An annular granular jet surrounding an excited air jet is studied experimentally by high-speed digital photography. Particle bubbles shaped more uniformly are periodically formed due to the addition of acoustic excitation to the central air jet. Two bubbling regimes, namely acoustic excitation regime and convergent nozzle regime, are considered to coexist in the bubble-formation process. Acoustic excitation regime plays a dominant role in the bubbling process at lower superficial air jet velocity. However, for larger superficial air jet velocity, convergent nozzle regime instead of acoustic excitation regime has absolute predominance in the bubbling process. The competition between the two regimes results in a non-monotonic variation of axial bubble size. In addition, the increases of excitation frequency and SPL can both encourage the growth of bubbles in the radial direction. (C) 2014 Elsevier Ltd. All rights reserved.