Q: How can you hear someone through a closed door?

[Cookies?]

A question about the simplest of things: Based on the physics of sound, how can you hear someone through a closed door?

I'm quite confused because someone once told me its was because the sound passes through the door; since its causing the air molecules to vibrate, when this vibration hits the door, it similarly causes the door to vibrate and transfer the sound energy through to the other side.

I've also read about diffraction as the cause so, what, is it a combination of both? Or just one. If its just diffraction, then if I was in a completely airtight room, would I then, technically, be unable to hear anything from outside?

 

[Curl]

They were talking about diffraction when the door is cracked open slightly.

The first explanation is the correct one when the door is closed/sealed.

 

[Drakkith]

I believe both effects happen, as most doors are probably not airtight, especially the ones in houses.

 

[Sophos]

Scenario 1: In this case, sound acts much like light in regards to the double split experiment. Sound -> crack in door. A new origin of the sound is created, and the process is repeated until the waves of sound are eventually deflected by particles in the air and the sound dissipates.

Scenario 2: In this case, sound waves interact directly with the door, causing it to vibrate. The door's vibrations create new sound waves that are similar to the ones that it was hit by. Smooth, hard, and some-what rigid objects tend to translate sound more effectively - which is why a cotton coat doesn't translate sound very well. ;)