Spontaneously flowing liquids have been successfully engineered from a variety of biological and synthetic self-propelled units^{1,2,3,4,5,6,7,8,9,10,11}. Together with their orientational order, wave propagation in such active fluids has remained a subject of intense theoretical studies^{12,13,14,15,16,17}. However, the experimental observation of this phenomenon has remained elusive. Here, we establish and exploit the propagation of sound waves in colloidal active materials with broken rotational symmetry. We demonstrate that two mixed modes, coupling density and velocity fluctuations, propagate along all directions in colloidal-roller fluids. We then show how the six material constants defining the linear hydrodynamics of these active liquids can be measured from their spontaneous fluctuation spectrum, while being out of reach of conventional rheological methods. This active-sound spectroscopy is not specific to synthetic active materials and could provide a quantitative hydrodynamic description of herds, flocks and swarms from inspection of their large-scale fluctuations^18,19,20,21.

自发流动的液体已经成功地由各种生物和合成的自推进单元^{1,2,3,4,5,6,7,8,9,10,11设计而成}。连同它们的取向顺序，在这种活性流体中的波传播仍然是激烈的理论研究的主题^{12,13,14,15,16,17}。但是，对该现象的实验观察仍然难以捉摸。在这里，我们建立和利用声波在破坏旋转对称性的胶体活性物质中的传播。我们证明了两种混合模式，耦合密度和速度波动，在胶辊流体中沿所有方向传播。然后，我们展示了如何从它们的自发波动光谱中测量定义这些活性液体的线性流体动力学的六个材料常数，而这却是常规流变方法所无法达到的。这种有源声波谱仪不是专门针对合成活性物质的，可以通过对它们的大范围波动^{18、19、20、21的}观察来提供对畜群，群和群的定量水动力描述。