The present invention provides a protective headgear having a multi-layered shell to attenuate amplitude of incident mechanical waves of a blunt trauma to a human head by inducing destructive interference with the incident mechanical waves through phase reversal of reflected mechanical waves reflecting off a boundary between two adjacent layers of the protective headgear, and by coaxially converging the phase-reversed reflected mechanical waves with the incident mechanical waves. The multi-layered shell is configured to equally protect people having a similar size of head but with a range of body weight.