The use of an impedance tube and horn for the absolute calibration of a 180‐m3reverberant room for the measurement of sound power in tones is discussed and demonstrated. The results of the calibration procedure are found to compare quite favorably with results obtained using a one‐third‐octave‐band decay method. For a modal overlap index greater than 5, corresponding to frequencies greater than 400 Hz, the difference in the predictions of the two methods is less than 0.8 dB while at a modal overlap index as low as 1.9, corresponding to a frequency of 264 Hz, the difference is less than 1.6 dB. The results of these tests suggest that the one‐third‐octave‐band decay method is quite sufficient for the qualification of a reverberant room with a modal overlap index of 5 or greater for tone‐power measurements. The results of these tests also suggest that any room can be qualified for the measurement of sound power in a tone if it is possible to substitute the mouth of an impedance tube and horn for the source whose sound power is to be measured. The required apparatus is simple and the measurements are easily performed.Subject Classification: [43]55.65; [43]85.20, [43]85.24.