The cracking or popping sound is thought to be caused by the gases rapidly coming out of solution, allowing the capsule to stretch a little further.

But how can releasing such a small quantity of gas cause so much noise? There is no good answer for this question. Researchers have estimated the energy levels of the sound by using accelerometers to measure the vibrations caused during joint popping.

The amounts of energy involved are very small, on the order of 0.1 milli-joule per cubic millimeter. Studies have also shown that there are two sound peaks during knuckle cracking, but the causes of these peaks are unknown. It is likely that the first sound is related to the gas dissolving out of solution, whereas the second sound is caused by the capsule reaching its length limit.

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