Abstract: In the paper two kinds of primary envelopes which are close to actual conditions are suggested,and the transient parametric pressure waveforms as well as thier spectra resulting from the self-demodulation of pulses are investigated.It is shown that the spectral components referred to the first kind of primary envelopes are in the higher frequency domain.However,for the second kind of primary envelopes,besides the above-mentioned spectral components more abundent components in lower frequency domain are contained.When a pulse train is available,the spectrum of parametric pressure has a periodic variability with its pulse duration ratio.