  Subharmonic response from contrast microbubbles as a function of ambient
overpressure is numerically investigated for subharmonic aided noninvasive
estimation of local organ level blood pressure. Three different interfacial
rheological models for the encapsulation is used with material parameters
appropriate for a common lipid coated contrast agent Sonazoid. The subharmonic
response is seen to either decrease, increase or vary nonmonotonically with
increasing ambient pressure. Compared to a free microbubbles important
differences arise due to the encapsulation. Specifically due to the enhanced
damping due to encapsulation, the range of excitation over which subharmonic is
seen is broader than that in free microbubbles. This results in different
trends of subharmonic response at the same excitation frequency for different
excitation pressures. The observed behaviors are explained by investigating
subharmonic generation threshold and resonance frequency.
