system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How does a horn amplify sound without adding any energy to it?

I was watching a tuba player the other day.  The buzz of a guy's lips can be amplified to the point where it shakes you.  Horns seem to defeat the first law of thermodynamics.  What is going on here?
gpt
Your lips, instead of just vibrating nearby air, vibrate the horn which in return vibrate the air inside the horn, thus making the amount of air originally vibrated much larger, and the sound stronger. In addition, the shape of the horn focuses the sound wave in a certain direction.