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Reload Thread: Is this a dumb idea

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    nutcasevi's Avatar
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    Is this a dumb idea

    I was gonna build a box like this for my 15L7. Its gonna be 3.0 cu ft ported to 48hz. I have 2 L7s but I want to build 2 boxes for them. This design works the best for fit in my car. but i want to know if this design sounds like crap before I do it.
    Attached Thumbnails Attached Thumbnails Is this a dumb idea-box.jpg  







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    yea it would work from my point of view.

    let somebody else like rangerman or jmac. they're box geniuses




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    Well, is the port going to have ample clearance?

    THe shape of the box will not matter, so long as the volume is intact. Some swear better output with uglier-looking boxes...



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    SPLThunder's Avatar
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    if i understand the drawing right i would say dont have the port directly in front of the sub. You never want to be able to see your woofer through a port. Just a good rule of thumb




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    nutcasevi's Avatar
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    Yea there's enough port clearance, the port is only 9 inches deep and the box is like 24 inches deep. The sub is mounted normal its just looks backwards according to the picture.




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    yeh, u look like ur in good shape.. but to me, i would think it would be good to see the sub.. is that "rule of thumb" from a reliable source, or just a made up one, haha?



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    SPLThunder's Avatar
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    The rule of thumb is based on the physics of the pressure that sound waves create. You dont want the head on pressure from a cone pushing directly into the port. It will not have distributed evenly inside the box. You are also more likely to get air moving in and out of the port Which we all know is not what a port is used for. Air inside the port is just suppose to resonate. In fact if you had a perfect port you shouldnt be able to feel and air move in or out at all.
    When a port is aimed in a different direction than the cone the pressure should have (i say should based on design) distributed itself through the whole box first which is why we make the boxes a certian size.




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    u seem to BE the reliable source.. great explanation, and it sounds like my hunch was right.. and ide take SPLthunder's words into consideration..



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    Will do, so pretty much stick the port anywheres but there.




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    Originally posted by SPLThunder
    The rule of thumb is based on the physics of the pressure that sound waves create. You dont want the head on pressure from a cone pushing directly into the port. It will not have distributed evenly inside the box. You are also more likely to get air moving in and out of the port Which we all know is not what a port is used for. Air inside the port is just suppose to resonate. In fact if you had a perfect port you shouldnt be able to feel and air move in or out at all.
    When a port is aimed in a different direction than the cone the pressure should have (i say should based on design) distributed itself through the whole box first which is why we make the boxes a certian size.
    Not to start a battle....but I have a bit of a physics background as well. Though your point about even distrobution is well taken, I disagree that a port is not intended to move air. With your physics background I am sure you understand that the port is simply a open ended tube with a fixed length. Given the fixed demension, you also understand that only a limited number a frequencies are able to resonate at that length (more than one because of first, second third harmonics /resonate frequencies. Since we know that only a few out of the hundreds of frequencies have a potential to resonate, I find it hard to belive that soooo many people would see increases in spl over a range of frequencies if the port was not also intended to move air through it. Thought i need bit more time to think about it, most manufacteres suggest the port be firing paralell with the driver. Now if you point about even distrobution is correct (and it very well may be), this person could fix that by simply bending the port inside the box, correct? Please correct me if I made a mistake in my logic, again not trying start a battle, just having a civl discution.



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    Originally posted by evo2k3
    Not to start a battle....but I have a bit of a physics background as well. Though your point about even distrobution is well taken, I disagree that a port is not intended to move air. With your physics background I am sure you understand that the port is simply a open ended tube with a fixed length. Given the fixed demension, you also understand that only a limited number a frequencies are able to resonate at that length (more than one because of first, second third harmonics /resonate frequencies. Since we know that only a few out of the hundreds of frequencies have a potential to resonate, I find it hard to belive that soooo many people would see increases in spl over a range of frequencies if the port was not also intended to move air through it. Thought i need bit more time to think about it, most manufacteres suggest the port be firing paralell with the driver. Now if you point about even distrobution is correct (and it very well may be), this person could fix that by simply bending the port inside the box, correct? Please correct me if I made a mistake in my logic, again not trying start a battle, just having a civl discution.
    Agreeed, We use a port pretty much to hear the back side of the cone as well as the front. Generally a 3db gain is achieved from just having a port. Wave dynamics is not my thing. I am more interested in electrical engineering. (by the way i am a physics undergrad) However from the professors i have talked to this seems to be my conclusion. When a frequency resonates it becomes very loud in a port however dont you still hear other frequencies even if they arent at a certian harmonic. If a port is tuned to 30HZ the longest wave to resonate will be 30 hz. You will still hear (just not as loud) higher than that even if the end doesnt catch on a node or antinode of the wave.
    you have to have heard a port "farting" before. This is not good. This is when the speed of air moving in and out in the port moves faster than i think 5% the speed of sound. However when you put you hand in front of a port you dont feel it move in and out at 30 times per second. You feel the "farting" which is not the spl that you want to be hearing. If you could feel the SPL it should be resonating at 30 times per second and only as much as you woofer is moving. It should be equal to the back of the cone. Again this is not my major just how i have come to understand it. If it is not right i would love to be corrected. I always enjoy learning new things.




  12. #12
    evo2k3's Avatar
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    i will be an entering computer engineering major of U of Illinois this august, I to have little background in wave dynamics.....i think you hit on the key that porting allows you to "hear" both the compressions and rarefractions from the front of the cone (like you always hear), but alos those created by the back of the cone. You definitly hear the frequencies other than the resonant frequencies, just not nearly as loud as a standing wave is not created in those cases. I would like to know more about the wave distribution and how placment of the port effects it, thats just beyond my knowledge base at the moment. btw....where you getting your degree from?



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    UW-LaCrosse
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    no thats not so bad as long as when you build a rectangular enclosure you build it with the ratios of
    1.62-1-0.62 or
    1.49-1-0.65 or
    1.25-1-0.8

    This will eliminate almost all standing waves in a box.




  15. #15
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    does this even need to be accounted for in a sub box though? that would have to be a huge box to get standing waves inside it.



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