In this Letter we show that a recent hydrodynamical model of superfluid turbulence describes vortex density waves and their effects on the speed of high-frequency second sound. In this frequency regime, the vortex dynamics is not purely diffusive, as for low frequencies, but exhibits ondulatory features, whose influence on the second sound is here explored.Hydrodynamics of superfluid turbulence, characterized by a tangle of quantized vortex lines [1,2], aims to describe the couplings between pressure and heat perturbations, and the vortex density dynamics [3][4][5][6][7]. Such hydrodynamics is a lively topic, with recent emphasis, for instance, on nonlinear features such as the influence of intense heat pulses on the vortex tangle [8-10], or in multi-scale formulations allowing to eliminate the fast processes to derive evolution equations for the slow processes [11]. In the present Letter we will focus our attention on a different topic, namely, the behavior of linear heat waves and vortex density waves in the highfrequency domain. This has not received much previous attention, but we think it is worthwhile because, whereas at low frequencies the behavior of the vortex distortions is mainly diffusive, at high enough frequencies, the vortex lines behave as an elastic medium, and are able to propagate waves by themselves, i.e. they behave like a viscoelastic medium: diffusive at low frequencies, elastic at high frequencies. Their effect on second sound propagation is of much interest to provide a suitable physical interpretation of the experimental data based on second sound.Collective vortex waves in rotating superfluids have been studied in depth for a long time [12], and the so-called Tkachenko transverse elastic waves in the vortex arrays with crystalline order arising in rotating cylinders have been theoretically discussed since 1966. However, vortex density waves in counterflow situations have not been studied, up to our knowledge.Usually, one considers homogeneous vortex tangles in counterflow situation (i.e. under vanishing barycentric speed) and the evolution of L, the vortex-line density, is assumed to be the well-known Vinen's equation [1,2,6,13] (1)where q is the absolute value of the heat flux q, A and B parameters linked to the dimensionless coefficients appearing in Vinen's equation by the relations A = α v /ρ s T s and B = κβ v [7], ρ s being the density mass of the superfluid component, T the absolute