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Thread: Where to set up the monitors?

  1. #11
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    Jan 2009
    Jakarta, Indonesia
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    Rock wool 100mm thick does make a good bass trap especially if it has a membrane or it is placed in a corner - sometimes with more absorptive material behind it. But it also makes a very good absorption panel.

    The quarter-wavelength (λ/4) rule dictates that a porous absorber for normal incidence must be at least a quarter-wavelength thick at the frequency of interest - this rule changes with the angle of incidence. For example, for a frequency of 1kHz, the minimum absorber thickness should be about 90mm. Lower frequencies require thicker material. So, 100mm of rock wool just about takes care of frequencies from 850Hz and up. Also note that an absorber is less effective at glancing angles and is sometimes 100% reflective at these angles. This often depends on the fabric used.

    The above is only written as a theoretical example. In actual practice a 100mm thickness of glass fiber material of 48kg/m3 density has essentially perfect absorption over the 125Hz to 4kHz region. Only 4kHz is listed because the STC curve is only rated to that frequency. The absorption at higher frequencies is also near perfect.

    Here's a handy little excel calculator for determining where to put the absorption at first reflection points. Enjoy!

    John H. Brandt Acoustic Designs - ABOUT US - OUR WORK - RESOURCES "Twenty thousand dollars worth of Snap-On tools does not make you a Professional Diesel Mechanic"

  2. #12
    Join Date
    Dec 2008
    Melbourne Australia
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    Right, I've had read and a ponder. So let me get this straight...

    If I use 100mm of rock wool (or high density insulation 48kg/m3) ay my first reflection points (say one on each side wall and one on the roof) covered in the "correct material", this would not only address the first reflections, but also help in trapping some of the bass. Then, some more bass trapping in the corners, perhaps to address some of the sub 850Hz frequencies would round off treating the room, at least for the first pass.

    This would be a good start to adressing the room, yes? Then I'd maybe measure the response with a mic/software setup and try to address any issues from there?

    "especially if it has a membrane" , "This often depends on the fabric used"

    Material sounds quite relevant. What sort of material are we talking about?


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