Ported box calculations

Discussion in 'Home Theater Projects' started by JoeFish, Aug 5, 2003.

  1. JoeFish

    JoeFish Stunt Coordinator

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    Hello again. I'm still playing with Unibox, and trying to learn about building a vented design. I've built sealed enclosures before, but vented is a new thing for me.

    I understand that below a box's tuning frequency, the driver will "unload" because there is no airspring behind it like in a sealed box. Response and power handling drop way down.

    The thing I'm not sure of is, how does one choose a tuning frequency? I've heard of "tuning" a ported box. Does this mean I'd build the box and tune it later by changing the length of the port?

    Any hints, advice, links to more info, will be greatly appreciated.

    -Joe
     
  2. Chris Tsutsui

    Chris Tsutsui Screenwriter

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    I'd use a software program to model the response of a speaker that you're designing. Play with the box volume and the tuning frequency until you achieve a frequency response you like.

    I'd also keep in mind the "Q" of the sub which is like how chesty "Impact" or smooth your bass will have.

    I think of it as playing with a few variables and making any sacrifices until you've designed a subwoofer you think you'll like.

    If you post your driver model and how big the box you'd like to build i'm sure you'll get some recommendations on a tuning frequency. What'll also help is what type of bass sound you're looking for.
     
  3. JoeFish

    JoeFish Stunt Coordinator

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    I haven't decided on a driver yet. I've been looking at the Adire ShivaIII or DPL-12. I just started looking at some of the Dayton drivers, as well. The Shiva has a reference design for the EBS alignment that ends up at 142.5l. 22.5" cube. I can live with that size, I think.

    I'm looking to use the system about 50/50 HT/Music. I'd like really deep bass extension, but I don't require high SPL. So, I'd like accurate, deep bass.

    I'm using Unibox to model the design. It's just a lot of new variables for the ported system.

    How does my Q relate to impact or smoothness? ie, high Q = ? And do you mean the Q of the sub itself, or the system? I notice ported systems don't have a Qtc like a sealed system does.

    Thanks,

    -Joe
     
  4. JoeFish

    JoeFish Stunt Coordinator

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    OK, I've been playing with the DPL-12. This looks a little too good to be true...Let me know if I'm on the right track here. The freq response graph looks dead flat until about 20hz, then rolls off smoothly.

    Click here to see the plot from Unibox.

    Here's the stats I came up with. Fb of 15.5hz! I used 2x 3" ports. They end up being 14" long. The box is 22.5". The driver's Qts is 0.40.

    No of ports 2
    Inside port dia. 7.62cm
    Port area 91.21cm2
    Port end correction 0.646

    Physical Vb 142.5l
    Absorption, Qa 20
    Leakage, Ql 30
    Port, Qp 120
    Alpha, a 0.986
    Vb 151.2l
    Fb 15.50Hz
    F3 14.72Hz
    Response peak 0.37dB
    Peak at 29.92Hz
    Port length 70.80cm
    Port 1. resonance 231Hz

    -Joe
     
  5. Brian Bunge

    Brian Bunge Producer

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    Joe,

    I think you need to double check your port lengths. If I use 2 3" ports I get upwards of 30" lengths.
     
  6. JoeFish

    JoeFish Stunt Coordinator

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    Brian,

    The information above is cut n' pasted from Unibox. The port length is 70.80cm, which is 27.87". Based on the fact that the length doubled when I switched from 1 to 2 ports, I assumed that value was the TOTAL length of both ports. Which is where I came up with 14" per port.

    Is that a bad assumption?

    -Joe
     
  7. Chris Tsutsui

    Chris Tsutsui Screenwriter

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    If the software tells you to use 28" then that's what you'll have to use for both ports.

    I'd try and get those port lengths shorter, even if you have to create a slight hump in the response by tuning it higher.

    Remember that the in-room frequency response will probably be different than that software sim.
     
  8. Patrick Sun

    Patrick Sun Studio Mogul

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    Strangley enough, the more ports (i.e. surface area for the ports) you add, the longer the ports will be (for a specific tuning frequency in the same sized enclosure). That's just how it works.

    So it comes down to a trade-off between port noise and port length. Use a small port, and you could get port noise, but the port length will be shorter. Use a wider port, get less port noise, but the port length gets longer. (using multiple ports can be treated like using a wider port).
     
  9. Brian Bunge

    Brian Bunge Producer

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    Joe,

    Nope, that's 27.87" for each port. Which means you'll have smoke stacks sticking up out of your box![​IMG]
     
  10. JoeFish

    JoeFish Stunt Coordinator

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    HA! The image of the subs with huge stacks sticking out is pretty damn funny [​IMG]

    I'll finegle the numbers and let you know what happens. Thanks for the help,

    -Joe
     
  11. JoeFish

    JoeFish Stunt Coordinator

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    Well, if I keep the box the same size (142.5l) and make a single port 4" dia, 14" long, it tunes to 19.50hz. There's a 3db bump at 24hz. What does that mean for me, sonically speaking?

    -Joe
     
  12. Brian Bunge

    Brian Bunge Producer

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    Joe,

    You don't want a peak in the response like that. It'll increase the port air speed at 24Hz and possibly push it into the audible range.

    Also, do you have some kind of fixation with 14" long ports?
     
  13. JoeFish

    JoeFish Stunt Coordinator

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    LOL Actually yes...yes I do...

    No, I don't have a fixation. It just worked out that way. I didn't want to make it so long that it butts up against the other side of the cabinet, but the shorter I make it, the higher Fb gets, and the bigger the bump gets. I'm still learning, take it easy on me [​IMG]

    [Edit] That said, is there a general minimum distance the end of the port should be kept from the other side of the cabinet? [/Edit]

    -Joe
     
  14. Patrick Sun

    Patrick Sun Studio Mogul

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    At least the width of the port opening.
     

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