Technology

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Video

Materials

• Glass bottles and drinking glasses
• Plastic wrap
• 1 pencil
• Water

Short explanation

You have now created a kind of xylophone. However, this xylophone actually works as a wind instrument, or rather lots of wind instruments, where each bottle or glass is a wind instrument tuned in a specific note. The note in which it is tuned depends on how much air is in the bottle or glass.

Long explanation

Sound is waves in the air, which consist of alternating high pressure and low pressure areas of air moving in the direction away from the sound source. To create sound, you need to put the air in waves, which can be done, for example, by letting a speaker diaphragm hit the air. The human ear has the ability to detect these sound waves and convert them into nerve signals, and our brain can then interpret these nerve signals as sound.

The air in the glass bottle vibrates when you hit the bottle. Just as water waves are created if you hit a water surface, sound waves are created when you hit the glass bottle, which in turn hits the air. The air, in turn, hits the back wall of the bottle, which in turn hits the air outside the bottle. Actually, all this happens almost simultaneously, because a standing wave is formed in the glass bottle. Imagine water waves that are in the same place all the time, even though the water moves through the wave. This is exactly the case in the glass bottle, the alternating high pressure and low pressure areas of air are always in the same place. But the air molecules themselves move, just like a car moving through a traffic jam that sometimes gets denser and sometimes thinner.

The air column in the glass bottle, and everything else that can vibrate, can only do this at one or a couple of speeds and can thus only be heard as one or a couple of tones. Exactly how fast something can vibrate depends on a number of factors, but the factors that can change here are the dimensions of the air column. A long air column (i.e. just a little water in the bottle) generally produces a fast vibration and short sound waves, which we humans perceive as a high note.

Test and improve

You have now built your first prototype. There are probably improvements to be made. In order to keep working on your design, try answering any of the following questions.

• How does the sound change if you replace the "drumstick" with something else?
• How does the sound change if you use something with thin glass, such as a fine wine glass?
• How does the sound change if you replace the water with something else?
• How does the sound change if you place the glasses and bottles on top of a hollow box (preferably in wood)?
• How does the sound change if you remove the plastic wrap?

Variation

If you only use bottles, you can hang them on strings. This can make it easier to play on them.

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