Blowing on bottles
|A bottle, or if you have got them a selection.||A jug of water|
Blow across the top of a bottle until you make a note, remember this sound.
Add some water
Blow across the bottle again. Is the sound the same?
Try different bottles, see what effect this has.
You should find that the pitch of the note gets higher as you add more water.
If you try other bottles you should find that the large ones give a lower note than the small ones.
If you are musical and have you may be able to play a tune.
When you blow on the edge of the bottle you are making a jet of air, if you hold your fingers nearby you can feel it. If you are making a noise this jet of air could either be deflected into the bottle or over the top of it, and it only takes a small force to change between these two paths.
If the air starts off going into the bottle, there is nowhere for it to escape so the pressure builds up, at some point the pressure builds up so much that the jet is pushed out of the bottle. Now the fast moving air sucks air out of the bottle until the pressure is low enough to suck the jet back in. You have made a vibration.
|The air stream is sucked into the bottle, which pushes extra air into the bottle||Eventually the pressure inside the bottle gets high enough to push the jet outside again. Now the fast moving air of the jet sucks air out of bottle, reducing the pressure inside.|
Why does adding water make the pitch change?
Pitch is just a measure of how fast the vibration is that reaches your ear, rapid vibrations mean high pitches. In a large bottle there is a lot of space so it takes more air flowing in to build up enough pressure to push the jet out again. Getting more air into the bottle takes a relatively long time so the air vibrates slowly (at maybe 100 vibrations a second).
If you add water or use a smaller bottle there is less space available so it takes less time for the pressure to build up. This means the vibration happens more quickly and so produces a higher pitch.
|An empty bottle has a large volume which means that the air behaves like a weak spring, because a lot of air has to move in to change the pressure much.||A half full bottle of water has less space for air inside, so it behaves as a stiffer spring, so it will vibrate faster.|
This is the principle that a flute works on, but instead of adding water you change the effective length of the tube by opening and closing holes.