  In order to achieve low Noise-Equivalent-Powers (NEP < 1 aW Hz$^{-1/2}$)
Transition Edge Sensors (TES) require high levels of thermal isolation between
the superconducting bilayer and the heat bath. We propose that short
micro-machined acoustic interferometers can be used for low-noise thermal
isolation, avoiding many of the difficulties inherent in conventional, long-
legged TES designs. In this paper, we present a detailed elastic wave model of
interferometric phononic legs, and demonstrate the successful fabrication of
TESs that are thermally isolated by interferometric structures.
