experimental limitations at very low temperature that are not taken into account in Eq. \[eq:Eres\_lim\], and [@Tali_11_CNS], or the current generation of massive calorimeters could be significantly improved.

To gain insight into this energy resolution discrepancy, we will carefully study both the dynamics and noise of the original [@CRESST_2009_Commissioning] and composite [@CRESSTII_2014_LowMass] CRESST phonon detectors as well as other calorimeters and then motivate and develop 6 design criteria that are applicable to very low-temperature massive detectors:

1.  The signal bandwidth (frequency scale at which energy transfers from the absorber to the sensor) must be greater than or equal to sensor bandwidth,

2.  The transition edge sensor (TES) must not phase separate,

3.  The sensor bandwidth must be larger than the $1/f$ noise threshold,

4.  The sensor bandwidth must be large enough to satisfy event rate requirements, and

5.  Microfabrication techniques should be used only on standard thin wafers.

6.  For applications that require minimal position dependence of energy estimators, we require that that signal bandwidth $\gg$ than the sensor bandwidth.

Finally, we present 2 prototype designs that simultaneously achieve all of these design requirements: a single-photon-sensitive light detector and a 1.75kg ZnMoO$_{4}$ double-beta decay detector.

Simulating CRESST Phonon Detector {#sec:DymNoise}
