Beer bottle flute

Making nice sounds with a beer bottle – an essential skill in student life.

The pitch of a sound depends on the frequency of the compression wave.

Try to make a constant sound by blowing over the opening of an empty glass bottle.

1. Why does the pitch change if you fill water into the bottle?

Where does the air go which you blow over the bottle opening?
› Sometimes into the bottle, sometimes over it.

What does this depend on?
› On the air pressure in the bottle. When blowing at the proper angle, the air pressure changes cyclically.

How long does one of these cycles take?
› That depends on how much space the air has to build up pressure and the bottle shape.

When does the pitch of the sound increase?
› Reducing the 'free' space in the bottle by adding water leads to faster oscillations of the air pressure and a higher pitch.

As your students blow over the bottle opening, the air can either go into the bottle, or pass over it. At the right speed and angle, the air will first enter the bottle and increase there the pressure. This pressure forces the air out again and pushes against the airstream passing over the bottle. This airflow over the opening sucks even more air out of the bottle. Once the pressure in the bottle is low enough, the air blown over the opening is sucked in again and the circle repeats.

By filling water into the bottle, the space for the air to build up pressure is reduced, and the cycle of in- and outflow accelerates. The pressure waves are therefore generated at a higher frequency – which we perceive as higher pitch.

If done with beer bottles, this experiment will be one of the few things your students will certainly remember from school – once they are at a student party...