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Thread: Positioning of ports and driver?

  1. #61
    Senior Member RMC's Avatar
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    When all is factored in volume calculation and if still too large then you get say a cubic foot of cheap old books and put them at the bottom of the box, therefore taking some excess volume.

    That's an example of box salvage mode.

  2. #62
    Member Greg_M's Avatar
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    no other drivers in the boxes.
    if I use 2x2s that would be another 279 cu in.

    No idea how to figure port displacement.

    We're down to 6.1915 cu ft not counting the ports.

  3. #63
    Senior Member RMC's Avatar
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    Greg,

    RE Port Displacement

    Volume of a cylinder: V= Pi X radius squared X height or length of cylinder


    Start with area of a circle: Pi X radius of circle squared, so 3.1416 X the circle's radius squared, so you get square inches

    From that result multiply by vent length in inches which will give you cubic inches

    e.g. Assuming your down to 6 cu.ft. then put a cubic foot of books in the speaker box and you got 5 cu.ft. net, the beauty of books or wood pieces is you can play with volume by removing some or adding some if need be.

    Or you may decide to stick to a 6 cu.ft. box, then have to model it in Win ISD to see if the results can be made to your satisfaction while playing with tuning frequency.

  4. #64
    Member Greg_M's Avatar
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    Ok
    Down to 6.145 cu ft

  5. #65
    Senior Member RMC's Avatar
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    then you decide what net volume you want to use 5 or 6 cu.ft., read post # 63

  6. #66
    Senior Member Ian Mackenzie's Avatar
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    Hi Greg,

    If you are not familiar with calculating the bracing here is an example using 2 x 2 lumber

    This bracing represents shelf bracing that is typical of the enclosure bracing used in the vintage systems. Having built a number of enclosures for these drivers over the past 30 years l can attest to the structural integrity. Adding too much bracing is bad and can cause more problems than it solves.

    Qty 10 x 17 inch braces for sides, top and bottom. 170 x 4 = 680
    Qty 5 x 22.75 inch braces for front and rear 113.74 x 4 = 455

    Total 1135 cu inches 3
    Cu Ft3 0.656

    Add
    Driver 0.2
    Ports 0.145
    Total 1.00 cu ft3

    Gross enclosure volume 6.7 cu ft3
    Less 1.00 cu ft3

    Sub total 5.70 cu ft 3

    Add
    Volume gained with 1 inch fibre glass on 5 sides
    0.57 cu ft3 (10%)

    Net Volume 6.27

    QL = 7 (assuming no box leaks and a well braced box. Note a QL = 5 is typical for large enclosures due to panel flexing, in this instance there enclosure is over volumed from 5-10%. So you are in the ballpark. By doing some modelling of the port tuning and comparing this to the actual measured port tuning you can then start to fine tuning the design. )

    You could make 6 cu ft 3 your target volume and see how it goes. The tuning Fb will be a but lower than the 5 cu ft 3 but you might like it. If you don't like it you can always go for smaller target volume by asking some solid mass inside the enclosure.

    Edit

    Attached in top image is a snap shot of the jbl 2235H spec driver in Leap Enclosure Shop
    Blue is the 6.27 cu ft 3 enclosure tuned to 28 hertz.
    The f3 is 29.75 hertz and is -6 dB at 26 hertz.

    Red is the 5 cu ft3 enclosure tuned to 30 hertz.
    The F3 is 32.5 hertz and is -6 dB at 28 hertz.

    These results are models only and demonstrate the differences in one tuning to another.

    As you can see there is no free lunch but if you want low bass the 6.27 cu ft3 enclosure is an interesting design. In the bottom image the original Jbl 4343 was 5.5 cu ft3 in red was tuned empirically for the best overall subjective results. The f3 is 31 hertz with a 29.5 hertz tuning. The blue curve is the recommended 5cu ft3 enclosure tuning.

    The scale is 2 dB per division so you can see the effect of different tunings and enclosure size.

    What is interesting is that the human ear is quite sensitive to low frequency amplitude variations. Given your larger than recommended enclosure size you can work out with some experimentation what works best for you in your listening environment.

    The science as Richard refers to it is well documented in terms of working out an enclosure and tuning from scratch. Given JBL recommendations l felt it was appropriate to
    Illustration the effect of some different tunings and enclosure volumes to give you an idea of what to expect.

    The graph scale is 2 dB per division.
    Attached Images Attached Images   

  7. #67
    Member Greg_M's Avatar
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    Thank you for your time and effort Ian Mackenzie.

    I was worrying about making stupid arithmetic errors, so this morning I did a simple spread sheet.
    There are several variables like internal dimensions and bracing dimensions etc. (Note two different sizes of corner bracing)
    Here's where I am as of this morning (while the glue is drying from yesterday).

    Name:  Enclosure Calc.jpg
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  8. #68
    Member Greg_M's Avatar
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    Quote Originally Posted by Ian Mackenzie View Post
    Hi Greg,

    Add
    Volume gained with 1 inch fibre glass on 5 sides
    0.57 cu ft3 (10%)

    Net Volume 6.27
    I think it's extremely interesting that volume can be gained with insulation.
    If there is a formula or way to estimate this, I'd love to include it in my little spreadsheet.
    Thanks again

  9. #69
    Senior Member Ian Mackenzie's Avatar
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    http://www.cieri.net/Documenti/JBL/D...re%20Guide.pdf




    If you refer to the links below you will pretty well find everything you need to get a good results.

    Jbl use and recommend fibre glass with guidelines on where and how much to use. If you decide to do something else your on your own. My recommendation is to follow the Jbl recommendations. Attempting to come up with your design using your own methods could easily result in a situation where you end up being not on the playing field.

    You can comfortably go up or down one cubic foot in volume from the recommendation of 5 cu feet as long as you adhere to the tuning process outlined by Jbl. The trade off is enclosure size versus system LF output and other performance criteria outlined by Jbl. There are articles on the lossy properties of fibre glass. Just google.


    http://www.lansingheritage.org/image...ual/page21.jpg


    http://www.lansingheritage.org/image...ual/page29.jpg


    http://www.lansingheritage.org/image...ual/page30.jpg


    http://www.lansingheritage.org/image...ual/page31.jpg


    http://www.cieri.net/Documenti/JBL/D...re%20Guide.pdf

  10. #70
    Member Greg_M's Avatar
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    Interesting reading.
    Thank you

  11. #71
    Member Greg_M's Avatar
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    Quote Originally Posted by RMC View Post
    Greg,

    RE Port Displacement

    Volume of a cylinder: V= Pi X radius squared X height or length of cylinder


    Start with area of a circle: Pi X radius of circle squared, so 3.1416 X the circle's radius squared, so you get square inches

    From that result multiply by vent length in inches which will give you cubic inches

    e.g. Assuming your down to 6 cu.ft. then put a cubic foot of books in the speaker box and you got 5 cu.ft. net, the beauty of books or wood pieces is you can play with volume by removing some or adding some if need be.

    Or you may decide to stick to a 6 cu.ft. box, then have to model it in Win ISD to see if the results can be made to your satisfaction while playing with tuning frequency.
    I was expecting different calculations other than a cylinder since it's hollow.
    Easy enough.

  12. #72
    Senior Member Ian Mackenzie's Avatar
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    No worries.

    Have fun and enjoy your project.

  13. #73
    Senior Member RMC's Avatar
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    RE "I was expecting different calculations other than a cylinder since it's hollow. Easy enough."

    Since its a cylinder volume calculation, i.e. calculation of space taken or air displaced in the box by the cylinder, then it goes without saying, but maybe I should have said, its the cylinder's outside diameter that's considered here, so the radius used isn't based on tube's inside diameter, but rather on outside diameter.

    When an effective port calculation is made i.e. for a device actively passing air in and out when in use, the relevant radius to use would be based on tube's inside dia. However, here we're only concerned about space occupied by the tube inside the cab (btw excluding front panel thickness), regardless if tube size is appropriate or not acoustically speaking. Hope this provides the nuances.

  14. #74
    Senior Member Ian Mackenzie's Avatar
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    Thatís a good point.

    Back in the day used 4 inch cardboard tube.

    If the port was made of 3/4 inch mdf there would be an error with the external volume versus internal.

    I would go bigger than 3.5 inch internal diameter. If the rear of the port is 4 inches or less from the rear put a bend in it. Jbl did this as routine in a number of systems. Itís easy to do with 4 inch pvc pipe fittings.

    Likewise keep the ports at least one diameter from the sidewalls. The effect messes with the port function and you will end up having to compensate with the port length (as Jbl did with the 4343 and 4345 monitors.

  15. #75
    Member Greg_M's Avatar
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    Quote Originally Posted by Ian Mackenzie View Post
    Likewise keep the ports at least one diameter from the sidewalls.
    Does this include one diameter minimum from the top or bottom?

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