Fact: A barrier’s claimed acoustic performance may be more misleading than helpful
An acoustic barrier works by making sound diffract (bend) around the barrier to reach the noise receptor, during which process the level of sound at the receptor location is reduced in comparison with that without a barrier in place.
The amount of screening attenuation depends upon the amount of diffraction. A barrier that just intersects the direct path (line of sight) between a source and receiver can provide 5dB attenuation at all frequencies. As the projection into the direct path increases, the amount of attenuation also increases, with better performance at higher frequencies than at the lower frequencies.
For most applications an acoustic barrier that significantly intersects the direct path may provide up to around 10dBA to 15dBA attenuation, depending upon a range of other factors. This is different to the amount of sound that can break directly through the barrier material, which must be somewhat lower than that diffracting around the barrier if its acoustic screening performance is not to be compromised.
Where it is apparently claimed that an acoustic barrier can provide higher levels of performance, this is probably due to confusion between the barrier materials’ sound reduction index (how much sound is prevented from breaking through the material) and any screening attenuation (which is location specific and depends upon the source, barrier, receptor locations, etc). As noted above, even a relatively high performance barrier, such as a small building may only provide around 15dB screening attenuation, depending upon these and other factors.
Sound that impinges on an acoustic barrier is either reflected back towards the source or absorbed by the barrier (with a proportion of this sound passing through the barrier material). Unless sound reflected back towards the source may create other problems, there is no benefit in the barrier having an acoustically absorptive surface. This means that data regarding an acoustic barrier’s absorptive characteristics will probably be of little significance when determining how well it will work as an acoustic barrier.