text: Second sound is observed in liquid helium at temperatures below the lambda point, 2.1768 K, where He becomes a superfluid known as helium II. Helium II has the highest thermal conductivity of any known material (several hundred times higher than copper). Second sound can be observed either as pulses or in a resonant cavity.
  The speed of second sound is close to zero near the lambda point, increasing to approximately 20 m/s around 1.8 K, about ten times slower than normal sound waves.
  At temperatures below 1 K, the speed of second sound in helium II increases as the temperature decreases.
  Second sound is also observed in superfluid helium-3 below its lambda point 2.5 mK.
  As per the two-fluid, the speed of second sound is given by
  where  is the temperature,  is the entropy,  is the specific heat,  is the superfluid density and  is the normal fluid density. As , , where  is the ordinary (or first) sound speed.