The performance of free field rooms of the type used for precision acoustical measurements has been investigated in the frequency range 1 to 20 kHz. The root-mean-square deviation of the field of a point sound source from the ideal free field value is established as a convenient index of performance. Directly comparable data based on detailed field samplings are presented for a small rectangular enclosure lined with three different wedge treatments, together with data for a large enclosure lined with one of these wedge treatments. Detailed consideration is given to transducer requirements and other factors relating to practical implementation of the test method, and to the effect of wedge imperfections on overall room performance. A theoretical analysis in which the sound scattered from each individual wedge is summed to predict the resultant field at any point within the enclosure is shown to be in good accord with the experimental observations.