The sound generated by a circular cylinder in a flow at low Mach numbers is investigated by direct solution of the two-dimensional unsteady compressible Navier–Stokes equations. Results show that sound pressure waves are generated primarily by vortex shedding from the cylinder surface into its wake. When a vortex is shed from one side of the cylinder, a negative pressure pulse is generated from that side whereas a positive pressure pulse is generated from the other side; alternate vortex shedding from the upper and lower sides of the cylinder produces negative and positive pulses alternately and thus produces sound pressure waves on both sides. The dipolar nature of the generated sound is confirmed; lift dipole dominates the sound field. The Doppler effect is shown to play an important role at finite Mach numbers. The direct solutions are also compared with the solutions obtained by Curle's acoustic analogy. The results show that Curle's solution describes well not only the generation mechanism of the sound but also the propagation process if we take the Doppler effect into consideration.