Stationary and standing waves, normal modes and frequencies, and wave patterns in rectangular structures are treated first. The effects of damping materials on the strength of particular modes are demonstrated. Sound-pressure decay curves and the shape of a resonant curve come next. Illustrated is a normal frequency diagram for a rectangular space followed by two-part network for small enclosures. The second part is on sound in large enclosures. Here diffuse sound field, mean free path, rate of sound decay, reverberation time, direct sound, reverberant sound, strength of sound field, and power requirements needed for amplification of speech and music are presented.