Foam transmits certain lower frequency sounds very well, actually
accentuating them. I put 2" foam sheet on my exterior, now I can hear
the neighbors kids running upstairs ,[ at first I thought it was an
animal in the house] houses are separated by 8 ft.
It depends on what frequency you are trying to stop, lower
frequencies need more mass as sound waves get longer the lower sound
goes. That is why you hear the bass, long waves need mass to be
absorbed. It realy depends on the sound-source to be blocked