When air is blown through the reed, the reed vibrates producing turbulence with a range of vibrational frequencies. When the frequency of vibration of the reed matches the frequency of vibration of the air column in the straw, resonance occurs. And once more, the result of resonance is a big vibration - the reed and air column sound out toegether to produce a loud sound. As if this weren't silly enough, the teacher then began shortening the length of the straw by cutting small pieces off its opposite end. Higher and higher pitches were observed as the straw was shortened.
Woodwind instruments produce their sounds in a manner similar to the straw demonstration.
A vibrating reed forces an air column to vibrate at one of its natural frequencies. Resonance is the cause of sound production in musical instruments. In the remainder of Lesson 5, the mathematics of standing waves will be applied to understanding how resonating strings and air columns produce their specfic frequencies.