H. DING, R. HOFFMANN, and O. JOKISCH, “An Investigation of Tone Perception and Production in German Learners of Mandarin”, Arch. Acoust., vol. 36, no. 3, pp. 509–518, Sep. 2011, Accessed: Mar. 04, 2026. [Online]. Available: https://acoustics.ippt.gov.pl/index.php/aa/article/view/161