This is the fundamental difference between the flute and every other wind instrument. With other woodwinds, there is a mouthpiece with a reed or a double reed around which you wrap your lips. Brass instruments, on the other hand, have a cup mouthpiece that is in direct contact with the player’s lips. We flutists have a hole and it sits below the bottom lip where lip and chin meet. In fact, we even call the top piece of a flute a headjoint, not a mouthpiece.
Over many years working with my students, talking with band directors and answering questions about how to shape and direct the air, it began to dawn on me that a mouthpiece has a really important function, namely to create resistance against which the player blows. To make a sound on a clarinet or a trumpet, one needs a certain amount of air pressure to overcome the resistance of the reed or mouthpiece. As flute players, there isn’t anything in the instrument itself that can generate that resistance. After all, it’s just a hole!
So where does the resistance come from that creates great flute tone? Why from the lips themselves. It’s important to note that there is not a lot of difference in the size of the aperture we use for low notes vs. high notes. In fact, you can ask any flute specialist whether a looser aperture in the lower register and a tighter aperture for higher notes has any basis in reality as a strategy for a strong, open, focused and characteristic sound on the flute. In my experience a large aperture in the low register results in a wooly, flat and unfocused sound with little power. A tight aperture for high notes leads to a small, pinched and sharp tone. For a short demo of this, check out this quick video http://youtu.be/4MBZR3W6_UI
A good way to gauge the best size for the aperture in any register is to compare it to the opening in an oboe reed, or the opening of a flat coffee straw. And then you have to grip the opening around the air column. It’s tricky to imagine this without the air moving through the lips but is easy to relate to when the air is passing through the lips. This very grip around the moving air column is what creates the resistance to make a strong, open and focused tone on the flute. Essentially the gripping of the air column with our lips creates resistance that is akin to blowing into a cup mouthpiece or blowing to activate a reed.
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