Abstract: In this work, the characteristics of sound spectra in chocked jet noise are studied. We derived the relation of the first frequency in screech spectra to pressure ratios, and the directivity of the screech spectra in far field. In our experiment, a convergent nozzle with diameter 20 mm was used and the pressure ratio from 2.05 through 3.95 was taken. A new method which estimates the distance 8, from the first cell to the nozzle, and the ratio N of vortex convection velocity V_c to jet velocity V_c was found, based on the directivity of screech. The values of N obtained in this work are 0.49-0.74 (average 0.61), in good agreement with that obtained in low Mach number by other. authors.