Fig. 1. Comparison of the waveform of a 4 Hz driving voltage to the vibrator and the resulting acceleration and displacement of the chamber, as well as the pressure changes inside the chamber. The pressure was measured close to the leading and the trailing chamber wall, respectively. The figure reflects relative waveforms only, and all parameters are presented in arbitrary units. The driving waveform comprised a single cycle of a sinusoid that was d.c.-shifted one peak value and phase-shifted to start at –90°. The waveform of the initial acceleration approached a sinusoid of about 1.7 x the driving frequency, whereas the frequency of the initial compression or rarefaction inside the chamber was about 2.1 x the driving frequency.