sentences:  Second sound is also observed in superfluid helium-3 below its lambda point 2.5 mK.Second sound has been observed in solid 4He and 3He,
  and in some dielectric solids such as Bi in the temperature
  range of 1.2 to 4.0 K with a velocity of 780 ± 50 m/s,
  or NaF around 10 to 20 K.
  
  In 2019 it was reported that ordinary graphite exhibits "second sound" at 120 Kelvin.This was both predicted theoretically and observed experimentally, and
  was by far the hottest temperature at which second sound has been observed.