Key Concepts
Sounds, sound waves

Introduction

Have you ever blown across a bottle’s top and made a pleasant, resonant sound? If so, have you wondered how that note is made exactly? A bottle is actually what is called a “closed-end air column.” Clarinets and some organ pipes are examples of musical instruments that function as this type. 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. 

This activity is not appropriate for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.

Background

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 out 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 end.

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 increases in air pressure being followed by decreases in air pressure. 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. 

Materials

  • Three identical narrow-neck bottles. They can be glass or plastic.
  • Permanent marker
  • Ruler
  • Water

Preparation

  1. Make sure the bottles are clean and empty.
  2. Try blowing across the tops of the bottles you selected to make a resonant sound. Do this by touching your lower lip to the edge of the bottle, pursing your upper lip and blowing gently over the opening. When you get the 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 on the bottles, try using some different bottles for this activity.

Procedure

  1. Measure the height of one of the bottles. Using a permanent marker, make a small mark at exactly halfway up the bottle. Fill this bottle with water up to the mark you made.
  2. On another bottle, use the permanent marker to make a small mark at exactly three-quarters up the bottle. Fill this bottle with water up to the mark you just made. 
  3. Leave the third bottle empty.
  4. 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 to the note made by the empty bottle? Is the note from the half-full bottle higher or lower in pitch?
  5. 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 that this bottle makes compare to the note made by the half-full bottle? Is it higher or lower in pitch than the half-full bottle?
  6. Overall, how do the notes made from the three bottles compare to each other?

Extra: If you have a piano, electronic keyboard or other musical instrument (or an electronic chromatic tuner), you could try comparing the notes from the bottles to the notes on a real instrument. Alternatively, you could try filling a bottle slowly up with water, checking what notes it makes as it becomes fuller, and compare those to 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 top of the bottle, try tapping the bottle (below the waterline) with a wooden mallet. How does the note produced by tapping the bottle change with water level in the bottle? 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, like 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 is left in the bottle. 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 (i.e., 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 made a sound wave with a lower frequency than the other bottles, 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-quarters full bottle should have produced a frequency that was twice the half-full bottle’s frequency. When one sound wave is twice the frequency of another sound wave, the pitches made are one octave apart. (For example, the middle C note on a piano has a frequency of 262 Hertz, while the C note 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-quarters full bottle should have made a note one octave higher than the half-full bottle.

More to Explore

Credits

Teisha Rowland, PhD, Science Buddies

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Key Concepts
Sounds, sound waves
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