Mix is a sound commonly used in contemporary music that you hear on the radio or in musicals. If middle singing voice is a transition between chest voice and head voice and uses a balance of those muscle groups, then mix occurs when you choose to vary the balance. If middle voice is a 50/50 balance of head voice and chest voice, the mix may shift those percentages.
For example, you may want to use 70 percent chest voice and 30 percent head voice, to create a sound that is fuller or thicker than middle voice. Other times you may want the mix to be 40 percent chest voice and 60 percent head voice; with more head voice mixed in, this sound is lighter than chest voice but fuller than head voice.
You create these variations by changing the combination of resonance, weight, and breath compression. Mix is appropriate for almost any style of singing, but it isn’t as commonly used in classical music.
The terminology can be confusing, but you need to know the phrases you may hear from other singers:
A heavy mix or a mix that has more than 50 percent chest voice is often called a chest-dominated mix because it feels full like chest voice and sounds similar to chest voice. The difference is that it isn’t pure chest voice; it has some head voice mixed in.
Likewise, a mix that’s lighter or has more than 50 percent head voice in it is often called a head voice–dominated mix.
No measuring scale can tell you the percentages; you discover how to hear the amount of weight or resonance in a sound and know that it’s a mix.