Acoustic. Sound Barrier
Sound barriers are high density layers that reflect air borne noise backwards towards the source.
In contrast to sound absorbers which reduce the sound power level, barriers reflect noise but do not reduce the energy carried by the sound wave.
Barrier materials are high density materials that reflect most noise that hits them. As sound waves hit the barrier most of their energy is reflected backwards.
Unfortunately a percentage of the noise will always pass through a barrier. This is why barriers should be used with absorbing foams.