Slotted port help...

Discussion in 'Home Theater Projects' started by Eric Eash, Oct 17, 2003.

  1. Eric Eash

    Eric Eash Second Unit

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    i've started building my sub, and have started to actually think about what could go wrong, and now i'm questioning my slot design. i building a big box for 2 av15s, and in order to port it correctly, slotted seems the only way to go. the inner dimensions of my box will be 22" wide and 36" long. i had planned on a 4"x22" slot, using the bottom wall as well as the side walls to form the port, with another sheet of plywood built into the box to form the top side of the port. for my tuning frequency of 19Hz, i need to make the port 30" long. what i'm wondering is, with the port stopping short of the back wall of the box by only 6", will the port act as a longer port than what it is cut at? should i stray away from using any of the box walls as port sides? this is what i'm talking about...speaker design

    eric
     
  2. Eric Eash

    Eric Eash Second Unit

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    do you guys understand what i mean? i just don't want the back of the speaker being so close to the port that it throws off tuning. another reason i was going to use a slotted port is because finding pvc pipe that's 10" in diameter is though.

    eric
     
  3. Pete Mazz

    Pete Mazz Supporting Actor

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  4. Eric Eash

    Eric Eash Second Unit

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    thanks for the link pete, some good reading. although the article states an end correction factor for using only one wall of the enclosure, i wonder about using 3. i think the info has helped me alot though so thanks once again. what i'd really like is 2 spun aluminum 8" flared ports to match the cones of the AV15's.

    eric
     
  5. Brian-K-Owens

    Brian-K-Owens Stunt Coordinator

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    From the JL site mentioned above:

    "The effective length of the port is found by adding an end correction factor. An end correction factor is necesary because more often than not, one wall of the port is also one wall of the enclosure and this wall extends beyond the end of the port thus effectively adding length to the port (remember, the driver can't "see" the length of the port, it can only go by what it "feels" is going on).

    Calculating end factor may sound like it would be more trouble than it's worth, but it's actually quite simple. To calculate end correction factor, simply add one-half of h to the physical length you calculated above (L1 + L2)."


    Is this right? I have been trying to find info on this for a couple of weeks. 1/2 "h" seems like a very gross approximation. Looks like the ratio of width to the height of the slot would have to come into play.

    If a slot port is 10" wide and 3" high, that formula says the effective length correction is +1.5". But if it is 3" wide, and 10" high, then the effective length correction is +5". Seems backward, since the first example above actually has more surface area that extends as part of the enclosure.

    Anyone know the real formula to determine end correction when one side of the port is part of the enclosure?

    Brian
     
  6. Eric Eash

    Eric Eash Second Unit

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    or what about using 3 of the enclosure walls?
     
  7. JamesFarina

    JamesFarina Agent

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    Hi Eric,

    You can get sonotube in just about any diameter you want. So don't rule out the circular port (it doesn't have to be pvc)

    Jim F
     
  8. Eric Eash

    Eric Eash Second Unit

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    yeah, i know. but i live in japan. not sure about aquiring sonotube over here.

    eric
     

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