Engstrand (1990), p.43. Engstrand, Olle (1990). "Swedish (Illustrations of the IPA)". Journal of the International Phonetic Association. 20 (1). Cambridge University Press: 42–44. doi:10.1017/S0025100300004060.
Mayr (2010, pp.93–94), citing Catford (2001) Mayr, Robert (2010). "What Exactly Is a Front Rounded Vowel? An Acoustic and Articulatory Investigation of the NURSE Vowel in South Wales English". Journal of the International Phonetic Association. 40 (1): 93–112. JSTOR44527004. Catford, J. C. (2001). A Practical Introduction to Phonetics (2nded.). Oxford University Press. ISBN978-0199246359.