Abstract: This papaer presents an analytic formula for the locus of shock front of spherical blast waves with back pressure. Meanwhile, it is shown that the wave velocity is inversely proportions to the area of shock wave in the Lagrangian coordinate space. If shock wave is propagating into the gas at rest, the wave velocity is also inversely propagating into the gas at rest, the wave velocity is also inversely proportional to the area of shock front in the Eulerian coordinate space. The results obtained are generally applicable and may be suited to the wave velocity of contracting spherical shock wave.

Key words: Blast wave, Wave velocity formula, Area