Considering all three polarization states together also means that an effective sonic velocity c e f f must be determined and used as the value of the standard sonic velocity c s . The Debye temperature T D defined below is proportional to c e f f ; more precisely, T D − − 3 ∝ ∝ c e f f − − 3 := 1 3 c l o n g − − 3 + 2 3 c t r a n s − − 3 , where longitudinal and transversal sound-wave velocities are averaged, weighted by the number of polarization states. The Debye temperature or the effective sonic velocity is a measure of the hardness of the crystal.