*Sure it works - it is simple physics.
You take a glass: hard glass is best.
You press the rim tight to the wall.
The wall vibrates from the yelling on the other side.
But the air scatters the sound and your ear misses it.
The glass changes everything - it acts as a bridge. The rim takes the tremor-from the drywall.
The bottom of the glass shakes-because air inside is trapped now, a prisoner.
It has nowhere to go but your ear - a thing that amplifies the mumbles into words.
It works on hollow walls, thin dry wall - A waste of time on brick, on concrete.
You hear nothing because stone kills the wave - A mean thing too dense to shake.