What is music theory?
Music theory is essentially the technical explanation for how music works. This covers the relationships between melodies, scales, chords, and keys. It also covers the rhythmic aspects of music. If you want to explore regarding the private singing lessons in Sydney, then visit https://www.unlockyourvoice.net/sydney-singing-lesson/.
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Is it necessary?
To some degree, this depends on the level that you aspire to as a musician. The higher the level you wish to achieve, the more important your technical understanding matters. Having said that, even if you don't aspire to be world-class, theory can bring huge benefits to you.
On a very basic level, having meaningful names to give to musical features makes them easier to remember. With just a little grounding in theory a musician can describe a chord sequence as saying 'a vi ii V I in the key of D'.
This description carries a lot of very useful information with it and a theory-literate musician will understand – before they have even heard the song – how they could approach the song.
Could you get away without knowing any theory?
Yes, and there are certainly examples of great musicians that have little technical understanding of music.
Note though, that these musicians have not achieved what they have because they haven't learned theory. They have achieved their level of musicianship despite not having learned theory.
What resources are available?
There are numerous music theory books available. You can take lessons from music theory tutors. You could buy a music theory app to work on in your spare time. You could work through video lessons online.