why can we hear through a wall with a cup?

Question: why can we hear through a wall with a cup?
- Keywords:
  cup,
  hear,
  sound,
  wall
Asked by maevelewisxo to Damien, Rachael, Simon, Suzi, Tim on 21 Jun 2011.
- Suzi Gage answered on 21 Jun 2011:
  
  Hi @maevelewisxo
  I don’t know the actual reason, but I think it’s something to do with the column of air inside the cup or glass, which amplifies the soundwaves as they come through the wall.
  
  Here’s a neat trick, if you want to listen to your mp3 player but you don’t have any speakers, if it has a built in speaker, you can put the mp3 player in a glass, and the music coming out of the built in speaker will be louder! Cool huh?

Question: why can we hear through a wall with a cup?