1. Glossary of singing terms. Available at: https://www.successfulsinging.com/singing-guides/glossary-of-singing-terms/. (Accessed March 25, 2020).
2. Sing! The Vocal Power Method;Howard,2006
3. The Art of singing;Mánén,1974
4. Tunne Kehosi-Vapauta Aänesi, Äänitimpurin Käsikirja, [Feel Your Body – Free Your Voice], in Finnish;Koistinen,2004
5. What is Vocal Placement and How to Project Your Voice? Available at:https://www.becomesingers.com/techniques/vocal-placement. (Accessed March 25, 2020).