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  1. #21

    Default Re: Different Slot Port ratio's ,..equal diff lengths ??

    Quote Originally Posted by isaeagle4031 View Post
    I just tried that calculator. Using a known build, I tried a 3 cube enclosure with two 4in ports.This came out to 33in. A slot port of 25in came out to about 25in. What we don't know is how that site is doing it's calculations. In contrast, my slot port is 31in and tuning is right at sim in wbcd.
    What does this mean " A slot port of 25in came out to about 25in. ?? and what is wbcd ?

  2. #22
    Join Date
    Sep 2005
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    Default Re: Different Slot Port ratio's ,..equal diff lengths ??

    A pair of 4"id (round) vents have an area of 4*pi*2, or 25.1 sq.in.
    Your calculator wanted a 33" long vent to tune that 3cf box to (whatever).

    Same software (or website) using a 25 sq.in. slot vent (maybe 1" x 25", or 2" x 12.5") wanted a length of only 25" for the same box/tuning.

    eagle BUILT that box with an equivalent slot vent, and made it 31" long, and it came out correctly to wbcd's predictions (should NOT be 25"). wbcd is another one of Jeff Bagby's softwares (stands for WooferBox?Design - not sure actually, I use WinISD). maybe it's WooferBoxDesignCalculator? - which I guess would be "wbdc".

    chris

  3. #23

    Default Re: Different Slot Port ratio's ,..equal diff lengths ??

    Quote Originally Posted by Chris Roemer View Post
    A pair of 4"id (round) vents have an area of 4*pi*2, or 25.1 sq.in.
    Your calculator wanted a 33" long vent to tune that 3cf box to (whatever).

    Same software (or website) using a 25 sq.in. slot vent (maybe 1" x 25", or 2" x 12.5") wanted a length of only 25" for the same box/tuning.

    eagle BUILT that box with an equivalent slot vent, and made it 31" long, and it came out correctly to wbcd's predictions (should NOT be 25"). wbcd is another one of Jeff Bagby's softwares (stands for WooferBox?Design - not sure actually, I use WinISD). maybe it's WooferBoxDesignCalculator? - which I guess would be "wbdc".

    chris


    Try it with a square port 5"x5" and it should be much closer.

    Try an example with just one port to make things less complicated maybe

  4. #24
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    Default Re: Different Slot Port ratio's ,..equal diff lengths ??

    I used an example for a box that I have actually built and a program, wbcd-woofer box & circuit designer, that I know to be accurate. I am not sure how or why the program you linked has a difference other than it maybe compensating for the volume taken up by the port.

    ime, I would not trust the accuracy of that site. Neither of the port types are correct in their lengths. Surface area and length vs net volume determine tuning freq of the enclosure. While there is some validity to friction of the two port types based on the perimeter, IMO it is for our purposes a non issue. For high SPL car audio, it maybe, but not in home audio.

    Oh and I meant 25sq in came out to a length of 25in which I know to be about 5 in short for my tuning of 25hz.

  5. #25

    Default Re: Different Slot Port ratio's ,..equal diff lengths ??

    Quote Originally Posted by isaeagle4031 View Post
    I used an example for a box that I have actually built and a program, wbcd-woofer box & circuit designer, that I know to be accurate. I am not sure how or why the program you linked has a difference other than it maybe compensating for the volume taken up by the port.

    ime, I would not trust the accuracy of that site. Neither of the port types are correct in their lengths. Surface area and length vs net volume determine tuning freq of the enclosure. While there is some validity to friction of the two port types based on the perimeter, IMO it is for our purposes a non issue. For high SPL car audio, it maybe, but not in home audio.

    Oh and I meant 25sq in came out to a length of 25in which I know to be about 5 in short for my tuning of 25hz.
    Just a question : the wbcd site you calculate on , does it have a Specific calculator for Slot ports or do u just convert from a round port calculation??

  6. #26
    Join Date
    Oct 2010
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    Grand Blanc, MI
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    Default Re: Different Slot Port ratio's ,..equal diff lengths ??

    Let me this time preface what I am about to say with "This is my opinion, I have nothing to back it up."

    The calculator that was used at the beginning of this thread has a selector titled "Are You Using a Slotted Port?" This is adding in an end correction of calculated port length minus half the width of the port. This is not well coded and will give different answers if you switch the height and width. At this point I would not trust anything calculated by this site.

  7. #27
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    Default Re: Different Slot Port ratio's ,..equal diff lengths ??

    wbcd is an excel based program, not a site. I am doing ths conversion from round port to slot port and adjusting the net volume for my enclosure. I have also used winisd and it agreed with wbcd.

    I think that Duane maybe correct with the site adding in end correction. Except in the example I gave, it called for a shorter slot port than it did for a pair of 4" Ports.

  8. #28
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    Default Re: Different Slot Port ratio's ,..equal diff lengths ??

    WBCD is Jeff Bagby's "Woofer Box and Circuit Designer" software. Runs in Excel on the PC. A popular and comprehensive box / circuit designer.

  9. #29
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    Kings Mountain, NC
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    Default Re: Different Slot Port ratio's ,..equal diff lengths ??

    Good god. The formula does not care about the shape...all it cares about is the area of the port to calculate the port volume.

    Go look at the equation yourself.

    Why are you being so obtuse?

  10. #30
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    Default Re: Different Slot Port ratio's ,..equal diff lengths ??

    Quote Originally Posted by winslow View Post
    Good god. The formula does not care about the shape...all it cares about is the area of the port to calculate the port volume.

    Go look at the equation yourself.

    Why are you being so obtuse?
    If this was directed at my last post- the web site calculator IS using shape to try to apply an end correction. Try it out.

  11. #31
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    Default Re: Different Slot Port ratio's ,..equal diff lengths ??

    Quote Originally Posted by randyc1 View Post
    This calculator uses the "long-side flush" approximation when calculating port length, that is, it subtracts 1/2 slot width from port length, under the assumption that the slot walls continue past the port mouth. This is purely geometric, no cosideration that I can see for non-linearities resulting from perimeter lengthening.

    Have fun,
    Frank

  12. #32

    Default Re: Different Slot Port ratio's ,..equal diff lengths ??

    Quote Originally Posted by fbov View Post
    This calculator uses the "long-side flush" approximation when calculating port length, that is, it subtracts 1/2 slot width from port length, under the assumption that the slot walls continue past the port mouth. This is purely geometric, no cosideration that I can see for non-linearities resulting from perimeter lengthening.

    Have fun,
    Frank
    Not really , try with 1 port,.. (.84) enclosure,..tuned to 34hz ,..1.77 x1.77 Port opening ,... then with .5 x 6.28, Opening ?

  13. #33
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    May 2008
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    Fort Wayne, Indiana
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    Default Re: Different Slot Port ratio's ,..equal diff lengths ??

    Quote Originally Posted by fbov View Post
    This calculator uses the "long-side flush" approximation when calculating port length, that is, it subtracts 1/2 slot width from port length, under the assumption that the slot walls continue past the port mouth. This is purely geometric, no cosideration that I can see for non-linearities resulting from perimeter lengthening.

    Have fun,
    Frank
    Yep, when choosing a "slot" port, it is assuming that one side of the box shares a side with the port. This makes the port effectively longer. The same is true if you were to put a round port directly adjacent to a wall. It would make the effective length longer than the length of just the round port itself. I'm not saying the calculation is correct, but I'm pretty sure that's what it is TRYING to account for.

    If you de-select the "slot" port option (this would assume a rectangular port in the middle of the box), then you will see that the calculations come out to the same length for the same cross-sectional area, regardless of what the two dimensions are.

  14. #34
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    Default Re: Different Slot Port ratio's ,..equal diff lengths ??

    Quote Originally Posted by randyc1 View Post
    Not really , try with 1 port,.. (.84) enclosure,..tuned to 34hz ,..1.77 x1.77 Port opening ,... then with .5 x 6.28, Opening ?
    Actually, it works out perfect on that site (with a small amount of error in precision):

    0.84 cubic foot box tuned to 37 Hz,

    1) Round port, 2" diameter (3.14 sq in), = length of 5.9"
    2) Rectangular port (NOT "slotted"), 1.77" x 1.77" (3.13 sq in) = length of 5.88"
    3) Rectangular port ("slotted"), 1.77" x 1.77" (3.13 sq in) = length of 5" (5.88" - 1/2 of 1.77" width = 5.88" - 0.885" = 5")
    4) Rectangular port ("slotted"), 0.5" x 6.28" (3.14 sq in) = length of 2.76" (5.88" - 1/2 of 6.28" width = 5.88" - 3.14" = 2.74")

    Edit: If I remember correctly, I believe the terms width and height are "reversed" on the site. You should subtract 1/2 the "thickness" of the slot, so in the 0.5" x 6.28" slot, you should only subtract 0.25". If you switch the numbers in their calculator, this is shown.

  15. #35
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    Default Re: Different Slot Port ratio's ,..equal diff lengths ??

    It's cool, and not directed at you.

    Quote Originally Posted by duanebro View Post
    If this was directed at my last post- the web site calculator IS using shape to try to apply an end correction. Try it out.

  16. #36
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    Kings Mountain, NC
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    Default Re: Different Slot Port ratio's ,..equal diff lengths ??

    Been doing car audio for over 20 years, built my share of ported enclosures...seen ports go from round, square, dollar bill shapes, "L"s for Lexus, "A"s for Acuras, and "H"s for Hondas...and everything in between. And as long as the area is preserved, the tuning remains the same.

  17. #37

    Default Re: Different Slot Port ratio's ,..equal diff lengths ??

    Quote Originally Posted by randyc1 View Post
    Not really , try with 1 port,.. (.84) enclosure,..tuned to 34hz ,..1.77 x1.77 Port opening ,... then with .5 x 6.28, Opening ?

    Sorry ,you are right fbov ,..it is Subtracting 1/2 of Width, of the calculation (when Slotted Port is Checked)

    (Slotted Port) UN-CHECKED , the lengths measure very close to Round port.

    ...But who the heck uses a Slot Port in the middle of the box, WITHOUT using an box wall ??

  18. #38
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    Default Re: Different Slot Port ratio's ,..equal diff lengths ??

    I've done this a few times. Different reasons. Didn't want to use a round port. The box was 9" wide and 24" or so tall. Port was 4.5" x .75" and located about 4" from the top. Bracing also attached to the port.

  19. #39
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    Default Re: Different Slot Port ratio's ,..equal diff lengths ??

    Quote Originally Posted by randyc1 View Post
    ......But who the heck uses a Slot Port in the middle of the box, WITHOUT using an box wall ??
    No problem...

    You ask a seemingly obvious question, and I'll be the second to say I've done this... sort of. I made a removable slot port, removable so I could adjust length if desired, and to keep aspect ratio down. Both ends were flared, a 3/4" spacing to the walls on 3 sides, and I still included the 1/2 width correction. Tuning's real close to the target.

    HAve ufn,
    Frank

    PS the attached shows the relationships among box F3/F10 and port length for a given box and driver. Note that the F3 curve has a broad flat region... in this range, port length has very little effect on F3, making it a poor tool for verifying port length. Better F10, at least it's monotonic!
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

Name:	Port Length vs Tuning Anarchy.jpg 
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Size:	13.0 KB 
ID:	35084  

  20. #40
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    Sep 2005
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    Default Re: Different Slot Port ratio's ,..equal diff lengths ??

    Quote Originally Posted by winslow View Post
    Good god. The formula does not care about the shape...all it cares about is the area of the port to calculate the port volume.

    Go look at the equation yourself.

    Why are you being so obtuse?
    Actually it uses the cross-sectional area to calculate the length for a given tuning, not to calculate port volume. Port tuning for a given length (or length for a given tuning) is directly proportional to the cross-sectional area of the port. It is not proportional to the volume of the port.

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