  Using three-dimensional magnetohydrodynamics simulations, we show that the
efficiency of cosmic-ray (CR) production at supernova remnants (SNRs) is
over-predicted if it could be estimated based on proper motion measurements of
H$\alpha$ filaments in combination with shock-jump conditions. Density
fluctuations of upstream medium make shock waves rippled and oblique almost
everywhere. The kinetic energy of the shock wave is transferred into that of
downstream turbulence as well as thermal energy which is related to the shock
velocity component normal to the shock surface. Our synthetic observation shows
that the CR acceleration efficiency as estimated from a lower downstream plasma
temperature, is overestimated by 10-40%, because rippled shock does not
immediately dissipate all upstream kinetic energy.
