The apparatus includes a housing, a compression chamber disposed in the housing and having at least a first interface operable to vary a volume of the compression chamber, an
expansion chamber disposed in the housing and having a second interface operable to vary a volume of at least the
expansion chamber, and a
thermal regenerator in fluid communication with each of the compression chamber and the
expansion chamber. The
thermal regenerator is operable to alternatively receive
thermal energy from gas flowing in a first direction through the regenerator and to deliver the
thermal energy to gas flowing in a direction opposite to the first direction through the regenerator. The compression chamber, the expansion chamber, and the regenerator together define a working volume for containing a pressurized working gas. Each of the first and second interfaces are configured for
reciprocating motion in a direction aligned with a
transducer axis, the
reciprocating motion being operable to cause a periodic exchange of working gas between the expansion and the compression chambers. In one aspect, at least one of the first and second interfaces includes a resilient diaphragm, and a
cylindrical tube spring coupled between the diaphragm and the housing, the tube spring being configured to elastically deform in a direction generally aligned with the
transducer axis in response to forces imparted on the tube spring by the diaphragm to cause the at least one of the first and second interfaces to have a desired
natural frequency. In another aspect the apparatus includes a first
heat exchanger in communication with the expansion chamber, a second
heat exchanger in communication with the compression chamber, the
thermal regenerator is disposed between the first and second heat exchangers, and each of the first and second heat exchangers are peripherally disposed within the housing with respect to the
transducer axis and configured to receive working gas flowing to or from the respective chambers and to redirect the working gas flow through the regenerator.