I had an interesting conversation with a guy in a pub who asked me why it is that blowing into a tube with some holes in it makes a sound. I said I thought it was something to do with the breath being split into two streams by the fipple, travelling at different speeds and interfering with each other. Then I realised that I didn’t know. Another guy at the bar, who said he was an accoustics expert, said it was all to do with reverse something or other, but he lost me about thirty seconds in.
Anyone out there know so next time I see them I can give the definitive answer?
Another thing. About 90% of the air comes out of the sound hole, so why does putting your fingers over the holes lower down change the pitch?
Splitting the airstream makes the air in the column of the pipe resonate. You get the same effect by blowing over the top of an empty bottle. The size and shape of the bottle determines the note you get.
You can get resonance, and a note, by blowing over a blade of grass. There it is just the angle of the grass that determines the resonance and the sound wave.
In a whistle, the altered speed of the air causes the resonance, and the length of the resonating wave determines the note. The longer the wave, the lower the note. The longer the pipe, the lower the note. When you open one of the lower holes on a whistle, this allows some air to escape, and alters the length of the wave - and so alters the note being played.
So all musicians fly in the face of the advice “don’t make waves”.
The physics of resonance in a closed chamber, like a bottle, pan pipes, referee’s whistle, or a boson’s call, and of an open tube like a flute or whistle aren’t the same thing at all, but splitting the column of air is something they have in common.
True, they’re not the same, but it does make an easy explanation in a pub. Practically everyone has blown across their beer bottle sometime, so the idea of getting a sound that way and changing the note by making the tube longer makes sense to most folks. Most of my drinking buddies don’t go in for complicated physics after a pint or two.
Here’s how I understand it: The initial rush of air over the blade causes low pressure inside the whistle, which causes the airstream to flip to the inside of the whistle. This in turn causes lower pressure on the outside of the window, pulling the airstream out again. The is the Bernoulli effect which also causes lower pressure above an airplane’s wing. The rate at which the airstream can flip back and forth is governed by the length of tube with closed holes (or the effective length in the case of cross-fingering). The longer the tube, the more difficult it is to flip the stream of air back and forth and the lower the frequency of the note.
The splitting of the airstream by the blade sets up what’s called a Karman Vortex Street, basically a fluid interference pattern. The vortices create alternating regions of high and low pressure above and below the blade. This has the effect of “flapping” the airstream up and down very rapidly - very much like the vibration of the reed in a reed instrument. In fact, this is often known as an “air reed.”
By itself, the air reed generates a broad spectrum of different frequencies and harmonics - basically noise. But the whistle tube (really, the air column in the tube) then acts as a kind of resonant filter, and selects some of those noise frequencies to pass through while suppressing others. The particular way this happens depends on the length of the tube and its geometry - cylindrical or conical, closed-end or open-end.
And here’s the magic … The resonant tube also couples with the air reed in such a way as to “lock it in” to the same vibrational mode as the tube. This turns the air reed from a random noise generator into a particular pitch (plus harmonics) generator. This is what makes the whistle sound a specific note.
You can hear this effect for yourself. Remove the head from a Generation-type whistle, and blow gently. You hear a random whistling noise. Then, continue blowing while inserting the tube back into the head while fingering, say, a G note. As the tube seats into the head, you suddenly hear the whistling noise “snap” into a definite pitch.
This also explains the apparent paradox that most of the air comes out the fipple hole. Your breath supplies the air for the air reed and the energy to set it in motion. Once this happens, its work is done and it can exit the instrument. In fact, very little air goes into the tube; it’s mostly the vibrational energy of the air reed that sets the static air column in motion.
When air runs across the fipple it initially creates a vacuum. Eventually, the vacuum will become great enough to pull the airstream down into the whistle. At this point, the whistle fills up with air until it is great enough to allow the stream to return to normal (shooting across). The length of bore affects the amount of time it takes to fill with air and deplete, it you do this 440 times per second, you have an “A” note. Filling and depleting air really fast becomes a tone. (Old sirens operated by chopping air at rates that are in the audible frequence spectrum.)
There are additional complexities that take place:. When overblowing, two pressure systems form, the lower pressure system can influence the upper pressure system by moving walls of vibrating air pressure. Thus all the weird cross fingerings occur, some are predictable and some are not.
Some whistle designs have special behavior that modifies the intake cycle and other modify the exhaust cycle which changes the quality of the instruments tone instead of a sine wave.
Some cars do the “subsonic fipple”, you drive with one window down and feel your ears beating - your driving a subsonic whistle.
I was hoping someone would post that wikipedia page with the animated illustration. I couldn’t remember the name of the Karmann Vortex. I’d read a couple hundred explanations, and as soon as I saw the animation recently, suddenly, it all made sense!
