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Sound Science: Do-Re-Mi with Straws

A musical physics problem from Science Buddies 


Musical straws
George Retseck

Key concepts
Sound
Music
Physics
Sound waves
Frequency
 
Introduction
Have you ever wondered how a musical instrument produces the beautiful sounds that it does? It all comes down to science! To make a certain note, the instrument has to generate a particular-sized sound wave. So what you hear as pitch is really just a collection of similarly sized sound waves moving through the air and hitting your ear.
 
Of course, most instruments are capable of producing many different notes—or sizes of sound waves. Many instruments accomplish this by changing the length of part of the instrument, such as  the strings in a piano or on a guitar, or a trombone's adjustable air column or the different-length keys on a xylophone. In this science activity, you'll make your own musical instruments using drinking straws and explore how changing the length of the straws changes the notes that they produce.  
 
Background
Sound is produced by vibrations. The vibrations push and pull on air molecules, changing the air pressure around them. The pushes cause a local compression of the air (increase in pressure), and the pulls cause a local rarefaction of the air (decrease in pressure). The compressions and rarefactions are rapidly transmitted through the air from the original source as a  wave, making sound. Sound itself is a wave, a pattern, of changing air pressures.
 
For a sound wave, the frequency of the wave corresponds to the perceived pitch of the sound. The higher the frequency, the higher the perceived pitch. Technically, the frequency of a wave describes how many cycles of the wave happen during a certain amount of time. This is measured in hertz (Hz), which is in cycles per second. On average, the frequency range for human hearing is from 20 Hz at the low end to 20,000 Hz at the high end.
 
On a stringed instrument, such as a guitar or piano, when the string is plucked (guitar) or struck (piano), it vibrates and produces a standing wave on the string. These vibrations are transmitted to the soundboard of the instrument, which amplifies the sound.
 
Materials

  • At least two plastic or paper drinking straws
  • Scissors
  • Piano, electronic keyboard or other musical instrument that can produce a scale of notes (optional)
 
Preparation
  • Cut one of the two drinking straws so that it is half the length of the other straw. How do you think the different lengths will affect the sounds the straws make?
  • Take one of the straws and flatten about one inch at one end of the straw. You can use your teeth or pinch it between your fingers or fingernails to flatten it.
  • On the same straw, use scissors to make two small, angular cuts, one on each side of the flattened end. This should make the end of the straw be similar to a "V" shape when flattened, but without a pointed tip at the end (the end should have a short, flat, uncut segment left).
  • Repeat this with the other straw so that both have small, angular cuts on one end.
 
Procedure
  • Insert the cut end of the longer straw into your mouth. Position the cuts so they're just inside your lips. Then curve your lips down and inward a little and apply light pressure on the straw with your lips. Why do you think you need to apply pressure to the straw?
  • Blow through the straw. What do you hear? The cut ends should vibrate and produce a tone. You may need to move the straw around slightly to locate the best position for creating your musical note. It might take some practice and repeated tries to produce a constant, single note.
  • Now blow through the shorter straw using the same method. What do you hear this time? How does it compare to what you heard with the longer straw? Again, you might need to try blowing through the straw a few times to make it produce a constant, single note.  
  • Extra: If you have a piano, electronic keyboard or other musical instrument, you could try comparing the notes from the straws to the notes on the real musical instrument. Can you figure out which notes the straws are making? Can you tell what the relationship is between the two notes? How does the note from the shorter straw compare to the note from the longer straw?
  • Extra: You could try repeating this activity but use eight straws and try to cut them so that each straw produces a note from a scale. For example, for a scale starting with C, these eight notes correspond to the white keys on a piano: C, D, E, F, G, A, B, C. See the Science Buddies project idea in the "More to explore" section for details on how you could do this. Can you make a scale using eight straws cut to different lengths?
  • Extra: Other instruments are easy to make as well! You can make multiple notes by filling soda bottles with different amounts of water and blowing across the tops of them, or by filling goblets with different amounts of water and using a clean, slightly wet finger to stroke around the rim of the glass to cause vibrations. How do the notes produced by these instruments change when you change how much water is in them? Can you use them to play a scale or a song?

 

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