Have you ever blown across a bottle's top and made a pleasant, resonant sound? If so, have you wondered exactly how that note is made? A bottle is actually what is called a "closed-end air column." Clarinets and some organ pipes are examples of musical instruments that work in the same way. In this science activity you will use bottles to investigate how the length of a closed-end air column affects the pitch of the note that it makes.
Some musical instruments produce sound from vibrating strings, others from vibrating reeds and still others from resonating columns of air. In this activity you'll try a simple example of the latter type of instrument: narrow-necked bottles that are partially filled with water. These bottles will function as closed-end air columns, which are basically tubes that are open at one end but closed (or covered) at the other.
How do musical instruments make the sounds that they do? All sound is made by vibrations that travel through the air. Specifically, these vibrations cause patterns of air compression that travel as a wave, with air pressure increases being followed by decreases. This is how sound itself is a wave. The pitch of the sound we hear depends on the frequency of the wave—how quickly an increase in air pressure is followed by a decrease. Higher pitches have higher frequencies.
- Three identical narrow-neck bottles (They can be glass or plastic.)
- Permanent marker
- Make sure the bottles are clean and empty.
- Try blowing across the tops of the bottles you selected to make a resonant sound. Do this by holding the bottle upright (so it is perpendicular to your face). Touch your lower lip to the edge of the bottle, pursing your upper lip and blowing gently over the opening. When you get the angle and airflow just right, you will hear a musical note as the air column in the open bottle resonates. How does the bottle sound? If you cannot make a note by blowing over the bottles, try using different bottles for this activity.
- Measure the height of one of the bottles. Using the permanent marker and ruler, make a small mark at exactly halfway up the bottle. Fill this bottle with water up to the mark you made.
- On another bottle make a small mark at exactly three quarters up. Fill this bottle with water up to the mark you just made.
- Leave the third bottle empty.
- Blow across the top of the empty bottle, as you did before. Make sure you can make a clear note. Then blow across the top of the half-full bottle. How does the note that the half-full bottle makes compare with that made by the empty bottle? Is the note from the half-full bottle higher or lower in pitch?
- Then blow across the top of the bottle that is three quarters full. It may take some practice to make a note from this bottle. How does the note this bottle makes compare with the one made by the half-full bottle? Is it higher or lower in pitch than the half-full bottle?
- Overall, how do the notes made from the three bottles compare with one another? Why do you think that is?
- Extra: If you have a piano, electronic keyboard or other musical instrument (or an electronic tuner), you could try comparing the notes from the bottles with the notes on a real instrument. Alternatively, you could try slowly filling a bottle with water, checking what notes it makes as it becomes fuller, and compare those with a real instrument. What notes does it sound like the bottles are making? Can you figure out a relationship between the three notes from the bottles used in this activity?
- Extra: Try repeating this activity but use bottles that are different shapes and sizes. Does the shape or size of the bottle affect the note it makes? What about the height of the bottle or how full with water it is (or the level of remaining air)?
- Extra: If you have narrow-neck glass bottles, instead of blowing over the tops of the bottles try lightly tapping them (below the waterline) with a wooden mallet. How does the note produced by tapping a bottle change with its water level? Can you explain how this works?
Observations and results
Did the empty bottle produce the lowest pitch? Did the bottle that was filled three quarters full with water make the highest pitch?
When playing a musical instrument that is a closed-end air column, such as the bottles in this activity, the pitch of the note that is made depends on the length of the air column. In other words, the pitch depends on how much water has filled up the bottle and how much empty space remains. This is because the pitch of the sound we hear depends on the frequency of the sound wave that can be created within the bottle's air. The shorter the air column (that is, the shorter the height of the air in the bottle) the higher the frequency. And the higher the frequency the higher the perceived pitch. This is why the empty bottle should have produced a sound wave with a lower frequency than the others and the bottle that was nearly full (three quarters full) should have made the highest pitch.
In fact, because the air column in the half-full bottle was half the length of the air column in the empty bottle, the half-full bottle should have produced a frequency that was twice the empty bottle's frequency. (For more on the mathematics behind this, see the "More to explore" section.) Similarly, the three-quarter-full bottle should have produced a frequency that was twice that of the half-full bottle. When one sound wave is twice the frequency of another, the pitches made are one octave apart. (For example, the middle C note on a piano has a frequency of 262 hertz whereas the C that is one octave higher has a frequency of 524 hertz.) This means that the half-full bottle should have made a note one octave higher than the empty bottle, and the three-quarter-full bottle should have made a note one octave higher than the half-full one.
More to explore
Sound Waves and Music, from the Physics Classroom
Air Column Resonance, from HyperPhysics
Blowing Bottle Tops: Making Music with Glass Bottles, from Science Buddies
Science Activities for All Ages!, from Science Buddies
This activity brought to you in partnership with Science Buddies