Still more questions RE: Sonosub tuning...

Discussion in 'Archived Threads 2001-2004' started by Robin Smith, Jun 22, 2002.

  1. Robin Smith

    Robin Smith Stunt Coordinator

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    Hi,

    I am so close to completing my sub I can feel it! I am painting my endcaps this weekend if the weather holds up, then its construction time.

    The final question I am trying to solve is the tuning (via port length).

    I cut my tube today, and the net volume will be 214L
    My original intention was to tune to 15.4 HZ which would require a 14.92" port

    I bought the 4" Precision Port and just for kicks I entered it into LspCAD at full length (17"). The tuning came out to 14.6 Hz.

    I am still learning the significance of many of the numbers in LspCAD (thanks to many on this board for their previous help) but noted the following comparisons:

    15.4 Hz Tuning:
    30 mm Excursion: 13.17 Hz
    15m/s Airspeed in port: 18.7 Hz
    20m/s Airspeed in port: 15.59 Hz
    50ms Group Delay: 13.74 Hz
    100ms Group Delay: 13.24 Hz

    14.6 Hz Tuning:
    30 mm Excursion: 12.48 Hz
    15m/s Airspeed in port: 17.26 Hz
    20m/s Airspeed in port: 14.34 Hz
    50ms Group Delay: 13.02 Hz
    100ms Group Delay: 12.61 Hz

    Now, to my untrained eye, the 14.6 Hz tuning looks "better" than the 15.4. What am I missing? What are the negatives to the lower tuning. Obviously there must be some trade-offs somewhere, I just want to understand what they are before finalizing my decision.

    Thanks in advance


    Robin Smith
     
  2. Patrick Sun

    Patrick Sun Studio Mogul

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    You do realize that the port's volume displacement (you treat the tube as a solid object inside the sonotube) is going to eat up the some of volume of that cut section of sonotube, so the tuning will be slightly off, but probably not by much, just a little lower than predicted.
     
  3. Robin Smith

    Robin Smith Stunt Coordinator

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    Doh!
    I do now! [​IMG]
    Okay, so the 214L was based on the volume of the cut port. Thanks Pat!
    So if I left the port at 17" it would be a ~212L sub.
    LspCAD didn't show much in the way of change in the numbers I cited above. So what's the trade-off??
    The airspeed in port, cone excursion and group delay numbers all look slightly better. What gets worse?
    Thanks
    Robin
     
  4. Patrick Sun

    Patrick Sun Studio Mogul

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    You get a little less output in the 20's Hz range, the lower tuning "pulls" the SPL curve down a wee bit. In practical terms, you won't miss much.
     

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