Whistling wind
Today I was doing standing waves in pipes with my standard level class. I was demonstrating the usual array of tubes and whistles when one of the students, Fredrik seen here whistling through his teeth, asked if I could explain what is happening when we whistle. The explanation is that air vibrates as it passes though the lips, this vibrating air makes the air in the mouth vibrate which resonates at certain frequencies causing standing waves to be set up. To change the frequency of whistle the shape of the mouth is changed as the rate of air flow is altered to match the new resonance. Well something like that. The point that Fredrik was making was that in other instruments air is blown into the tube but when whistling it's blown out of it. This highlights nicely the fact that it is the sound wave that is reflecting off the end of the pipe not the air. I wonder if there exists a musical instrument that has the resonator before the reed? I'm sure I've see some sort of bag pipe with reeds at the end of the pipes but my quick bit of research suggests not.