WE PROPOSE PHONONIC CRYSTALS TO MEET THE REQUIREMENTS OF ULTRA-LOW-NOISE THERMAL DETECTORS. BY DESIGN A PHONONIC CRYSTAL EXHIBITS PHONON BANDGAPS WHERE HEAT TRANSPORT IS FORBIDDEN. THE SIZE AND LOCATION OF THE BANDGAPS DEPEND ON THE ELASTIC PROPERTIES OF THE DIELECTRIC AND THE GEOMETRY OF THE PHONONIC UNIT CELL. A WIDE-BANDWIDTH LOW-PASS THERMAL FILTER WITH A CUT-OFF FREQUENCY OF 1.5 GHZ AND EXTENDING TO 10 GHZ CAN BE REALIZED WITH QUASI-PERIODIC PHONONIC STRUCTURES. A FEW 1019 WHZ1/2 DETECTOR SENSITIVITY IS READILY ACCESSIBLE WITH PHONONIC FILTER THERMAL ISOLATION. PHONONIC FILTERS ARE NATURALLY COMPACT <20 M IN LONGEST DIMENSION AND CONTRIBUTE NEGLIGIBLE HEAT CAPACITY TO A THERMAL SENSOR. WE PROPOSE A THREE-YEAR EFFORT TO FABRICATE AND TEST PHONONIC-ISOLATED TRANSITION- EDGE SENSOR ARRAYS SUITABLE FOR BACKGROUND-LIMITED OPERATION IN A FAR-IR SUR- VEYOR. WE EMPHASIZE THAT PHONONIC THERMAL ISOLATION OFFERS A VIABLE PATH TOWARDS DETECTOR SENSITIVITIES AN ORDER OF MAGNITUDE ABOVE THAT ACHIEVED WITH CURRENT STATE-OF-THE-ART THERMAL DETECTOR TECHNOLOGIES. OUR EFFORT ADDRESSES THE APRA SOLICITATION FOR ADVANCING DETECTOR DESIGN AND OPERATION TOWARDS HIGHLY SENSITIVE COMPACT AND ROBUST CHARACTERISTICS.