Why can thin sponge lining soundproof earmuffs but not walls? Every webpage that I can find says that sponge is bad soundproofing material for walls because it's light and physics says that materials with more mass isolate better.
So my question is how come thin sponge lining can isolate earmuffs with noise reduction even up to 35 decibels but the same material can't isolate sounds coming through the walls?
If a whole wall would be covered with the exact same thin sponge lining used inside earmuffs, would it isolate the wall like earmuffs do? E.g 35 decibels. If not, why?
 A: It's because the effectiveness of different noise-abatement treatments depends on the frequencies that the noise source contains, and whether the objective of the treatment is to absorb reflected sound or prevent its conduction through a barrier.
Foam on walls is effective in controlling reflection of high frequencies but is ineffective at preventing conduction of low frequencies. So objectionable noise containing frequencies below 500Hz will pass straight through the foam on a wall while noise containing frequencies above 3kHz will be effectively absorbed upon impingement by foam or so-called "acoustic tile".
So to protect your ears, ear muffs stop the low frequencies with the mass of their plastic casings (which makes the incoming sounds bounce off them instead of penetrating through them) and then have foam inside to absorb any highs that get through. A wall is good at absorbing highs but is relatively transparent to lows; stopping the lows requires mass in the form of drywall, thick plaster, sand or brick.
