Musical Wind Instrument Registers
Date: Winter 2012-2013
I am a flautist with the local Youth Symphony Orchestra. I always wondered, how is it possible for a woodwind instrument to reach high notes in the upper octave? Unlike a violin (or other string instruments), since the process is less visual, I have trouble understanding it.
The reason you are having difficulty understanding the acoustic mechanism of woodwind instruments is simple-- IT IS NOT SIMPLE. A search on the term: ?acoustics of woodwind instruments? describes the mechanism(s), but the factors involved are too many and involved to address in a short venue such as NEWTON. The ?hits? in the search above will take you as deep or as shallow as you wish to probe the subject matter.
It is simple to envision with a string instrument since you can see a violin string (or a guitar string) vibrate back and forth and realize that its vibration rate corresponds to the frequency of the resulting sound. Consider, though, what that string is doing: it is pushing and pulling on the air around. When the string pushes, the air becomes slightly crowded (compressed), and when the string returns, the air becomes slightly less crowded (rarefied). The vibration sets up a wave that propagates in the air to your ears. This resulting wave causes your eardrum to vibrate in response, and the eardrum stimulates nerves that we subsequently perceive as sound. The vibration rate corresponds to our perception of pitch.
This explanation is elementary, but now consider removing the string and leaving the sound wave. Given another means to set up this wave, our ears will perceive it as a pitch regardless of how it is created. If you blow into a flute, air flows across the holes and sets up vibrations within the flute chamber. These travel back and forth along the chamber and "tune" the vibration to a pitch based on the geometry of the flute. In some sense, it is similar to a guitar in that the frets represent the ends of the flute. In a guitar, the string is clamped at the two ends, and only certain vibrational patterns can exist on the string. These patterns correspond to the pitch. Similarly, the ends of the flute "clamp" the sound waves traveling in the chamber only allowing certain vibrational patterns. These patterns correspond to the flute pitch. Changing the hole pattern (covering and opening different holes) causes different acoustic patterns leading to different pitches. Generally, longer and larger instruments lead to lower pitches, although there is an interaction among the locations of the holes within the chamber and the chamber dimensions.
The actual pitch is not important as the pattern set up in the flute chamber. It is not necessarily more difficult for a woodwind instrument to create a higher pitch. The vibration pattern of the air within the instrument is critical to the pitch. Consider, for example, that dog whistles create pitches beyond the human range of hearing, and they are not particularly complex.
Kyle J. Bunch, PhD, PE
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Update: November 2011