This document defines and compares structure-borne sound and air-borne sound. Structure-borne sound spreads through solid objects like walls via vibrations, while air-borne sound spreads through the air between 20-20,000 Hz. Common causes of structure-borne noise are vibrating sources that transmit energy through structures, while air-borne sound travels through air and may reverberate in enclosed spaces. The document also discusses sound propagation and methods to reduce structure vibration and noise transmission, such as using dampers, barriers and sound absorbing materials.