A characteristic frequency of two mutually interacting gas bubbles in an acoustic field

Transition frequencies of two spherical gas bubbles interacting in an acoustic field are discussed theoretically. In the present study, transition frequency is defined as the frequency of external sound for which the phase difference between a bubble’s pulsation and the external sound is π/2. It is shown by a linear theory that a bubble interacting with a neighboring bubble has three (or fewer) transition frequencies but only two natural frequencies. This result means that the bubble has a characteristic frequency besides the natural frequencies.