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    Reference01's Avatar
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    hertz

    I've heard many people talking about tuning to however many hertz, I was wondering how you do this







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    Re: hertz

    I have the same question cuz i know nothing about ported sub boxes n stuff but im sure if u ask this same question in Box Construction and Fiberglass help section i bet u will get alot more responses. Hope this helps




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    Re: hertz

    i'm gonna ^bump this cause I'd like to know also




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    Re: hertz

    theres a big equation to figure it out, it is on JL's website under tutorials i think, it relates to port area and length and volume of the box



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    Re: hertz

    www.bcae1.com Has tons of info/tools on stuff like this



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    Re: hertz

    Quote Originally Posted by nweibley
    www.bcae1.com Has tons of info/tools on stuff like this
    Absolutely, this site is great.

    But to help out those who are scanning, I'll give a short explanation.

    First:
    Understand how a speaker works.
    A speaker is fed AC electricity of some frequency. Or more accurately, a bunch of frequencies that create a complex waveform. But let's picture a simple sine wave, a single frequency, let's say 40hz.
    In this case, the motor will push the cone in and out, 40 cycles per second.

    But let's think slow-motion for a second. Super slow motion:
    When the cone moves out, it compresses the air in front of the cone...
    ...but it is decompressing the air behind the cone.
    And when the cone moves back in, the opposite happens... it decompresses the air in front of the cone...
    ...but it compresses the air behind the cone.

    So, we begin to
    Understand why a box is needed in the first place.
    If there was no box, when the cone moved out, the compressed air in front of the cone would bleed right around to the backside of the cone, into the vacuum created behind the cone... and vice versa when the cone moved back again.
    This is the definition of cancellation... sound waves by definition are air disturbances, alternating nodes of pressure and vacuum travelling through the air.
    If each time the speaker tries to create a pressure node, it bleeds away into some adjacent vacuum... and each time the speaker tries to create a vacuum node, it is filled by some adjacent pressure source... there won't be any sound... or at least not much.

    So the box, essentially, manages the rear sound wave so that it can't cancel with the front sound wave, since they are 180 degrees out of phase and will cancel each other.

    A sealed box is the simplest.
    It simply constrains the rear wave, so it can't escape and cause the front wave any harm. It's that simple.
    It also contributes to the sound, of course, as the trapped air in the box has a "spring" effect, that affects cone motion... augments the suspension of the subwoofer... so you can manipulate the sound, and the speaker's efficiency, both dramatically, by varying enclosure size.

    And then we get to the enclosures in question here:
    A ported box allows you to use the rear wave.
    A ported box has a port in it.
    Obviously, it's not just a hole, or else you'd have the same cancellations you'd have in the scenario where you didn't have a box at all.
    A port has a port opening area, and a length.

    Think of a port as a delay device.
    Sound travels at a certain speed (the speed of sound), no matter what the frequency is.
    But, sound travels at different speeds through different materials.. and even in air, the speed varies with temperature - and density.

    By creating a port that has a given diameter (or area, I should say, because you can make rectangular or other shape ports too), you can manipulate the port length and create a scenario where the air density in the port is denser, more restrictive if you will, than the ambiant air.
    So, simply put, it slows the air that enters and exits the enclosure, as the speaker is in operation... or, I should say, the sound that exits and enters the enclosure via the port.

    With a given port opening area, by varying the length of the enclosure, you are manipulating the tuning of the enclosure.
    By making the port shorter, you are making the delay smaller, because the port as a whole is less restrictive.
    By making the port longer, you are making the delay longer, because the port as a whole is more restrictive.

    How port tuning corresponds to port length:
    If you tune a port in such a way that it results in a delay sized in such a way that a 40hz tone is effectively 'in phase" with the front of the cone sound, you've tuned the port to 40hz.
    And this effectively means that as you move away from that 40hz tone in either direction (playing music higher in frequency, or lower in frequency), you become more and more out of phase... but not too bad, just a bit. It won't reach the degree where it begins to cancel out for a while in either direction.

    If you were to make the port longer, you'd impart more of a delay, "tuning" to a lower frequency.

    What effects a port has on the sub behavior:
    A sub needs to excurt back and forth to make sound.
    Interestingly, with the port putting this delay on the air trying to enter and exit the box, at some frequencies, the air pressure inside the box will be greater than what a simple sealed box would be, because you have the sub trying to compress the air in the box (or decompress it), and you have the air momentum of the air rushing in the port trying to compress the air in the box (or decompress it) at the same time.
    This makes it harder for the sub to move, and you'll notice that when you play frequencies that are near the tuning frequency, sub excursion is minimized.
    And as you move away from that frequency, sub excursion is increased.
    This makes the sub able to handle more power near the tuning frequency, since you aren't likely to bottom it out.

    But, if you play frequencies that are much below the tuning frequency of the port, you will bottom the sub out rather easily, because the port and sub are now not only out of phase again... but the port is actually behaving more like a simple hole in the box, and that "air spring" that you had with the sealed box (or the ported box operated near it's tuning frequency or above) is simply missing. This makes the sub able to handle very little power below the tuning frequency, as very little power woudl be needed to bottom it out, just like if it had no enclosure at all.
    Something to be aware of.

    Ported boxes can be good, as they allow a subwoofer to play louder than it would be able to otherwise... since you are able to use not only the air that the front of the cone is stimulating to contribute to the sub's output, but you are able to use the air that the rear of the cone is stimulating too... productively, rather than cancelling as it naturally would.

    Hopefully that is clear and helpful!




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    Re: hertz

    Just scanning hu? lol just kidding




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