  Brittle failures of materials and earthquakes generate acoustic/seismic waves
which lead to radiation damping feedbacks that should be introduced in the
dynamical equations of crack motion. We present direct experimental evidence of
the importance of this feedback on the acoustic noise spectrum of
well-controlled spring-block sliding experiments performed on a variety of
smooth surfaces. The full noise spectrum is quantitatively explained by a
simple noisy harmonic oscillator equation with a radiation damping force
proportional to the derivative of the acceleration, added to a standard viscous
term.
