  ``Transition frequencies'' of a system containing an arbitrary number of
bubbles levitated in a liquid are discussed. Using a linear coupled-oscillator
model, it is shown theoretically that when the system contains $N$ bubbles of
different sizes, each bubble has $2N - 1$ (or less) transition frequencies
which make the phase difference between an external sound and a bubble's
pulsation $\pi / 2$. Furthermore, we discuss a discrepancy appearing between
the present result regarding the transition frequencies and existing ones for
the resonance frequencies in a two-bubble case, and show that the transition
frequency, defined as above, and the resonance frequency have a different
physical meaning when $N \ge 2$, while they are consistent for $N = 1$.
