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Join Date: Nov 2006
Location: Bushnell's Basin, NY
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If you isolated the walls from the concrete, then you should be all set. I thought I saw that...
You and I are describing the same thing. I'm still focused on the big picture - what kid of device and overall dimensions - so I'm leaving some details out, like the need to seal the cavity. We're very much talking about a device like Everest shows in fig. 12-24. But with dimensions and construction determined by performance requirements.
As to your dimensions, I don't think they're realistic for a device tuned to 30Hz. Look at the formula, above, or the nonogram on Everest page 202. That's why I asked about the immediate surroundings of the listening area. You have a side wall that's open to the basement and has room behind it for an absorber's cavity.
And that brings us to the last global topic, before we can talk construction details - how much absorption do you need? How much surface area inside the theater needs treatment in order to reduce the 30Hz issue to acceptable levels? That all depends on "absorption coefficient," how much acoutic energy is absorbed per unit area. Will a single 4'x4' device suffice, or will you need three or four 4'x8' panels to do the job?
I seems intuitively obvious to me that your approach of setting depth and adjusting mass will result in very low absorption coefficients. It seems better to me to give a little on both ends. That's why I only suggested 1/2" sheetrock, not a double panel, as I suspect the latter would be very inefficient. It would only need to be 8" deep, but what's the advantage if you need to rebuild the entire right wall this way to get enough absorbance?
To step back a little, the acoustic goals for a good sounding residential HT is an RT60 of 0.2-0.3 sec. that's flat to slightly rising at low frequencies, but has no bumps. (There is discussion regarding first reflection points, but that's a detail at this level.) From a theoretical standpoint, we do that by:
- measuring room surface area
- measuring size and absorption coefficients for room contents
- calculating room absorbance and predicting RT60 - Everest Eq. 11-1 - as a function of frequency
... or in most practical cases, measuring reverbration as a function of frequency via waterfall diagrams and ETC analysis
- examine that function for level and bumps
- adjust the level with broadband absorption by trading areas of lower absorption (i.e. walls) for areas of high absorption (acoustic panel hung on the wall).
- attack the bumps with tuned absorption, also traded for areas of (hopefully) lower absorption.
The one thing I will not be able to predict is alpha, so I can't say how large an area needs to be dedicated to this device. That's where your measurements will be critical. The data leads you to what's needed. Perceptual factors (discussed in Toole's book) lead you to what type of divice ans where is best.