An experimental study of nonlinear frequency spectra of ultrasound transmission through two-dimensional microbubbles

The frequency spectra of transmission coefficients for ultrasound passing through a sheet of gas-filled micropores have been measured using incident waves with amplitudes up to 2.4×10⁴ Pa. It is found that as the amplitude of the incident wave is increased the peak frequency of transmission loss through the two-dimensional trapped-bubble ensemble shifts to lower values by as much as 47%. The experiments indicate that the shifting is caused by the net displacements of air-water-membrane triple lines and the air loss of the trapped bubble, which are due to the radiation force and microbubble production produced by the incident wave