The diagrams
show how a sound wave "fits in" to the size of tube that makes it. They also show the vibrations
of the molecules of the gas in the tube.
The note that a tube emits depends on three
things:
(a) the length of the tube
(b) the gas in the tube
(c) the temperature of the
gas in the tube
The longer the tube is the lower the pitch of the note that it can emit.
When a tube is heated it expands and so is longer!
So, if the temperature of the tube
goes down the length will be shorter and the pitch of the note should get higher. If the
temparture of the tube goes up the tube expands and the pitch of the note that it can emit should
go down.
However it is unfortunately not quite that simple. As the gas in the tube gets
warmer the molecules move faster, that means they can carry the vibrations of the sound wave
more rapidly and so the pitch goes up.
It is this second fact that has the greater
effect on the final pitch of the note.
So, an organ pipe in a cold church will be
shorter than normal but the air molecules in it will be moving slowly and so the pitch of the note
that comes from it will be lower than it should be – the note will be flat.
