L. S. Giap and O. S. Fen, “The Effectiveness of Teaching Non-Native Learners in Recognizing the Differences of 4 Tones using a Romanised Mandarin Web-based Instruction”, JEVR, vol. 1, no. 3, pp. 67–71, Jun. 2011, doi: 10.22610/jevr.v1i3.11.