Scientists disagree over why cracking your knuckles makes noise. Now, a new mathematical explanation suggests the sound results from the partial collapse of tiny gas bubbles in the joints’ fluid.
Most explanations of knuckle noise involve bubbles, which form under the low pressures induced by finger manipulations that separate the joint. While some studies pinpoint a bubble’s implosion as the sound’s source, a paper in 2015 showed that the bubbles don’t fully implode. Instead, they persist in the joints up to 20 minutes after cracking, suggesting it’s not the bubble’s collapse that creates noise, but its formation (SN: 5/16/15, p. 16).
But it wasn’t clear how a bubble’s debut could make sounds that are audible across a room. So two engineers from Stanford University and École Polytechnique in Palaiseau, France, took another crack at solving the mystery.
The sound may come from bubbles that collapse only partway, the two researchers report March 29. A mathematical simulation of a partial bubble collapse explained both the dominant frequency of the sound and its volume. That finding would also explain why bubbles have been observed sticking around in the fluid.