  We study turbulent Rayleigh-B\'enard convection over four decades of Rayleigh
numbers 4E8 < Ra < 2E12, while harmonically modulating the temperatures of the
plates of our cylindrical cell. We probe the flow by temperature sensors placed
in the cell interior and embedded in the highly conducting copper plates and
detect thermal waves propagating at modulation frequency in the bulk of the
convective flow. We confirm the recent numerical prediction [PRL 125, 154502
(2020)] of the significant enhancement of Nusselt number and report its
dependence on the frequency and amplitude of the temperature modulation of
plates.
