We have detected low-frequency acoustic waves generated by the plasma of optical breakdown in air at the intensity of incident radiation of the order of 106 W cm-2. Using fast Fourier transform analysis we estimated the spectrum of the acoustic signal detected. The acoustic frequency is from 103 to 104 Hz. Measurements made with an electric probe showed that in plasma there occur oscillations of the electron density at a rate of 105 to 106 Hz, with the electron density that corresponds to the acoustic signal maximum being 1016 cm--3.