Lee, Wai-Sum; Zee, Eric. Standard Chinese (Beijing). Journal of the International Phonetic Association. June 2003, 33 (1): 109–112. doi:10.1017/S0025100303001208.
Lee-Kim, Sang-Im. Revisiting Mandarin 'apical vowels': An articulatory and acoustic study. Journal of the International Phonetic Association. December 2014, 44 (3): 261–282. doi:10.1017/S0025100314000267.