## 1 Introduction  
Pitch is a feature employed at different levels of a phonetic token to fulfill various grammatical functions. In tonal languages, pitch indicates tone, thereby distinguishing different semantic meanings. Conversely, pitch at the word level can signify stress, similar to many stress-timed languages, and at the phrasal and sentence levels, it creates intonation patterns. It is, therefore unsurprising that Mandarin Chinese, as a tonal language, is often reported as challenging for speakers of non-tonal languages to perceive (cf. [1], for French; [2; 3], for English) and to produce (cf. [4; 5], for English, [6] for Japanese, English and Korean; [7; 8] for Polish). Several cross-linguistic studies have documented tone production errors among speakers of non-tonal languages. Some studies suggest that T1 and T4 are easier to acquire (cf. [9; 10; 11]), while others indicate that T2 and T3 are easier [5]. Most research focuses on monosyllabic words, showing that tone errors can be analyzed based on tonal register or tonal contour. When examining interlanguage tonal distribution, stress-timed languages appear to have a limited impact on learners' ability to discriminate and produce tones [12; 13; 14]. However, studies focusing on disyllabic words, particularly among Polish learners of Mandarin, are rare. Regarding the difficulties encountered by non-native Mandarin speakers in learning the tones of disyllabic Mandarin words, the most extensive research has been conducted on Japanese learners. For instance, ref. [15] found that Japanese students learning Mandarin frequently made errors when the first syllable was T2 or T3. Most tonal errors occurred in disyllabic words with the tone combination T2T1, especially when the first syllable was T2, which was often incorrectly pronounced as T2T3. The researchers attributed this to the Japanese language, where disyllabic compound words typically allow only one peak, meaning the first syllable must have an opposite stress to the second syllable. Similarly, ref. [16] examined 25 Japanese learners of Mandarin who were asked to pronounce 80 disyllabic Mandarin words with all possible tone combinations. Most errors involved T2 and T3, with the majority occurring in the first syllable when it was T2 or T3, especially when followed by high starting tones such as T1 and T4. Errors in the second syllable predominantly involved T3. A similar error pattern was found in [17], which involved 25 Korean learners of Mandarin. The difficulty for the first syllable was T2 = T3 > T1 = T4, with more errors when T2 or T3 was followed by the high starting tones T1 or T4. Ref. [18] investigated the difficulties of beginner Japanese students in perceiving and producing disyllabic words. They found that initial consonants were more challenging than vowels in perception, but there was no similar result in production. Ref. [19] examined nine German participants in Mandarin Chinese perception and production tasks, finding that they had the worst performance in perceiving rising tones. For the first syllable, T2 was the most difficult to produce, while T4 was the most challenging in the second syllable. Overall, no similar results were found between disyllabic and monosyllabic words in production performance, with participants appearing to use their preferred tone combinations to handle disyllabic words. Taken together, the production of Mandarin disyllabic words among L2 learners of Mandarin Chinese who speak Japanese, Korean, or German is gaining increasing attention. This trend likely results from the prevalence of disyllabic communication patterns in modern Mandarin [20]. Therefore, describing the production process of L1-dominant Polish speakers learning Mandarin (L2) cannot omit their current performance of disyllabic words. Previous research on high-variability training has yielded mixed results. Some studies have demonstrated significant advantages [21; 22; 23; 24], while others have found no significant benefits [25; 26; 27; 28]. For instance, ref.