  Surface acoustic waves (SAWs) provide a contactless method for measuring the
wavevector-dependent conductivity. This technique has been used to discover
emergent length scales in the fractional quantum Hall regime of traditional,
semiconductor-based heterostructures. SAWs would appear to be an ideal match
for van der Waals (vdW) heterostructures, but the right combination of
substrate and experimental geometry to allow access to the quantum transport
regime has not yet been found. We demonstrate that SAW resonant cavities
fabricated on LiNbO$_3$ substrates can be used to access the quantum Hall
regime of high-mobility, hexagonal boron nitride (hBN) encapsulated graphene
heterostructures. Our work establishes SAW resonant cavities as a viable
platform for performing contactless conductivity measurements in the quantum
transport regime of vdW materials.
