High and Low: column length effects on pitch





Purpose: To demonstrate how the length of the column of air in the wind instrument affects the pitch


1) glass of water
2) wind instrument model from the previous experiment (ie “Higher”)


1) Set the glass of water near the edge of a table.
2) Hold the wind instrument as in step 3 of “Higher”, but do not cover the end of the lower section with your finger.
3) Place the end of the lower section just below the surface of the water in the glass.
4) Blow through the straw while you lower and raise the straw in the water. Listen to the sound produced as the straw is moved up and down in the water.

Results: The pitch of the sound is lower when the straw is lowered into the water and higher when it is raised.

Why? :

Blowing through the straw causes the column of air in the lower section to vibrate. The shorter the column of air, the faster the air vibrates. The faster the vibrations, the higher the pitch. The sound produced depends on the amount of air in the tube. When the tube is out of the water, it contains more air, so the column of air is longer. When the straw is in the water, the column of air is shorter. As the column of air shortens, the pitch rises. Different length tubes in wind instruments behave in the same way.