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, candy wrappers (or other elastic sheets), 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.