Ethan Winer wrote on Tue, 17 February 2009 16:28 |
Thomas Jouanjean wrote on Tue, 17 February 2009 13:41 | Everything about acoustics is [unintuitive]
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It makes no sense that wood on a slab could absorb 80 to 90 percent around 1 KHz. The data I've seen is more like 6 percent. So this tells me the wood in that test was thin and suspended in the air. Or at least not mounted flat and bonded to cement as is done for floors, which is the intent of my article.
--Ethan
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Ethan, the test was done in an
impedance tube, like Andre said. So it can't be suspended in the air. But it's placed against something real hard and heavy (bottom of the tube) which implies high density, denser than cement. Don't confuse impedance tube test with reverb chamber tests.
The graph shows little absorption with perpendicular incidence. About what you mention, between 5 and 10%.
If the piece of wood were to resonate (and surely it does on it's own) it would be best excited by a perpendicular incidence. The graph tends to rule out that phenomenon to explain the absorptive behaviour with incidence as the piece is placed against something hard. It's therefore dampened.
It could be a measurement problem, I just had a chat about it with a colleague from Leuven University which does those tests now and then, and while unlikely, he said it could be the result of a leak in the tube seal around the sample. But don't you jump to conclusions too fast because this measurement was likely performed by the book or it would not have been published and I would not question it so far.
Although the test results show important coefficients when an incidence other than 0