Superfluid density of3He measured by fourth sound

Superfluid density of³He has been measured using fourth sound in two superleaks: “confined,” packed powder and “open,” parallel channels. The superleak pore size has little effect on the superfluid density near the melting pressure, but it has surprisingly large effects at lower pressures. The open, parallel channel superleak results show an unexpected pressure dependence of “strong coupling” effects and an unexpectedly small superfluid density change at the B→A transition.