The determination of the sound power output of small sources in reverberation rooms and the transmission loss of building partitions require the measurement of space-averaged sound pressure squared. Such measurements are contaminated by a direct field bias error. This article reviews the underlying theory and presents a number of graphs for determining the direct field bias error as a function of measurement distance, source directivity factor, room volume, and reverberation time. Also, an in situ experimental method for determining the effective directivity factor of a source is presented in graphical form. Finally, the concepts of ‘‘modal overlap’’ and Schroeder frequency are explored in terms of an elementary electrical resonant circuit and the more descriptive term, number of modes per modal bandwidth, is proposed.