A semi-empirical formula for predicting the noise spectra of axial flow fans in a free field is developed. In deriving this formulation it is assumed that sound radiation from an axial flow fan is primarily due to fluctuating forces exerted on the fan blade surface. These fluctuating forces are correlated to the total lift force exerted on the fan blade, and is approximated by pressure pulses that decay both in space and time. The radiated acoustic pressure is then expressed in terms of superposition of contributions from these pressure pulses, and the corresponding line spectrum is obtained by taking a Fourier series expansion. To simulate the broadband sounds, a normal distribution-like shape function is designed which divides the frequency into consecutive bands centered at the blade passage frequency and its harmonics. The amplitude of this shape function at each center frequency is unity but decays exponentially. The decay rate decreases with an increase in the number of bands. Thus, at high frequencies, the narrow bands merge to form a broad band-like spectrum. The noise spectra thus obtained are compared with measured ones from four different types of axial flow fans running under various conditions.