Nonlinear Effects in a Focused Acoustic Beam in Pressurized Superfluid 4He
Sound propagation in superfluid 4He can quite easily become highly nonlinear at a moderate sound energy, because its density and sound velocity are both very low. Furthermore, one can change the dispersion relation of phonon in superfluid 4He from anomalous to normal by changing the pressure. The propagation properties of the finite amplitude acoustic wave has been investigated on the basis of these aspects(1). Using a high frequency focused pulse, we studied the propagation of acoustic waves which have much higher energy density than used in previous works, and observed new nonlinear effects(2).
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