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calculations

Discussion in 'Home Cinema Building DIY' started by go5, May 11, 2003.

  1. go5

    go5
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    Maybe this would be an easier way if anyone is willing to help.

    I will post my specs for the speakers and any experienced cabinet makers maybe can give me what their dimensons would be based on these. I am getting confused by different results I am getting and I have never attempted this before so any input would be greatly appreciated.

    Vas 130.000 litres
    Qts 0.320
    Qes 0.450
    fs 26.0 Hz
    Xmax 7.62 mm
    Dia. 30.48 cm
    PEMax 250 watts

    Constants
    Ro 1.18 kg/m^3
    c 345.00 m/s



    Desired Qtc 0.70
    Qr 2.19
    Vr 3.80
    Vb 34.21 litres
    Fb 56.9 Hz
    F3 57.5 Hz

    Sd 7.30E-02 m^2
    Vd 5.56E-04 m^3
    n0 0.005
    SPL 88.9 dB/1W/1m
    K1 1.372
    K2 113.374
    Amax 1.000
    Par 1.372
    Per 280 watts
    PeakSPL 93 dB

    thanks for your time and help

    Mike
     
  2. themadhippy

    themadhippy
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    for a sealed box a very quick calculation gives a box size of around 40 liters and fb at 57hz pretty simalar to your figures.
    the "golden ratio" for box dimmesions gives around 21cm x 35cm x 57cm (internal measurements)
     
  3. go5

    go5
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    thanks for that quick calculation

    but 57 cm seems a little big or maybe it is just me
     
  4. shodan

    shodan
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    WOW!! What language was the first post in?
     
  5. themadhippy

    themadhippy
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    for a 12" driver the cabinet dont seem that big,says the man with 171 liter cabs and 8"drivers :D
     
  6. go5

    go5
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    here are some specs changes that were made, better than the original. does it change anything??

    Vas 130.000
    Qts 0.320
    Qes 0.450
    fs 26.0
    Xmax 7.62
    Dia. 30.48
    PEMax 250

    Constants
    Ro 1.18
    c 345.00



    Enclosure Specifications
    Desired Qtc 0.60
    Qr 1.88
    Vr 2.52
    Vb 51.68
    Fb 48.8
    F3 58.9

    Sd 7.30E-02
    Vd 5.56E-04
    n0 0.005
    SPL 88.9
    K1 0.741
    K2 110.696
    Amax 1.000
    Par 0.741
    Per 151
    PeakSPL 113
     
  7. go5

    go5
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    so i have spent all afternoon researching, playing with numbers, etc. and have come to a reasonable ( I think) calculation.

    1.82 cubic feet which equals 3144.96 cubic inches

    a 12 " x 15" x 18.75 " (golden ratio included) enclosure equals 3375 cubic inches.

    I read up to a 20% difference would not make a noticable difference.

    do these dimensions make senses according to the above t/s parameters?

    thanks for info!
     
  8. themadhippy

    themadhippy
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    best thing to do is a throw cabinet together and give it a listen,if you make it bigger than required then you can easily reduce the volume by adding bricks,books ect rather than having to keep rebuilding test cabs.
     
  9. MikeK

    MikeK
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    Have you bought this/these drivers yet?


    An F3 of 50odd Hz doesn't look impressive, at least on paper anyway. If that's a sealed design, you could EQ it, or Linkwitz it etc, but as it stands, if the figures are accurate, you may be in for a bit of a disappointment!

    Also , what type of subwoofer do you want to build?
    Sealed, ported, specific size, specific cost.............?
     
  10. go5

    go5
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    I already on the driver that is why I would like to hopefully use it. It is goingbe used for a home theatre sub. I read that sealed was the best option ( not according to all) , plus easy to build for a first timer like myself. As far as cost, just cost of materials as I already have an amp. Size depends on what the calculations tell me to do. The size I have came to is posted above.


    Mike
     
  11. MikeK

    MikeK
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    If it's just the box which you have to pay for, then even if the end result isn't all you'd hoped for, it's not exactly going to cost a lot - B&Q will cut up an 8x4 sheet of 18m MDF for around £15 (depending on the amount of cuts etc).

    If it's a sealed unit you are after, you're looking at around 2cu ft.
    Don't be too worried about being ultra exact - it's really not "that" important!

    You may be better off though, doing a ported box of around 2.5cu ft and using a port, 12"x4" (circular), giving an F3 of about 35Hz.

    However, given that increasing the volume of the sealed box to 2.5cu ft won't really affect it's performance that much, then why not build that - if it's not what you hoped for, you can the simply switch to a ported alignment by simply fitting the tuning port - no need for a new box!
    You may need to watch the excursion though, at 7.6mm, it's not that big for a 12" driver.
     

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