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.^[1] Crackling can be observed in many natural phenomena, e.g. crumpling paper,^[2] candy wrappers (or other elastic sheets),^[3][4] fire, occurrences of earthquakes and the magnetisation of ferromagnetic material.

Cracking noise contrasts with snapping noise and popping noise. Snapping noise is one large yielding event, while popping noise is a constant level of similar-sized, small yielding events. Crackling is between these. It occurs when connection strengths between components of the system is at a critical level, such that there are many yielding events with sizes spanned across several orders of magnitude.^[5]

Some of these systems are reversible, such as demagnetisation (by heating a magnet to its Curie temperature),^[6] 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.