How to measure absorption/diffusion coefficient?

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Absorbers are measured in a specially designed acoustically hard room - a reverberant chamber is the opposite of an anechoic chamber. You have to excite the room (probably with octave band pink noise) and measure the RT60

Without a reverberant chamber you can only measure RT60 in the room before and after install. Read up on Sabine.

With a diffuser you could measure the spectral return (impulse) of a flat piece of material the same size as the diffuser in an anechoic chamber, then you could measure the return of the diffuser at various angles of incidence - it would be a lot of data.

Without an anechoic chamber you could try gated measurements, but I think they would be very difficult to interpret.

In all cases above, you would typically want measurements from multiple microphone locations and elevations.
 
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