The time interval T between p2 and p3 is defined as the period of Helmholtz oscillation, and T experimentally obtained through pressure signals decreases from 8.8 ms for pv 1⁄4 20 kPa to 7.4 ms for pv 1⁄4 270 kPa, as shown in Fig. 8. T can also be estimated by the formula reported by Alster (1972):