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, Oct. 2013, Accessed: Jun. 15, 2025. [Online]. Available: https://acoustics.ippt.pan.pl/index.php/aa/article/view/161