The acoustic elements—acoustic mass, acoustic compliance, acoustic resistance—are treated in detail. Examples of mixed elements are given. Next come the reflection and radiation of sound—reflection from a plane; radiation from a pulsating sphere; radiation from a monopole source; radiation from a multiple source; radiation from a dipole source; radiation from an oscillating sphere; and radiation from multiple point sources vibrating with different phases. Directivity index is treated next. A section on radiation impedances follows for: pulsating sphere; oscillating sphere; and circular piston. The chapter ends on viscous and thermal losses, particularly in lossy tubes.