Quote:
Originally Posted by andrebrito  |
Excellent Andre! That saves me the effort of doing a home-made test that might not be accepted as valid by everyone here. I took the numbers for several surfaces from that site, and converted them to surface reflectivity in dB in an Excel spreadsheet. A screen-cap and the spreadsheet are attached. I'm not a big math guy, but I'm pretty sure the formulas are correct. Maybe Andre or others can confirm the basic formula:
Reflection dB reduction = 10 * Log(1 - AbsCo))
Where AbsCo is the stated absorption coefficient. I also formatted the results to two decimal places. A couple of notes on the data:
* Some of the surface materials were measure in air, rather than against a solid rigid surface. This is especially relevant for the glass results, because glass absorbs a fair amount of bass and low midrange. I used the data for ordinary window glass, but in hindsight I should have used the heavy plate glass because that is more like a mirror attached to a rigid wall.
* Likewise, the gypsum board is only 1/2 inch thick and mounted on studs, so that too gives some sympathetic bass trapping beyond what the surface reflects.
* With the above taken into account, all of the surfaces reflect the same to within less than half a dB. That is, all of these surfaces basically reflect 90 percent or more of the sound at all frequencies, and in most cases much more than 90 percent. To my mind, surfaces that reflect the same to within half a dB should sound pretty much identical.
--Ethan