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Passive cavitation detection was used to improve the experimental characterization of single ultrasound contrast agent microbubble responses to short, large amplitude pulses. Two situations were examined: isolated microbubbles in an unconstrained environment, and isolated microbubbles flowing through a tube. The microbubbles were categorized according to a classification scheme based on the presence or absence of postexcitation signals, which are secondary broadband spikes that may follow the principle oscillation of the ultrasound contrast agent in response to an insonifying pulse. Experiments were conducted for different frequencies, peak rarefactional pressures, flow rates, and types of microbubble. Postexcitation activity was found to increase as frequency decreased, acoustic pressure increased, and flow rate increased. Additionally, lipid-shelled microbubbles were found to exhibit greater postexcitation at lower acoustic pressure thresholds than albumin-shelled microbubbles.