In order to create mix you have to have two equally strong registers that automatically cross in the middle. AUTOMATICALLY.
If you do not develop head register, it will not be strong in the bottom pitches. It takes time to make it strong and clear in a lower pitch range because of the vocal fold behavior. An easy way to get it to be clear is simply to squeeze the throat. That can work on a short term basis, but eventually, it’s not a productive functional behavior. You cannot get head register to be strong as long as the sound is breathy. A tricky situation.
Chest register is the normal mode of speech (modal) in most people, but not in everyone. And even in people who have a strong chest dominant speech, as the pitch range goes down, it will also get weaker, as the folds shorter and loosen to get those pitches. Therefore, you must develop strength in chest for most singers as well and the vowels must remain undistorted and natural.
The way to measure strength is through volume. When it is easy to get louder without doing anything else but get louder, the sound can be considered strong. WITHOUT DOING ANYTHING ELSE other than creating more contraction in the belly muscles underneath an firmly open rib cage.
If you do not work to make the low notes strong in chest register, without vowel distortion or laryngeal manipulation, you will not do well in mix. If you do not work to make head register strong and clear on upper pitches, without distortion, you will also not do well in mix. Head register strength takes about twice as much time to develop as does that of chest. You cannot skip this development. It needs to be about twice as strong as chest in the middle notes to counter the “down and back” pull of chest register.
Mix is only possible when you can control the volume of the registration without manipulating any other parameters except the volume. The vowel sound /ae/can access register balance but relying on the vowel sound as a destination makes for a distorted and skewed result. Volume alone will measure how balanced the registration is. When it is possible for the larynx to handle a solid exhale without being breathy and to ascend and descend in pitch without any obvious breaks, the vowel sound shapes should be easy to adjust without distortion or change in any other parameter.
People who sing only in mix have undifferentiated registration. That means they have an undeveloped chest register in low range, an undeveloped head register in high range and an indistinct and usually immoveable amount of both registers in middle pitches. While this generally does not create vocal health issues it is nearly impossible to change anything in this default except possibly, pitch and volume. It might be possible to open the mouth more and it might be possible to change the volume, but “resonance” or vowel sound shape will not, NOT, move. Chest, in this mess of mix, is often loudest at A, Bb, B and C above middle C (approximately), after which the sound either thins out considerably and dies off or just stops, as if it was “shut off”.
I continue to hear “my student doesn’t understand how to sing in mix”. No, you, the teacher, do not yet understand how to develop mix. You need to know how to use the exercises such that the student does not have to understand anything but will discover, in the lesson, that the sound emerges all by itself because you have created the correct conditions for it to do so. When the sound shows up, you say, “There. Now, this is mix. This is the sound we have been seeking.” The student’s eyes will grow wide and she will be surprised and delighted that this new sound has somehow come from her own body without struggle. Because you have asked her to do specific exercises over a period of time and THEY have developed a conditioned response in her vocal mechanism that would not have been there had she not sung them. Mix is not something you “do”, it is something that happens.