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Tempest Enclosure Help with LspCAD

Discussion in 'Archived Threads 2001-2004' started by Aaron D, Jun 24, 2001.

  1. Aaron D

    Aaron D Stunt Coordinator

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    I am trying to find the best parameters for a Tempest sonosub that I plan to build. Could anyone tell me if these values are going to work?
    Volume = 230 L
    Ql = 7
    Qa = 30
    Fill = 25%
    Port Length = 57.9 cm
    Port Diameter = 6 in
    Res Freq = 17.9 hz
    I have the program set up for 100 W into 8 ohms. Cone excursion is around 27 mm (I think this accounts for both directions of travel) and port airspeed is around 13 m/s.
    Am I missing something, or does this look OK?
    Thanks for your help.
    Aaron
     
  2. Julian Data

    Julian Data Second Unit

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    Aaron,
    Ok, you have 200W @ 4 Ohms since the VCs are in parallel. The design looks pretty good.
    Your GD is around 22ms @ 20Hz.
    If it was me, I would've gone just a tad lower in the tune say 17Hz but you are fine as it should be a nice box! [​IMG]
    jd
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  3. Aaron D

    Aaron D Stunt Coordinator

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    Julian, thanks for the help.
    Do you think I should tune lower? The sub will be mostly used for movies, but I will use it alot for music as well.
    Also, could you quickly explain the group delay value to me? I'm not sure what it should be (or what it is for that matter).
    Thanks,
    Aaron
     
  4. Patrick Sun

    Patrick Sun Studio Mogul

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    GD - the lower the better (look for the highest value and what frequency it occurs for ). GD is kind like a measure of the "tightness" of the bass from the subwoofer. For example, for properly designed sealed subs, they will have very low GD values, and this translates into what people refer to a sub being "tight" and musical. Also, the lower the GD, the better the transient response.
    If you can keep the GD around/under 20ms from 20Hz and up you should be okay. Most of the time, the GD will tail off down to 7ms around 40Hz, so as long as it's not tremendous huge down low, you should be okay. Low tuned (13Hz and under) enclosures will mimic a "tight" sealed sub, but at the expense of SPLs down in the 20Hz range (always a trade-off in subwoofer design).
    ------------------
    PatCave; HT Pix; Gear; DIY Mains; DIY CC; Sunosub I + II + III; DVDs; LDs
     
  5. Julian Data

    Julian Data Second Unit

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    Aaron,
    I would tune lower as I prefer GD in the vicinity of 18ms or less but that's just me.
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  6. Brett Miles

    Brett Miles Stunt Coordinator

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    Are you using sonotube? If so and you don't have it yet, I can clue you in on a place in KC that has a great deal! I had to drive from Topeka, but it was about $40-60 less than everywhere else I found. Btw, the design looks good to me, but I don't know that much [​IMG] . (I've been learning everything from these two guys among others.)
     
  7. Aaron D

    Aaron D Stunt Coordinator

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    Brett, YGM.
    Pat, as usual, you are quite helpful- thanks!
    Julian, I went back and set the tuning frequency to 15 hz (port went to 86.7 cm long) and the GD went down to ~16.5 at 20 hz. SPL at that frequency didn't change much (I don't think).
    My remaining question is this: will a sub tuned this low still be suitable for music?
    Thanks Again,
    Aaron
     
  8. Jack Gilvey

    Jack Gilvey Producer

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    A sub with that low a GD tends to be very good for music, but that port length can lead to resonance problems.
     
  9. Julian Data

    Julian Data Second Unit

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    Aaron,
    I argree with Jack; you can, in fact, have too long of a port since pipe resonances are an issue. So pay attention to the length of the port.
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  10. Patrick Sun

    Patrick Sun Studio Mogul

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  11. Julian Data

    Julian Data Second Unit

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    With that rule in mind, your proposed tune has surpassed it.
    Pat, I thought it was 4 times the diameter?
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  12. Jack Gilvey

    Jack Gilvey Producer

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    With a low tuning like that, a 4" port might work ok, and be much shorter. If you're only using 100 watts, 4" would work fine at just about any tuning.
     
  13. Patrick Sun

    Patrick Sun Studio Mogul

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    JD, JohnJ from Stryke advocates 5x, and Dan Wiggins didn't quite disagree with him, and in my experience, 9x was too long, so there you go! [​IMG] If you push it to 6x, I don't see much of a problem, but there is a point where the pipe doesn't act like a slug/mass of air, and causes resonance problems.
    ------------------
    PatCave; HT Pix; Gear; DIY Mains; DIY CC; Sunosub I + II + III; DVDs; LDs
     
  14. Jack Gilvey

    Jack Gilvey Producer

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    This article discusses maximum recommended port length not as a function of its diameter, but as a fraction of the wavelengths the port might experience and be excited by, which are dependent on the crossover.
    http://www.adireaudio.com/Tech%20Papers/vent_sizing.htm
    Fwiw, Unibox is the only program I'm aware of which calculates organ-pipe resonances of port lengths.
     
  15. Julian Data

    Julian Data Second Unit

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    Isn't that something of an excerpt from an article at TrueAudio?
    IIRC, I read something like that there.
    Jack,
    I think you are right about Unibox.. I haven't seen any modeling in any other software that I am aware of.
     
  16. Aaron D

    Aaron D Stunt Coordinator

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    OK, after taking the suggestion to use a 4 in port, the length is down to 35.7 cm (less than 4x port diameter) and didn't significantly alter GD or SPL.
    However, port airspeed did change (peak of ~27 m/s at ~11 hz, ~23 m/s at 15 hz). Is this a problem? I thought I read somewhere that it needs to stay under 30 m/s.
    Jack, I will download Unibox today and see what happens. I must admit that I need more time to read the Adire article, though!
    As an aside, Pat, I read somewhere that "Rule of Thumb" originated as a guideline (from back in the day) for the acceptable width of an object that could be used by a husband to beat his wife. Weird, huh? (Of course, I don't condone this!!)
     
  17. Patrick Sun

    Patrick Sun Studio Mogul

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  18. Fred Seger

    Fred Seger Stunt Coordinator

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    Your should try to keep the port airspeed as low as possible(I shoot for under 15m/s), but at 11 and 15 hz you won't have to worry because not much, if any, source material that goes that low, so I wouldn't worry about it.
     
  19. DanWiggins

    DanWiggins Second Unit

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    To emphasize what Fred just posted, realize that what few sources HAVE signal down in that range, it's rarely above -10 dBFS. That means it's a good 10 dB down from stuff in the 20-50 Hz range. Means you're getting MUCH less delivered power down deep.
    All in all, a peak in airspeed of 20+ m/s down below 16 Hz isn't really an issue. Keep the airspeed in the 20-30 Hz range low, but below that, the need for low airspeed dramatically drops because of the loss in source signal strength.
    Dan Wiggins
    Adire Audio
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  20. Aaron D

    Aaron D Stunt Coordinator

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    Dan, thanks!! At 20 hz, airspeed is ~13 m/s and by 50 hz, it drops to
     

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