sentences:  The theoretical models, however, predict longer absorption lengths would be seen in isotopically-pure graphite, 
  and perhaps over a wider temperature range, e.g.even at room temperature.Measuring the speed of second sound in 3He-4He mixtures can be
  used as a thermometer in the range 0.01-0.7 K.
  
  Oscillating superleak transducers (OST) use second sound to locate defects in superconducting accelerator cavities.