Sound is caused by vibrations in air, which can be reflected and amplified when they are contained (for example, when an Inuit seal hunter’s whistles are trapped and echoed and amplified by sea ice). This is why a drum produces a louder sound than simply hitting a taught skin, the sound waves are amplified and reflected. When woodpecker taught the Dakota people how to make a flute, they learned that sound waves can also be generated by air moving across a hollow space. How does this work?
There are two different types of waves. Transverse waves where the medium is moving perpendicular to the direction the wave is moving (like the ripples when you drop something in water) and longitudinal waves where the direction of the wave and the direction of the motion of the medium is the same. Sound waves are longitudinal waves. Longitudinal waves are harder to imagine, but can be demonstrated using a slinky.
When air moves across the top of a hollow space (hollow branch, or bottle top) it creates a vibration that travels down the material, and reflects from the bottom (or other end in the case of a tube). This sets up a longitudinal wave in the air within the hollow space, and that wave bounces back and forth between the two ends, and creates a strong and amplified sound wave. This is the vibration that you hear from ‘wind instruments’. It can be made even stronger by creating a strong vibration using a reed, such as those used in the traditional Lakota and Dakota flutes, and in western orchestral instruments including the oboe and saxophone. In these cases the musician passes a vibration from their lips, tightly blowing air and sparking vibration between two reeds.
The pitch and tone of the instrument’s song on the shape and size of the ‘echo chamber, the instruments’ body, as well as the vibrational capacity of the material that the instrument is made of.
MATERIALS (per student): 2 straws, scotch tape, scissors
RELATED MEDIA: Video ‘Dakota Flute Origins’ (5 min); Sound Effects;
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Each student will need: a straw and scissors
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Flatten the end of a straw using the scissors
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Cut the flattened time into a point (a V shape) to make the reeds
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Gently chew on the point to bring the reeds close together
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Blow through the straw and voila! You have a musical instrument
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Gather in a circle and listen quietly to each student’s instrument – are they all the same pitch or are there differences?
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Discuss, how can you change the pitch? Try your ideas out in small groups then gather to share
Draw Conclusions about the characteristics and physical properties of sound, including pitch and loudness, based on observation
Changing the length of the straw (by clipping it off, attaching another straw or by making a straw trombone by sliding a larger straw on top) changes the time necessary for the vibration to travel up and down the straw, and so changes the pitch (changes the wavelength!).
Making a hole in the straw lets the vibration bounce off from where the hole is, which will also change the pitch!
Blowing faster, making a broader reed, and blowing across a hole (closing one end with tape) will all create new sounds. How might different media (metal, wood, thicker straws) affect the sound?
What hollow items might be fitted onto the plastic reeds that the students have made to create new symphonic instruments?
This hands-on activity was compiled by Jimmy Lam and Mary Tait, University of Saskatchewan Science Ambassador Program (spring 2013)