Sound waves through the body are unique enough to be a biometric:

"Modeling allowed us to infer what structures or material features of the human body actually differentiated people," explains Joo Yong Sim, one of the ETRI researchers who conducted the study. "For example, we could see how the structure, size, and weight of the bones, as well as the stiffness of the joints, affect the bioacoustics spectrum."

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Notably, the researchers were concerned that the accuracy of this approach could diminish with time, since the human body constantly changes its cells, matrices, and fluid content. To account for this, they acquired the acoustic data of participants at three separate intervals, each 30 days apart.

"We were very surprised that people's bioacoustics spectral pattern maintained well over time, despite the concern that the pattern would change greatly," says Sim. "These results suggest that the bioacoustics signature reflects more anatomical features than changes in water, body temperature, or biomolecule concentration in blood that change from day to day."

It's not great. A 97% accuracy is worse than fingerprints and iris scans, and while they were able to reproduce the biometric in a month it almost certainly changes as we age, gain and lose weight, and so on. Still, interesting.