I was not born with a perfectly functioning singing voice. I had no problems in chest voice or in the lower part middle voice, just above chest voice. I had a very distinct and annoying crack, which kept me from singing opera, popular, Broadway and R&B songs until I was trained. I have helped singers to no longer crack and also to help prevent vocal nodules (callouses of the vocal folds). Most people need training to gain control over the voice. A very few are born with it, perhaps 1%. I am not ashamed to have studied, because the results opened up a whole new world to me, which includes having sung professionally in Las Vegas.
Let's clarify some terms to start on the road to understanding more about the singing voice.
What is chest voice? Chest voice is the range in which you can feel vibration in your chest when you sing in that range. The reason is that the sound waves are larger in chest voice and they resonate in your chest, the size of which approximates the sound waves, pitch-wise. Have you ever been in a hallway or room and found a tone which loudly resonates with your voice? There will be one specific tone which will do this. Your chest will vibrate when you are in chest voice and that is how it got its name. Unless you are singing with a breathy tone, chest voice will be a full sound, which can be produced loudly when needed. This is called full voice.
Full voice is often confused with chest voice. They are not the same thing. Full voice is achieved when the vibrating vocal folds are in close proximity, to the extent that the tone is full, rather than weak or breathy. When the vocal folds are not vibrating in close proximity, excess air escapes and can be heard mixed in with the tone. It is not necessarily wrong to sing with a breathy tone, for effect when stylistically or emotionally appropriate. The breathy tone production does have the potential of causing dryness to the vocal folds (vocal cords, as they used to be called). The “lubrication” of the vocal folds is done by mucous secreting glands. It can become irritating to the delicate tissue, if it dries out.
Head voice is called head voice because your head vibrates when you are in the higher notes of head voice. In a properly trained and developed voice, the tone can be full and powerful, just as in chest voice. It can be called full voice in head or full head voice. It is not a breathy sound. When working properly, a singer can sustain a high note and crescendo from very soft to very loud, without a change in the sound of the tone. If there is a sudden change in the sound of the tone, that is what singers call a break or a crack. Things do not actually crack or break in the sense of the vocal apparatus but instead, the singer and listener hear an abrupt distinct change in the tone quality.
The hardest thing for many singers, including some professional singers, is to smoothly execute the tones throughout the entire vocal range. Most untrained singers have difficulty singing passages which extend from chest voice to head voice. Singers whose voices crack or break when transitioning from chest voice to head voice (or vice-versa) are actually experiencing a brief and sudden loss of adduction of the vocal folds. They “pop” open and then close again. Some singers will yell or scream out high notes but over time that can and has caused injury. There must be a coordination of adduction in such a way as to not hyper-adduct the vocal folds. It is a balancing act. It is the correct amount of pressure sufficient to achieve the desired tone quality. Most people need training to gain control over the voice. A very few are born with it, perhaps 1%.