Final answer:
Walls vibrate in a room with sound from a projector due to resonance and wave interference, where sound waves reflecting off surfaces create variations in loudness through constructive and destructive interference.
Step-by-step explanation:
The walls in a room may vibrate when sound is emitted from a projector due to the phenomenon known as resonance and wave interference. Sound waves travel away from the projector and bounce off surfaces such as walls, causing vibrations that can sometimes be felt as well as heard. Depending on how these reflected sound waves interfere with each other, areas within the room can experience different sound levels due to constructive or destructive interference. Constructive interference occurs when waves combine to create a louder sound, while destructive interference results in a quieter or dull sound. This is similar to what you might notice when you hook up a stereo system and hear variations in sound volume around the room.