Crackling noise
Crackling noise arises when a system is subject to an external force and it responds via events that appear very similar at many different scales. In a classical system there are usually two states, on and off. However, sometimes a state can exist in between. There are three main categories this noise can be sorted into: the first is popping where events at very similar magnitude occur continuously and randomly, e.g. popcorn; the second is snapping where there is little change in the system until a critical threshold is surpassed, at which point the whole system flips from one state to another, e.g. snapping a pencil; the third is crackling which is a combination of popping and snapping, where there are some small and some large events with a relation law predicting their occurrences, referred to as universality. Crackling can be observed in many natural phenomena, e.g. crumpling paper, fire, occurrences of earthquakes and the magnetisation of magnets. Some of these systems are reversible, such as demagnetisation (by heating a magnet to its Curie temperature), while others are irreversible, such as an avalanche (where the snow can only move down a mountain), but many systems have a positive bias causing it to eventually move from one state to another, such as gravity or another external force. (Wikipedia).
