length of the port. The idea is to keep the length of the port short enough that the resonant frequency is above the frequency at which the driver has any significant output, say above 200Hz (30in port) for a subwoofer crossed LR4 at 80Hz.

    I find pipe resonance more audible than port noise when it occurs, but it's pretty easy to avoid in subwoofers. It's much more difficult to avoid a bookshelf-sized speaker, so then you'll need to make a trade-off between port noise and pipe resonance.

    Play with Unibox. It gives you the port resonance, excursion, and port velocities at whatever power level you put in. You'll see many times, you won't get any more output out of a ported box than a sealed box before the excursion exceeeds Xmax at some frequency.
     
  13. Ryan Schnacke

    Ryan Schnacke Supporting Actor

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    "So it doesn't matter that your 20Hz tone isn't causing the woofer to move, you're still exceeding excursion because of the 50Hz tone. "

    Okay, that makes sense. But I still think that a sealed subwoofer using the same driver would be in even worse shape since it would need much, much more excursion to play the 20Hz note than the 50Hz note. No way it could match the volume of the vented sub before distortion really set in.

    "You'll see many times, you won't get any more output out of a ported box than a sealed box before the excursion exceeeds Xmax at some frequency."

    I only ever see this below the tuning frequency. Are you sure you're not thinking of power handling as opposed to SPL? Cause vented subs are usually larger than their sealed counterparts, leading to increased sensitivity and decreased mechanical power handling.
     
  14. Dan Wesnor

    Dan Wesnor Second Unit

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    For a sub, where you can tune at 20 or below, this is probably true. I don't see it happening with a lot of mid-woofs, though. Like I said, you just can't generalize.

    For subs, I still think it's best to figure out how much output you really need (in your room, counting room gain) and design to use 1/2 the Xmax to reduce distortion.

    It would be nice if Unibox would had the capability to add in a high-pass and low-pass filter so you could see what the actual response would be when you used it. The low-pass might cut out over-excursion around 50-60Hz (and also lower the F3), and a rumble-filter might add boost making the problem worse at low frequencies.
     
  15. Ryan Schnacke

    Ryan Schnacke Supporting Actor

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    Yeah, for mid-woofers its definitely another issue since you just can't make them cover the whole bass spectrum no matter how hard you try. Without bass management you'll always have some signals below Fb. I always kinda wondered what happens to the guys who buy a pair of small, high-tuned, ported main speakers and set them up as "large" since they don't have a sub yet. Seems like it'd be just a matter of time til they bottom.

    I guess when people talk "excursion" my mind immediately thinks "subwoofer".
     
  16. Shawn Solar

    Shawn Solar Supporting Actor

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