1. Sweet, Primer of Phonetics, p. 40
2. Panooncelli-Calzia, Die experimentelle Phonetik in ihrer Anwendung auf die Sprachwissenschaft, 1924, pp. 69 et seq.
3. “ English School of Phonetics ”, Transactions of the Philological Society, 1946, pp. 98, 127 ff.
4. A Study in the Phonology of an Igbo Speaker;Carnochan;BS0AS,1948