ω [1015 rad/s]

ka

  

a:

 [m]

b:

 [m]

c1:

 [m/s]

c2:

 [m/s]

ωmax:

 [rad/s]

Fig. 5. The dispersion relation. The frequency of the waves are plotted against k. The gray bands correspond to the frequencies where no waves propagate through the crystal. Source file.