The sound powers of small uncased centrifugal‐fan rotors were measured at low‐tip Mach numbers. Correlation of the data by use of a similarity relationship derived from theory has shown that the broad‐band rotor sound is a result of randomly oriented dipole sources caused by fluctuations in fluid momentum. Deviations of the dependence of sound power on speed from that theoretically predicted were found. Speed dependences less than that predicted can be explained as a nearfield effect; the lack of agreement between theory and the fan law for sound can be explained on the same basis. Observed speed dependences greater than that predicted appear to result from having fixed frequency limits on the measurement system. A sharp wedge placed near the rotor periphery was found to generate discrete‐frequency dipole sound. Quadrupole sound appeared to be negligible.