1. See for instance:H. Kolsky:Stress Waves in Solids (New York, 1958);N. Davids Editor:Int. Symp. on Stress Wave Propagation in Materials (New York, 1960);J. D. Ferry:Viscoelastic Properties of Polymers (New York, 1970).
2. L. A. Vainshtein:Sov. Phys. Tech. Phys. (English Transl.),2, 2420 (1957).
3. L. Brillouin:Wave Propagation and Group Velocity (New York, 1960), chapts. 4 and 5.
4. L. Brillouin:Wave Propagation in Periodic Structures (New York, 1953), chapt. 5.
5. S. C. Hunter:Progress of Solid Mechanics, edited byI. N. Sneddon andR. Hill, vol.1 (Amsterdam, 1960), p. 3.