  The attenuation of small-amplitude acoustic waves in a suspension containing
ultrasound contrast agents (UCAs, coated microbubbles) is determined by the
linear oscillation of the UCAs in the medium, which can be estimated via a
linear attenuation theory. Recently, several nonlinear phenomena of energy
attenuation at very low-intensity of acoustic pressures have been observed
experimentally, raising concerns on the validity of the linear attenuation
theory. Explanations of the nonlinear phenomenon are still lacking.
Particularly, the interpretation of the pressure-dependent attenuation
phenomenon is still under debate. In this note, we investigated the energy
dissipation of a single UCA via a nonlinear Rayleigh-Plesset equation and used
a formula capable of estimating attenuation coefficient due to the nonlinear
oscillation of the UCA. The simulation results show the linear oscillation of
an UCA at low excitation pressures does not always guarantee the linearity in
the energy attenuation. Although nonlinear oscillation of the UCA contributes
to the occurrence of nonlinear attenuation phenomena, it is not the only
trigger.
