Crackling noise, power spectra, and disorder-induced critical scaling

Crackling noise is observed in many disordered nonequilibrium systems in response to slowly changing external conditions. Examples range from Barkhausen noise in magnets to acoustic emission in martensites to earthquakes. Using the nonequilibrium random-field Ising model, we derive universal scaling predictions for the dependence of the associated power spectra on the disorder and field sweep rate, near an underlying disorder-induced nonequilibrium critical point. Our theory applies to certain systems in which the crackling noise results from an avalanchelike response to a (slowly) increasing external driving force, and is characterized by a broad power-law scaling regime of the power spectra. We compute the critical exponents and discuss the relevance of the results to experiments.