• These are “discontinuous” i.e. intermittent, “explosive” sounds.
• Laennec described them as sounding like the crackling noise made when salt is heated on a frying pan.
• They are caused by airway opening.

Crackles are intermittent explosive sounds that have been described as being similar to the crackling sound heard as wood burns. Crackles appear in the time domain as intermittent spike-like deflections. They can be seen in the unexpanded display but are usually seen more readily in the expanded mode as shown in the figure. Considerable evidence has been presented in support of the hypothesis that crackles are caused by the sudden opening of airways. It is likely that they are also caused by fluid in the airways. Crackles are divided into two types, fine and coarse by their acoustic properties. On auscultation fine crackles are in general higher pitched, less intense and of shorter duration than coarse crackles. Crackle waveform features such as initial deflection width, largest cycle deflection, number of zero crossings, etc. have been used to separate fine and coarse crackles objectively. These characteristics can be measured and utilized in computer algorithms allowing automatic detection, counting and classification at the bedside. Note that the crackling sound can be transmitted throughout the chest. In this case the crackling sound generated by the same event can be recorded and identified by multiple microphones. When crackles are counted one has to pay special attention to avoid counting the same event at multiple microphones. The term unrelated crackle rate underlines the fact that only crackles representing unrelated events are counted. While fine crackles can be heard in normals particularly after special maneuvers such as after breath holding at low lung volumes, they are usually a sign of disease.