Relations of 1/fand 1/f2power spectra to self-organized criticality

We find that the discrete sandpile type of models in finite dimension under a uniform particle addition must result in 1/${\mathit{f}}^2$ or 1/(f+a${)}^2$ power spectrum depending on the existence of particle conservation law or not. However, their continuous counterparts may exhibit 1/f instead. This difference in power spectrum behavior can be expoained in terms of rate of relaxation of the system in the continuous models.