A U-shaped pulse-tube refrigerator includes a regenerative tube and a pulse-tube that are juxtaposed with each other; a communicating path that connects a low-temperature end of the regenerative tube and a low-temperature end of the pulse-tube; a heat exchanger that is provided at the low-temperature end of at least one of the regenerative tube and the pulse-tube; and a flow smoothing member that is provided between a first exit of the communicating path at a side where the heat exchanger is provided and the heat exchanger.

System and method for converting energy; the system comprising: an acoustic resonator; a phase-exchange device configured for forming across a section of said resonator a concentration gradient of a first substance in a gaseous medium contained by said resonator, to thereby generate a pressure wave within said resonator; and a conversion device for converting mechanical energy constituted by said pressure wave to non-mechanical energy.