A vibrating medium. Frequency is proportional to pitch. For example, the greater the frequency, the greater the pitch. Distinguish between a compression and a rarefaction. Air is the slowest carrier of sound waves of the three.
Sound needs molecules bumping molecules to transfer the momentum and energy.
There are no molecules in a vacuum. Defend your answer. Water and steel are much faster carries of sound than air. Low frequencies of sound are emitted by larger objects. Larger objects have more inertia than smaller objects and therefore take more energy to vibrate.