reduce early reflections in meeting room

Discussion in 'Pro Audio' started by Tangy, Jan 7, 2004.

  1. Tangy

    Tangy Guest

    Hello,

    I need to reduce the reflections in a 20 x 30 room with a 15' ceilings. The
    room has wood floors, drywall and brick walls and a wood ceiling with
    exposed joists. We have a small sound system used for music and speech in
    the room but the reflections are clearly a big problem. I want to make some
    absorbing panels for the walls and maybe ceiling.Here is my plan but feel
    free to rip it apart and suggest alternatives. I am planning on using
    acoustic ceiling tiles grouped together to form 4 x 8 panels framed in wood
    and covered with fabric and placed around the room, maybe two panels per
    wall and on the ceiling. Do you think this will do the trick or would a
    softer fibreglass style insulation be a better solution for the panels.

    Thanks in advance for any help.
     
    Tangy, Jan 7, 2004
    #1
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  2. Tangy

    Chris Whealy Guest

    I need to reduce the reflections in a 20 x 30 room with a 15' ceilings. The
    First of all, I wouldn't worry too much about early reflections until
    you have first controlled the room's overall RT and the resonant modes.

    If your dimensions are accurate, then you will have some nasty modal
    resonances simply because the height and length of the room are exact
    multiples of each other, and the length, width and height all share a
    large common factor (5').

    A quick analysis of the first 120 room modes shows that there will be 5
    frequencies at which three modes coincide exactly:

    67.75 Hz, 93.95 Hz, 118.84 Hz, 135.5 Hz and 160.54 Hz

    And there are 15 more frequencies at which three or more modes coincide
    within 1 Hz of each other. This all adds up to form a nasty bass
    resonance problem.

    A first estimate of the RT60 of the room would be around the 2.5s - 3s
    mark (empty room). For a room of this volume, you should be looking at
    an RT60 of around 0.5s (I am assuming control room requirements here).
    You will probably need to have an empty room RT60 of around 0.8-0.9s, so
    that by the time it is full of people, the value will have dropped to
    around 0.5s.

    2" Fibreglass or Rockwool can be applied directly to the wall surfaces,
    but the bass absorbancy of this material is greatly improved by having
    an airspace (at least 4") between the treatment and the wall. Since
    bass resonance is a problem in this room, you will need to ensure that
    your absorbers are efficient as possible below 200Hz. Also, moving each
    wall surface in by 6" will disrupt the harmonic length/width
    relationship, which in turn will result in fewer exact modal coincidences.

    Without seeing the room, or knowing what budget you are working within,
    I would recommend treating around 75% of the wall and ceiling area, and
    leaving the hard, reflective floor alone.

    HTH

    Chris
     
    Chris Whealy, Jan 7, 2004
    #2
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  3. Tangy

    Scott Dorsey Guest

    Those tiles are basically no good below 1 KHz or so, but they are okay
    above that. And they are dirt cheap (usually free for the asking).

    Thick foam will allow you to make a device that will damp down to lower
    frequencies.

    Fibreglass batting behind acoustical cloth will allow you to go even
    lower.

    How low do you have to do, and how nice does it have to look? If you
    only need to go down to 1 KHz or so and you want it to look nice, some
    tapestries or hanging rugs are well worth it.

    Certainly putting some oriental carpets down should be your first step.
    Fix the floor problems first since they are the easiest to deal with.
    --scott
     
    Scott Dorsey, Jan 7, 2004
    #3
  4. Make sure to get a fire retardant grade of foam. Not all foam is alike!
    And it's collossally hard to set this stuff on fire either. Sure, the
    cloth covering might go up, but there is so much less 'fuel' in that
    cloth than an equivalent amount of foam, and the smoke from that cloth
    is a lot more benign too.

    As recent events in New England have shown, it's important to consider
    the fire proerties of acoustical treatments. The best part of this is
    that the better absorbing materials happen to be very hard to set fire
    to. All that remains is for you to choose the right stuff.


    Be safe,

    Monte McGuire
     
    Monte P McGuire, Jan 8, 2004
    #4
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