The laboratory evaluation of the acoustic microbubble spectrum analyzer has disclosed several types of persisting microbubble phenomena as a varying abnormal attenuation with frequency. Microbubble populations are detected in a tank filled with tap water only sporadically and may be influenced significantly by barometric pressure. Distilled water open to the atmosphere can become rapidly charged with persisting microbubbles. An increase of temperature or a reduction of local pressure can be used to expand microbubbles into the detection range of the instrument. For limited excursions of temperature and pressure, the change in attenuation is reversible, indicating a stability range for persisting microbubbles. The observed attenuation spectra include examples of sharp peaks, which indicate a narrow range of microbubble size, and broad peaks, which indicate a distribution of sizes. [This work supported in part by the Office of Naval Research.]