1. Beard, D. W. & Walters, K. 1964 Proc. Camb. Phil. Soc. 60,667.
2. Andres, J. M. & Ingard, U. 1953 J. Acoust. Soc. Am. 25,928.
3. Rayleigh, Lord 1883 Phil. Trans. A175, 1.
4. Riley, N. 1965 Mathematika,12, 161.
5. Stuart, J. T. 1966 J. Fluid Mech. 27,445.