Chest vs. Head voice and falsetto:
Your true voice involves your sound resonating in your natural resonating chambers in your body - the chest cavity and the sinus cavities. The Bel Canto technique is all about balance, so you will not use strictly head voice, nor strictly chest voice, but will use varying proportions of each, depending on the note you are singing.
Here's an analogy I like to use. Imagine a pair of stereo speakers with woofers and tweeters. The woofers and tweeters need to work together to create the sound, because individually, they will sound terrible!
Part of good singing technique involves developing what is called the "line of the voice"- which essentially means ensuring a consistency of tone as you work from your lowest notes and upwards towards your highest notes. Learning to produce this consistency of tone across the registers takes time and practice. This is part of the second stage of the Bel Canto technique known as "the mask of the face."
Your falsetto register is NOT the same as your head voice. Head voice is produced with a continued full resonant tone, whereas falsetto starts to take on a whole different character - the familiar small, squeaky sound often associated with chipmunks, elves, and singing ladybugs. Although you can sometimes "cheat" a little by using falsetto for a note here or there, the Bel Canto technique does not address the falsetto register, as it is not considered part of your real voice, and there is a very limited range of repertoire that calls for it.
Can I extend my range? How?
The short answer is yes, you can extend your range. There are a couple of qualifiers, though.
First, everyone's range has an upper and lower limit. Simple physics says that any string of a given length and thickness can only vibrate so fast. Since you cannot adjust the length or thickness of your vocal cords (though some techniques talk about "zipping them up", but whatever... more on that in another section), you will hit a point where it is physically impossible to produce notes lower or higher.
Second, improving your range takes time. You won't do it in two weeks, or two months, or even six months. A rate of a semitone every year or two is considered about right, until such time as you hit your limit, of course.
How to extend it? Slowly, and on a regular basis, start in the middle of your range singing intervals of only a few notes. Move downwards one semitone at a time until you can't comfortably (ie. properly) sing any lower. Now work your way back up to the middle, and then up to the top, one semitone at a time still, until you can't properly sing any higher. Eventually, these upper and lower limits will improve. The first thing you will notice is that your original upper and lower limits will become gradually easier to produce, so you will sing them with better tone and with less effort. As the original maximums become stronger, you will be able to build on them and extend upwards one semitone at a time.
When you hit your physical limit, continued practice will allow the notes on either extreme end of your range to become stronger and more focused with less effort.
What about people who have an 8-octave range?
Short answer.... they don't. Seriously.
Consider this: the lowest open string on a guitar to the highest playable note on the first string of an electric guitar is 4 octaves. Do you really think anyone has even that kind of range? Now consider that a full sized piano has 7 octaves. Do you really think anyone comes even close to 8 octaves now?
The reason people rattle on about the likes of Mariah Carey and Celine Dion having 8-octave ranges is that they don't know what an octave is, but they do know they have a great range, so it must be a lot. Rather than research it exactly, it is easier to just sensationalize their assertion with unsubstantiated hyperbole. It's kind of like saying that a Concorde jet can fly a million miles an hour.
It was recently brought to my attention that a Brazilian lady named Georgia Brown has an 8 octave range. Though she is listed in the Guinness Book of Records as having that, the methodology needs to be questioned. First, the so-called video evidence (check YouTube) shows an impressive range of an estimated four octaves - not eight. Second, it is alleged that she sings a note that would be considered as G10 (more than two octaves above the highest note on the piano). The frequency of that note is over 25Khz. Given that the human ear can only detect frequencies up to 20Khz, one has to ask if we really believe that she can sing notes that she herself can't hear, and subsequently, are we really to believe that she is singing notes if we can't actually hear them ourselves? Hmph. You get the idea. Thirdly, most of her singing is in what is called the "whistle register," which is another even more artificial means of producing pitch than falsetto. Neither falsetto nor whistle register are counted when defining a singer's range. (falsetto is Italian for "small, false voice.")
How much range do I need?
Most vocal ranges are defined by a range of two octaves. If you have a two octave range, you can sing most repertoire in most styles of music. There are always exceptions, but the two octave rule will hold true about 90% of the time.
Most people - even professional singers - don't have a range that exceeds much more than that - at least not what I like to call usable range. Sure, maybe they can hit notes above and or below the two octave boundary, but many of them are unusable due to poor control of tone and pitch. Anyone can whine in falsetto, but where would you use it?
There is a lot of simple pop repertoire that even falls into a single octave.
What range am I?
Voice types are divided initially into two broad categories - Adult males and Adult females/children. From there, each is divided further into three slightly narrower and somewhat more vague categories - high, medium, and low voices. At that point, each is defined with a label - tenor, soprano, etc. At this point, it is important to note that a male whose voice has not yet changed is classified similarly to females, as this is the range that they sing in. A young boy with a high voice would be a boy soprano, for instance.
Adult Males:
Children and Adult Females
Now, very few of us have a voice that fits these registers exactly, because everybody is different. The following is a general guideline of the ranges of each category. Note that these ranges are all typically defined as either from C to C, or from G to G, each covering two octaves. For the purposes of defining your range, falsetto and whistle registers are NOT counted.
Adult Females and Children:
Adult Males:
Bear in mind that this is not a definitive guide. There are differences of opinion, differences of application (ex. an operatic tenor is expected to reach a high C, whereas a choral tenor can sing most repertoire going only up to the A), extremes of repertoire demands (Mozart wrote a high F for a tenor part in one of his operas), etc.
Also, within all that, are different vocal characteristics. It's not just that you can *sing* the notes an alto will sing, you should also *sound* like an alto. It can get kind of confusing."