1.7.3.1. Video: 1 hour of English Conversation Training! #learnenglish - YouTube - https://www.youtube.com/watch?v=_LoERB1453c

1.7.3.2. Video: Learn English Pronunciation | Vowel Sounds | 23 Lessons - YouTube - https://www.youtube.com/watch?v=USKrBTvgY_s

1.7.3.3. Article: [PDF] Instrumental Analysis of English Vowels Produced by Male and ... - https://www.sciedupress.com/journal/index.php/wjel/article/download/23845/15063

1.7.3.4. Article: What Acoustic Studies Tell Us About Vowels in Developing and ... - https://pmc.ncbi.nlm.nih.gov/articles/PMC7893529/