  We investigate effects of upstream density fluctuations on the diffusive
shock acceleration by Monte Carlo simulations. The simulations show that
particles are reaccelerated in the shock downstream region by a sound wave
generated at the shock front. The time scale of turbulent acceleration by the
sound wave is estimated. We propose a new back reaction of particles
accelerated around the shock front. The accelerated particles generate the
upstream density fluctuations by the Drury instability, which are converted to
the downstream sound waves by the shock. The downstream sound waves modify the
momentum spectrum of particles accelerated around the shock front. This new
back reaction affects emission from the accelerated particles, which gives a
new constraint on the acceleration efficiency of the diffusive shock
acceleration.
