Key Concepts
Physics
Sound
Perception
Vibration
Pitch
 
Introduction
Making music can be a lot of fun—and it is also a great way to explore the physics of sound. With some string and a friend even an absolute beginner can play a nice tune. This experiment is especially interesting because what is audible to you can barely be heard by anyone else.
 
Background
You hear sounds when vibrations get inside your ear and stimulate your nerves to send electrical signals to your brain.
 
Sound waves carry vibrations into your ears. Inside your ear moving air pushes on your eardrum and starts it vibrating. Your eardrum, in turn, pushes on the bones of your middle ear, the tiniest bones in your body. These bones act like a set of levers, pushing against the thin membrane that covers the opening to your inner ear.
 
Materials

  • A helper
  • A piece of string about five feet long
 
Preparation
  • Put your hands over your ears.
  • Have your helper loop the string around your head and over your hands and gently pull the string.
 
Procedure
  • While pulling gently, have your helper pluck the string. What do you hear? Why do you think that is? What can your helper hear?
  • Have your assistant pull the string tighter. What do you hear? Has the sound changed? Why do you think that is?
  • Now have your helper slightly loosen the string. What is different about the sounds now? Why might that be?
  • Now let your helper take a turn listening. This time you pluck the string that's looped around your helper's head. What do you hear? Ask your helper what he or she hears.
  • Change the tension on the string and ask your helper to describe what he or she hears.
  • Extra: Try the same activity with different sizes or types of string (such as nylon string, cotton string or shoelaces). How does the size or type of material seem to affect how well the string transmits waves as sound? What about the pitch?

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Observations and results
Why did you have a much harder time hearing the sound when the string was wrapped around your helper's head than when it was wrapped around yours?
 
When you pluck on the string that's wrapped around your helper's head, the string starts vibrating. To reach your ears the vibrations in the string must push on the air to make sound waves that travel through the air. But the string isn't very large and it doesn't push on very much air so sound vibrations don't travel easily from the string into the air.
 
When the string is around your own head, however, the sound can take a more direct route to your ears. Rather than traveling through the air, the vibration can travel through your hands and skull bone directly to the fluid inside the cochlea in your inner ear. Instead of traveling from solid to air and back to solid, the vibrations move from one solid (the string) to another (your body) then into the fluid of your cochlea. As a result, the sound you hear is much louder and richer.
 
In vibrating strings there is a relationship between tension and vibration speed of waves in the string. So the pitch you hear is a result of the string tension; the tighter the string the higher the frequency of vibration. This high tension creates a tone with a high frequency. For a less tense string the reverse is true: lower frequency vibrations translate into the perception of a lower tone.
 
More to Explore from Exploratorium
Ear Guitar, from Exploratorium
Sound Bite: Listen with Your Head Bone, from Exploratorium
Find That Sound, from Exploratorium
Science of Music, from Exploratorium
 
Harmonies in Your Head was developed by Exploratorium and is featured in the book Exploralab: 150+ Ways to Investigate the Amazing Science All Around You. Created by Exploratorium, Exploralabis a book that takes curious kid scientists, ages eight to 12, through 24 hours’ worth of household investigations, experiments and discoveries.

This activity brought to you in partnership with Exploratorium

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