where r is the ratio of specific heats, R is the gas constant and T is the temperature of gas mixture in vessel. C, calculated using equation (2), ranges from approximately 360 m/s for unburnt gas to approximately 860 m/s for burnt gas mixtures. This yields a period of Helmholtz oscillation from 9.6 ms to 4.0 ms by equation (1). Measurement of the period ranges from 8.8 ms to 7.4 ms, which is within the range of theoretical values. In addition, Fig. 8 shows that T decreases when pv increases; a possible reason for this behavior is that the average sound speed of the gas mixtures in the vessel increase with the increase of pv(McCan et al., 1985).