The device described here consists of a millimeter-size tube immersed in a liquid, closed at one end, and partially filled with gas. A sound field in the liquid causes the gas volume to pulsate alternately expelling and drawing liquid through the open end of the tube. According to general fluid mechanical principles, the liquid exits the tube as a jet, while it enters it from the entire solid angle available. Averaged over a cycle, this flow pattern results in a net source of momentum which, by reaction, exerts a force on the tube. Possible applications include the self-propulsion of the tube, a pump and a rotary actuator. Thereby a single device can be made to respond to different frequencies, for example, switching the direction of the force by changing the sound frequency. The acoustic power required is well below biologically hazardous levels, which would permit, among others, powering the device remotely through living tissue.