Quote:
Originally Posted by andrebrito  Yes for 0ยบ incidence.. change the degree of incidence and absorption changes... the figure is perfectly clear |
Andre, I
just know you're not saying that a thick slab of wood bonded to concrete absorbs 80 to 90 percent at 1 KHz when sound arrives at a 90 degree angle. So what exactly are you saying?
Two more points need to be made about the graph you posted, but I'll make only one and leave the second to you (and others) to puzzle over.
You say this data was measured in an impedance tube, but all the impedance tubes I've seen are long cement "chimneys" two feet by two feet and 20 feet long or even longer. The longer the tube, the lower the frequency that can be measured accurately. When material is tested in an impedance tube it is placed at one end, and sound is sent from the other end far away. Clue #1: Therefore, measuring different angles means the sample itself must have been rotated.
Clue #2: Now here's the challenge to anyone who like to consider themselves knowledgeable about acoustics: Look at the graph again, especially the big dip in absorption around 3 KHz. What does that tell you about the wood sample that was measured?
--Ethan