The effect of three different modes of acoustic excitation on an unsteady separating flow over a circular cylinder at a Reynolds number (Red) of about 1.5×105 has been investigated. The acoustic exciters included (I) a speaker mounted inside the cylinder and blowing through a narrow spanwise slot; (II) a circular piezo-electric acoustic transducer mounted flush with the surface; and (III) strip shaped flush mounted acoustic transducers. The first mode of excitation was judged most effective at about 1.3 kHz, based on changes in the time-averaged surface pressure distribution on the cylinder. The corresponding frequencies for (II) and (III) were 7 kHz and 2.25 kHz respectively. The transducers (II) and (III) did not introduce the blowing and suction effects of transducer (I). Transducer (II) increased the mean surface pressure slightly, while (I) and (III) reduced it.