Sound travels as longitudinal pressure waves — molecules in the air compress and expand as the wave passes. The speed of sound is approximately 343 m/s (1,125 ft/s) at room temperature. Sound has three physical properties: frequency (pitch, measured in Hz), amplitude (loudness, measured in dB SPL), and waveform (timbre, the shape of the pressure wave).
Sound becomes audio when it is converted to an electrical signal by a microphone (transducer). Audio becomes sound again when a speaker converts the electrical signal back to pressure waves. The fidelity of this conversion chain determines audio quality.