Adire LspCAD Airspeed in port question?

Discussion in 'Archived Threads 2001-2004' started by Patrick Sun, Feb 4, 2001.

  1. Patrick Sun

    Patrick Sun Moderator
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    Okay, I finally downloaded and have started playing with the Adire version of LspCAD to model Sunosub II (version 2.0).
    I have a question on the Airspeed in port:
    Where should I be focused on when I look at the graph? What is the "danger zone" for the airspeed and at what frequency should I be canvassing when I look at this plot to determine if my design is "okay".
    Just to get some more feedback:
    I'm thinking about doing the following subwoofer design with the Tempest:
    10 ft^3 (285 liters)
    Tuned to 16.5Hz (Single 6" wide port, 22.75" long)
    LspCAD reports the F3 to be around 19Hz.
    (I'm thinking about adding 72 ounces of polyfil to give it a little more "volume", good idea or not?)
    If I could make heads or tales out of the airspeed plot, that would help me evaluate the design.
    Thumbs up or down for this design?
    TIA!
    ------------------
    PatCave ; HT Pix ; Gear ; Sunosub I + III ; DVDs ; LDs
     
  2. Greg Monfort

    Greg Monfort Supporting Actor

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    Max airspeed will be just below Fb. Normally, as long as it doesn't exceed 5% mach (17.22m/s), you're fine, but if you tune it below the lowest expected signal, then the 5% can apply to whatever that point is. IOW, let's say Fb = 12Hz/35m/s, but the lowest expected signal is 18Hz/15m/s, I would only be concerned with what it is at ~18Hz.
    WRT fill, it lowers box Q. If it helps with in-room or transient response, fine, but otherwise it reduces max SPL. 100% stuffed = ~a sealed response.
    GM
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    Loud is beautiful, if it's clean
     
  3. John E Janowitz

    John E Janowitz Second Unit

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    I believe 35m/s is the point at which it is impossible to get more output from the port no matter how much you increase the power input. Compression begins much before 35m/s though. Greg suggested 17.22 m/s which sounds like a good limit to shoot for. Klipsch also did testing on when distortion began to be audible. They found that 10 m/s was the point where audible distortion set in. This is what I always shoot for when doing a ported system. It's not always that easy to do. That's why I'm such a big PR fan now. They take away all the concern of vent velocity, distorion, and compression.
    John
     
  4. Greg Monfort

    Greg Monfort Supporting Actor

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    I wonder what parameters they used to define audible, as R. Small's tests with individuals set the limit at 5% mach. I know I can't tell any difference between them below this limit.
    I agree about PRs. Tom D.'s sharing his design insights a few years ago have really spurred consumer/DIY sub driver/PR design to get the bottom octave with low distortion/high SPL.
    GM
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    Loud is beautiful, if it's clean
     
  5. Patrick Sun

    Patrick Sun Moderator
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    Thanks for the info, guys!
    Okay, so with the current design, and 400W/4ohms (200W/8ohms in LspCAD), my airspeed is 20m/s at 16.5Hz, so that looks live-able. Dropping the power reduces the airspeed even more.
    Should I worry about my cone excursion hitting 40mm at 24Hz at the max 400W/4ohm (it's 28mm at 24Hz at 200W/4ohms)?
    And does 50% stuffed for a 10 ft^3 enclosure mean 5 pounds of polyfil?
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    PatCave ; HT Pix ; Gear ; Sunosub I + III ; DVDs ; LDs
     
  6. Greg Monfort

    Greg Monfort Supporting Actor

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    >Should I worry about my cone excursion hitting 40mm at 24Hz at the max 400W/4ohm (it's 28mm at 24Hz at 200W/4ohms)?
    ====
    That depends on what Xmax/Xsus is, and whether or not the distortion is audibly obnoxious to you.
    ====
    >And does 50% stuffed for a 10 ft^3 enclosure mean 5 pounds of polyfil?
    ====
    Yep.
    GM
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