We study the energy and magnetization noise spectra associated with first and second order phase transitions by
using Monte Carlo simulations of the Ising model and 5-state Potts model in 2D. For a finite size system, the total
noise power and the low frequency white noise S(f < fknee) increase as Tc is approached. In the thermodynamic
limit S(f < fknee) diverges but fknee → 0 and the total noise power vanishes. f-1knee is approximately the
equilibration time. At high frequencies S(f > fknee) ~ f-μ. For the Ising model, we relate μ to the critical
We show that 1/f noise is produced in a 3D electron glass by charge fluctuations due to electrons hopping between isolated sites and a percolating network at low temperatures. The low frequency noise spectrum goes as ω-α with α slightly larger than 1. This result together with the temperature dependence of \alpha and the noise amplitude are in good agreement with the recent experiments. These results hold true both for a noninteracting electron glass with a flat density of states and for a Coulomb glass. In the latter case, the density of states has a Coulomb gap that fills in with increasing temperature. For a large Coulomb gap width, this density of states gives a dc conductivity with a hopping exponent of ≈ 0.75 which has been observed in recent experiments. For a small Coulomb gap width, the hopping exponent ≈ 0.5. At low temperatures the noise amplitude of a noninteracting electron glass increases linearly with temperature while the noise amplitude of a Coulomb glass increases quadratically.