Ponencia presentada en el XIX Congreso Internacional de Acústica (ICA2007), Madrid, 2-7 Sep 2007.-- PACS: 43.35.Ei.The acoustic field and the emergence, distribution, and motion of bubbles in two rectangular water channels is investigated at ultrasonic frequencies around 20 kHz. The transducers are positioned at one channel end. We identify a near-field and a far-field region by measuring amplitude and phase of the acoustic pressure within the channels at low intensity. At higher ultrasonic power, cavitation bubbles close to the transducer show net drift into the channel, while bubbles in the far field form streamers fixed in space. Inbetween, a hopping bubble motion is observed. This can be reproduced by a calculation of bubble drift taking into account Bjerknes forces and rectified gas diffusion into the bubbles.Peer reviewed