ANSI Standard SI.21‐1972, “Methods for the Determination of Sound Power Levels of Small Sources in Reverberation Rooms,“ represents a significant departure from its predecessor S1.2‐1962 in that it requires a performance evaluation to demonstrate the qualification of a reverberation room measurement system for measurement of discrete frequency sound. Experience in preparing this type of performance requirement was provided to many members of the working group through their association with ASHRAE and ISO writing groups who prepared or were in the process of preparing similar types of standards. Data on a number of facilities were available from round‐robin evaluations sponsored by ISO. A forthcoming paper by Ebbing and Maling [“Reverberation Room Qualification for Determination of Sound Power of Sources of Discrete‐Frequency Sound,” J. Acoust. Soc. Am.54, No. 4 (1973)] provides an extensive discussion of the theoretical basis for the standard and presents some of the round‐robin data. In this presentation, the data available to the writing group at the time of preparation of the standard are reviewed. As might be expected, the performance requirements represent a compromise between desirable measurement accuracy, and achievable measurement conditions in reasonable size rooms. The selected performance requirements of S1.21‐1972 are discussed in the context of this compromise, and the data available on existing rooms. Finally, some estimates of the achievable accuracy of power level measurement of an arbitrary source are related to the performance requirements.