In addition, Helmholtz oscillation was absent for K=V2/3/A>4.2 in the experiments of McCan et al. (1985), and other research studies also found that it occurs only for large vent areas (Kordylewski and Wach, 1988; Rocourt et al., 2014). However, in the current study, Helmholtz oscillation of the internal flame bubble occurs in the vessel with Kz11. Thus, whether Helmholtz oscillation can occur depends on other factors in addition to K, such as the type and concentration of the gas mixtures because Kumar et al. (1989) also detected Helmholtz oscillation for lean hydrogen-air explosions in a 2.3-m-diameter sphere connected to a duct with a 45-cm internal diameter.