When a dynamic system undergoes a cyclic evolution, a geometric phase thatdepends only on the path traversed in parameter space can arise in addition tothe normal dynamical phase. Such geometric phases have profound impacts in bothquantum and classical physics. In classical systems, the geometric phase effectis commonly associated with the polarizations of vector waves, which are absentin scalar wave systems. Here, by exploiting the orbital angular momentum ofsound vortices as a new degree of freedom, we successfully observed thegeometric phase effect in airborne sound, which is a scalar wave. We show withboth theory and experiments that these effects, associated with the helicaltransportation of sound, can be used to control the flow of sound vortices.This finding opens new possibilities for the manipulation of scalar wavepropagation by exploiting geometric phases.
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