A single acoustic resonance, excited by vortex shedding from the trailing edge of a flat plate, has been studied in detail. The resonance was generated in the working section of a low-speed wind tunnel and the scale was large enough for accurate measurements to be made. The trailing edge was designed so that it could be made to oscillate slightly to simulate the correlating effect of the resonance on the vortex shedding. The mode was traversed at resonance and the modal shape and natural frequency were found to agree well with computations. At and a little above the air-speed for maximum pressure amplitude the sound pressure level was steady at around 125 dB, the wake was fully correlated and the wake and pressure maintained a steady phase relationship. Outside this speed range the amplitude was unsteady; at slightly lower speeds the acoustic pressure and vortex shedding were found to occur at different frequencies, but at slightly higher speeds both frequencies were the same. The frequency at the maximum amplitude was some 2% higher than the natural shedding frequency appropriate to the velocity at which the maximum occurred. No instance was found in which the frequency was reduced below the natural shedding frequency. This finding is in marked contrast to results by Parker and Wood. A theory is given which enables the amplitude of the acoustic mode to be predicted successfully, working from a knowledge of the pressure field well below resonance where the fluid is nearly effectively incompressible, and the measured damping factor for the acoustic mode. The acoustic mode is not driven directly by the pressure fluctuations within the wake. Associated with the eddy shedding when the fluid is effectively incompressible are low amplitude pressure fluctuations which are distributed over a large area, and it is these which excite the acoustic mode, the excitation coming mainly from a region upstream of the trailing edge.