Source 1: https://youtu.be/4IXOM4Q2Apc?t=6
Source 2: https://youtu.be/4IXOM4Q2Apc?t=8
Source 3: https://ramseyvoice.com/wp-content/uploads/2018/03/vibrato.jpg
Source 4: https://youtu.be/j6TmogXhOZ8?t=17
Source 5: https://youtu.be/Ve75y5NbqL4?t=57
Source 6: https://youtu.be/B5qULV6x_cE?t=68
Source 7: https://youtu.be/DgGtW1NWzfE?t=42
Source 8: https://youtu.be/UeRSqekHh1g?t=51
Source 9: https://www.merriam-webster.com/dictionary/vibrato
Source 10: https://cvtresearch.com/description-and-sound-of-vibrato/
Source 11: https://www.youtube.com/watch?v=TGXBzNV8b2U
Source 12: https://ramseyvoice.com/how-to-sing-vibrato/
Source 13: https://www.youtube.com/watch?v=4IXOM4Q2Apc
Source 14: https://www.youtube.com/watch?v=B5qULV6x_cE
Source 15: https://cvtresearch.com/description-and-sound-of-vibrato/