A group of physics students has been given a small stereo speaker and told to use it for investigating sound waves. After much experimentation, the students discover that the speaker works by quickly moving in and out. This process pushes the air around it in little bursts that travel in the same direction as the speaker's motion. Based on these observations, how should sound waves be categorized?
An electromagnetic wave that has transverse vibrations
A mechanical wave that has longitudinal vibrations
An electromagnetic wave that has longitudinal vibrations
A mechanical wave that has transverse vibrations