Abstract: The invention can be used to monitor the cleaning effectiveness of a cleaning system in which vibrations (such as, for example, vibrations having a megasonic or ultrasonic frequency) are applied to a volume of fluid (e.g., water) in which an object to be cleaned (e.g., a substrate, such as semiconductor wafer or other semiconductor substrate) is at least partially immersed. Generally, the invention monitors the state of a physical characteristic of the fluid during the application of the vibrations, thus providing a more direct and accurate measure of the effectiveness of the cleaning than has heretofore been obtainable. For example, the magnitude of the acceleration of a pressure wave produced in the volume of fluid by the vibrations can be monitored, thus enabling the amplitude and/or frequency of the pressure wave to be determined.