Single burst acoustic streaming

On a previous occasion we reported observations of a new type of acoustic streaming which only occurred around a needle hydrophone positioned near the focus of a focusing ultrasound transducer. The streaming was observed even at burst repetition frequencies as low as l Hz. In an attempt to find the origin of this streaming attention was focused on possible excitation of evanescent waves on the probe surface. Presently we report results front a new experiment on this subject. Streaming was observed and measured using tiny tracer particles of polyamid which were recorded with a CCD-video camera and a frame grabber in a PC. The frames were analysed with a Matlab-program for obtaining stream lines and velocity measurements in a vertical plane through the sound axis. The results show that we also observe streaming in free field when using burst repetition frequencies lower than l Hz, and even when using single bursts.