Bass cancellation due to room modes?

Discussion in 'Archived Threads 2001-2004' started by StephenMSmith, Oct 16, 2002.

  1. StephenMSmith

    StephenMSmith Stunt Coordinator

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    Here's something I've been wondering about regarding bass cancellation due to room modes:

    Say I measure some room nulls from my listening position. Are these only null from my listening position? ie. if I can't hear it, does that mean that my neighbors on top and below my apartment don't hear (or feel) it either?


    Thanks,
    Steve
     
  2. Chris Tsutsui

    Chris Tsutsui Screenwriter

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    Good question.. hmm
    Lets take an example of the first (order) standing wave mode for an 8 foot high room which happens to be 70hz.
    This axial room mode occurs between the ceiling and the floor.
    Since this is the first order room mode there will be 1 null and 2 peaks in it. The high pressure levels of the wave (or peaks) will be at the floor and ceiling surface while the low pressure point will be smack in between. So if you sit with your ear directly at the 4 foot high mark you will notice a drop in SPL near the 70hz region.
    So in this case, the listener doesn't hear it, yet the walls and ceiling experience maximum SPL due to them being in the high pressure zones thus the neighbors may notice the vibrations more. Actually in EVERY standing wave when a listener experiences the "null", the walls will always be experiencing the "Peak" of that resonance.
    So I guess "no" would probably be my short answer. Standing waves primarily alter the acoustics within the room but I guess they "could" alter the outside volumes due to structural resonances of walls or something.
    Standing waves have both cancellations and reinforcements in the same wave. Lets not forget about the other orders like how the 2nd order standing wave for the 8 foot high room would be 141hz. This has a different configuration than the first order and would actually cause a peak in SPL at the 4 foot high listening level in addition to peaks at the floor and ceiling.
    Think of the standing waves as uniform hills, rollercoasters, or "waves", with the same number of dips as the order of resonance. So 1st order = 1 dip, 2nd order = 2 dips, etc. with peaks at the side walls. Sorry if I confused you, it's a lot easier to read and see pictures like at the:
    http://www.harman.com/ white papers. I also have a mode calculator at my site as well.
     
  3. StephenMSmith

    StephenMSmith Stunt Coordinator

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    Hey Chris. Nice site. Are you doing all that w/your soundmeter and Excel?? If so, you should take a look at SpectraLabs, so you can plug your sound meter into your computer for instanteous feedback on freq response changes.

    Steve
     
  4. Chris PC

    Chris PC Producer

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    Where can SpectraLabs be found?
     
  5. StephenMSmith

    StephenMSmith Stunt Coordinator

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  6. Chris Tsutsui

    Chris Tsutsui Screenwriter

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    Good Idea, too bad I'll have to buy a mic or borrow one of my friend's good ones.

    I've just been so busy with school lately I havn't had the time to "play".
     
  7. StephenMSmith

    StephenMSmith Stunt Coordinator

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    You don't a mic. Just plug your RadioShack SPL meter into your soundcard and use a mic compensation file with the known RS SPL adjustments.

    Steve
     

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