Sound we hear is caused by vibration of the air. However, vibration can be transmitted through any substance, where solid, liquid, or gas, in which adjacent particles of the substance come into contact. A single pulse can be started off by a single event - when hands are clapped and the air between them is forcibly compressed. Regular pulses can be started by an object that is vibrating, such as a stretched wire inside a piano. The number of pulses in the air matches the rate of vibration in the wire.
Sound waves are vibrations in matter.
Human Chain Push vs. Pull Tight vs. Loose Connection Push is the compression phase of a sound wave. Pull corresponds to the expansion phase of a sound wave. A loose connection is like a sound wave traveling in air - the molecules are not close packed so the sound does not travel as efficiently. A tight connection corresponds to sound traveling through a metal where there is a lattice of molecules which allow for a very efficient transfer of energy. A sound wave is a longitudinal wave.