## 2 Background of This Research
### Studies about Mandarin Chinese Tone Pronunciation  
There are several languages called tonal languages [7] that distinguish the meaning of spoken words based on pitch change patterns included in each syllable. It takes a great deal of effort and time for second language learners to recognize tone correctly and learn to pronounce the words with the right tone, especially for the non-tone native-language speakers. Furthermore, there is a large individual difference in tone learning speed, which places a heavy burden on educators who must help learners deal with tone leaning difficulties and keep learners motivated. We targeted Mandarin Chinese, which has four main tones, and investigated the study of Mandarin Chinese tone pronunciation. The four main tones used in Mandarin Chinese conversation are the flat-high tone (tone 1), medium-rising tone (tone 2), low-dipping tone (tone 3), and high-falling tone (tone 4), as shown in Figure 1 using [PERSON]'s five-level tone mark [8]. Compared with languages (like Vietnamese) that have six or more tones, the features of Mandarin Chinese tones are more obvious and may seem easier. However, the acquisition of Mandarin Chinese tones is difficult for non-Mandarin Chinese learners, and many studies on Mandarin Chinese tones and tone pronunciation have been conducted to solve this issue. There are several studies on Mandarin Chinese tonal pronunciation targeting learners whose native language is Vietnamese, which has more kinds of tones [9; 10; 11]. Vietnamese speakers, whose native language is a tone language, have the knowledge of a tone language, and they have mastered the Vietnamese tones that seem to be more difficult than Mandarin tones. However, due to the negative transfer of the mother tongue, the Vietnamese tones that learners are used to speaking may cause pronunciation errors. In addition, if the similarity between the native language and the Mandarin Chinese tone is high, specific pronunciation errors are likely to occur frequently, such as that the pitch should be slightly higher or that the falling of the pitch should be more obvious. The majority of the research on the pronunciation of Mandarin Chinese tones that we investigated focused on error analysis and error corrections by phonetic experiments. Though some research may be conducted without restrictions on the subject's native language and learning experience [12], most of the research is to specify the subject's native language, discuss the differences between the native language and Mandarin Chinese, then discuss the pronunciation errors based on the results of the experiments [13; 14; 15; 16; 17; 18; 19; 20; 21]. However, the research on Mandarin Chinese tone pronunciation has little content on tone learning method, and the conclusion of the experiment is mainly the error analysis of tone pronunciation of Mandarin Chinese learners, such as the order of difficulty of each tone and the cause analysis result of pronunciation errors. As a result, almost every study focuses on the negative transfer of the learners' mother tongue. In addition, according to the related works, the tone pronunciation of subjects with more than two years of learning experience is far from ideal, and there is an experiment result even showing that the tone pronunciation ability of learners with more than six years of learning experience is even lower than those of less than one year [15]. Regarding tone learning, a lack of guidance and learners not being conscious of tones, especially fossilized errors due to the neglect of tone learning in initial education, are the reasons that even advanced learners have such a high tone error rate. The general method of Mandarin Chinese tone learning is to practice the pronunciation of words, phrases, short sentences, etc. The tone learning method mainly used in previous research and the remedial learning after error analysis is pronunciation practice or practicing pronunciation of two-syllable words focused on the combination of tones and short sentences.