From water to vine to bourbon and vodka, most of the fluids we are familiar with are characterized by two material constants: their density and their shear viscosity. In stark constrast, all hydrodynamic theories of active liquids involve a host of hydrodynamic coefficients that have been out of reach of conventional rheological characterization.
In this talk I will demonstrate a method dubbed active sound spectroscopy which makes it possible to fully characterize the hydrodynamics of spontaneously flowing liquids. To do so I will first experimenatlly evidence the free propagation of two sound modes in polar active liquids assembled from colloidal rollers. I will then show how to infer the material constants of this active material from the dispersion relation and the spectral width of these active sound modes.