It is important to differentiate between acoustic absorption and sound insulation. We will use the following examples to illustrate the difference :
A material with a high acoustic absorption such as fibre glass (α =0.85); to verify the characteristic, if r=0.05 then τ=0.1. The standardised isolation R=10log(1/); with which R=10 dB; a very low isolation.
On the other hand a very reflective material (plaster wall) will have a very low acoustic absorption (α =0.01), a high reflection (r=0.989), leaving τ =0.001; that means a sound insulation of R=30 dB; that is an acceptable value.