These formulas are intended for maximum comfortable speech amplification and best sound intelligibility. Schwa deletion is important for intelligibility and unaccented speech. The higher the linguistic distance, the lower the mutual intelligibility. Mutual intelligibility between some dialects can be low. "Ethnologue" follows general linguistic criteria, which are based primarily on mutual intelligibility. Intelligibility or grammatical correctness are not taken into account. The variables studied were "intelligibility" and "fidelity". The absolute measurement of speech intelligibility is a complex science. STI has gained international acceptance as the quantifier of channel influence on speech intelligibility. The Ukrainian language retains a degree of mutual intelligibility with Belarusian and Russian. This is very acceptable for communications radios, where compression of the audio aids intelligibility. The mutual intelligibility between the Continental Scandinavian languages is asymmetrical. There is a high degree of mutual intelligibility between these languages. There is a high degree of mutual intelligibility among Belarusian, Russian, and Ukrainian languages. Standard Slovak shares high degrees of mutual intelligibility with many Slavic varieties.