One involves not sound waves but ripples on the surface of a liquid or along the interface between layers of superfluid helium, which is so cold that it has lost all frictional resistance to motion.

• 其中一种牵涉到的并非声波，而是液体表面或是沿著两层超流氦之间介面传播的涟漪（超流氦冷到在运动时完全没有摩擦阻力）。