The production of tones by an isolated rotor interacting with fluctuating and steady flow distortions is first discussed and a method of separating the steady components is explained. Circumferential, radial, and axial microphone traverses have been made in the duct of a 1‐m‐diam four‐bladed subsonic ventilation fan with no stators. The steady components of the rotor tones were measured and phase related to the shaft rotation. The results show that the acoustic field close to the rotor is dominated by its steady force field: at any multiple of rotational frequency, the basic mode is present, having a number of lobes equal to the rotational frequency harmonic order. At greater axial distances from the rotor, the basic mode decays, leaving only modes of low circumferential order at the intake. The estimated power in these modes is in agreement with the acoustic power measured from farfield traverses. The loading spectrum of the fan is determined from the acoustic modes in the intake and compared with the spectrum used in helicopter noise theory.