Open ended notes

Open ended notes

The discussion of wind column instruments such as trumpet, trombone french horn or tuba requires that we have a basic concrete understanding of very simple wind columns first. We'll start off with penny whistle, a very simple cylindrical instrument with holes cut in it, open at both ends with a fipple at one of the open ends to create the notes we're to play.



First, let's consider a penny whistle with no holes in it or one in which all of the finger holes are covered up.



The mouthpiece of a penny whistle, called a fipple, has a flat, narrow tube that we blow through. This directs a high speed air stream at a slanted hole with a sharpened edge that splits the air stream causing a batch of eddy currents or vortices at the mouthpiece. These eddys have a lot of frequency content in them -- they're noisy! Some of the noise has notes in it that will just fit the tube, so to speak, and the tube begins to resonate. The resonance "talks back" to the fipple (again, so to speak) causing the resonant note to "capture" the system, and we hear a nice pure tone of a cylindrical pipe.



Now I have used several terms and voiced several ideas in the last paragraph that might seem a bit mysterious. Let's begin to dispel that mystery. It'll be easy!

First, let's make sure we see how sound or compression waves/pulses move in a pipe.

Let's consider our simple cylindrical tube. We'll hold our hand against one end of the tube effectively closing it off leaving the other end wide open. If we suddenly and vigorously pull our hand away from the end of the tube, we will leave a tiny region of low pressure right at the end of the tube. The low pressure we have left behind wants to suck air into its place in order to restore that tiny region to normal room pressure. Some air molecules from the room will move into that place, to be sure, but some will come from inside the tube. Since the tube is a closed environment, the molecules of air that moved into the low pressure...