The control of cavity tone by utilizing a plasma actuator composed of electrodes elongated in the streamwise direction was investigated, where the upper electrodes were arranged symmetrically in the spanwise direction with respect to each lower electrode. The experimental investigation was carried out with a low-noise wind tunnel. The depth-to-length ratio of cavity was D/L = 0.5 and the approaching lami-nar boundary layer thickness was δ/L = 0.07. In this condition, the occurrence of intense tonal sound was observed. Because the cavity tone is influenced by the incoming boundary layer, the actuator was in-stalled in the upstream of the cavity. The boundary layer thickness was found to be thicker by the control at the spanwise position corresponding to the lower electrode. The investigation for the effects of the driv-ing voltage shows that the tonal sound was reduced to the same level as that of the broadband noise with the peak-to-peak voltage of 5.7 kV in continuous driving. Moreover, the control effects by the intermit-tently driven actuator were investigated with varied intermittent frequency and the ratio of the driving time in the intermittent period (duty ratio). At the low duty ratio such as 0.25 with the voltage of 6.0 kV, the control effects on the cavity tone were negligibly small independently of the intermittent frequency. By increasing the duty ratio, the control effects on the sound became larger, where the reduction level of 30 dB was obtained with the duty ratio of 0.75 and the ratio of the intermittent frequency to the fundamental frequency of cavity tone of 0.08. Also, the influence of the effects of the intermittent frequency on the control was found to be particularly large at the moderate duty ratio of 0.5, where the reduction level of the cavity tone was increased for the intermittent frequency in the range of 0.02 – 0.08.
