1. H. Engan, K. A. Ingebrigsten, and A. Tonning, Appl. Phys. Lett. 10, 311 (1967).APPLAB0003-6951
2. F. R. Rollins, Jr., T. C. Lim, and G. W. Farnell, Appl. Phys. Lett. 12, 236 (1968).APPLAB0003-6951
3. G. W. Farnell, in Physical Acoustics, Principles and Methods, Vol. VI, edited by W. P. Mason and R. N. Thurston (Academic, New York, 1970), p. 109.