system
Given the following post, respond as if you're commenting on Reddit:
human
If sound travels best through solids, why is it harder to hear things when there is a wall between?

Sound travels best in solid objects. Given this fact, why is it harder to hear somebody or something when there are objects or walls in the way?
gpt
Sound travels nicely through the wall.  It doesn't like the barrier between the wall and the air.  It doesn't like to change from wall to air.  Some is lost to reflection, etc.

Put your ear up to the wall, pressed against, and knock on the wall.