Abstract: The speech broadcasting system for a long range is a problem of speech propagation in at-mospheve. The principal parameters concerning this system are the average spectrum of speech, the function of signal pre-processing ,the characteristics of electro-acoustic transformation and the attenuation of sound transmitted in air. In most cases, it is complicated to analyse interrelations among these parameters, and therefore designing the broadcasting system seems difficult. In this paper, on the basis of theoretical derivations and experimental results on these parameters, the approximate analytic formulas of the relation between these parameters and frequency are presented by an analysis method of curve fitting. It is convenient to design the broadcasting system by these formulas. As an example, a designing method of the broadcasting system for 8-10 km is introduced.