In some embodiments, a method for sonically cleaning a reactor (for example, a fluidized bed reactor useful for the production of polyolefins) using a set of sonic sources, including by varying the operating mode of the set of sources to reduce or prevent cleaning problems that would otherwise result from weak spots if the operating mode were not so varied. Other embodiments are methods for determining positions and operating parameters (e.g., duty cycle and output acoustic wave frequency) of each source of a set of sonic sources to be used for sonically cleaning a reactor, and methods including the steps of determining a position (relative to a reactor) of each source of a set of sonic sources, positioning each said source in the determined position, and then sonically cleaning a surface of the reactor including by varying the operating mode of the set of sources to reduce or prevent cleaning problems that would otherwise result from weak spots if the operating mode were not so varied.