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The Physics of sound in tubes


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.





schoolphysics waves in tubes animation

To see an animation of waves in tubes click on the animation link.



schoolphysics waves in tubes (molecular vibrations) animation

To see an animation of molecular vibractions in tubes click on the animation link.


 

A VERSION IN WORD IS AVAILABLE ON THE SCHOOLPHYSICS USB
 
 
 
© Keith Gibbs 2020