Blowing on bottles

02 March 2008

Ingredients

A bottle A bottle, or if you have got them a selection. A jug A jug of water

Instructions

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.

Result

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.

Explanation

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.

Air going into the bottle The air flows over the top
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.

Empty Bottle Half full bottle
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. 

Comments

Why is the note produced by striking the bottle different than when you blow across the mouth of it?

Hello Dan.

The reason is that, when you hit the bottle, you are making the glass or plastic resonate (vibrate); the mass and shape of the glass/plastic, and the contents of the bottle, will affect how fast the material can vibrate and how quickly the vibrations die away (damp), both of which affect the pitch of the sounds transmitted back into the air as vibrations you can hear.

When you blow across the top of the bottle you are setting up a wave inside the bottle, the wavelength of which is determined by the height of the space between the bottle neck and the fluid level or bottom of the bottle. The bigger the distance the longer the fundamental wavelength that can "fit" into that space and hence the lower the note. So the sounds (of blowing across the neck versus hitting the sides) arise through two totally different and independent mechanisms and, hence, you would not expect them to be the same.

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