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Waves of Sound

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Author: Janice VanCleave

If a tree falls in a forest and there is no one around to hear it, is a sound produced? The answer to this interesting question could depend on how you define sound. By the physics definition—sound is a mechanical vibration that travels through a medium—the answer is yes, sound is produced. But if you go by the physiological definition—sound is the sensation produced in hearing organs by vibrations—then no sound is produced.

In this project, you will investigate the physics definition of sound by producing sound waves and determining how the length and the material of the vibrating object affects the frequency of the sound produced. You will determine the ability of sound, which is a type of energy, to do work. You will also compare the efficiency of sound travel through the different phases of matter.

Getting Started

Purpose: To determine how the length of a vibrating material affects the frequency of sound produced.

Material

  • flexible plastic ruler

Procedure

  1. Lay the ruler on a table so that about three-fourths of it extends past the edge of the table.
  2. With one hand, keep one end of the ruler held securely to the table.
  3. With the other hand, push the free end of the ruler down, then release it. Note the sound produced and how fast the free end of the ruler vibrates.
  4. Repeat step 3. As the ruler vibrates, slowly move the ruler so that less of it extends over the table's edge. Note what sound the ruler makes and how fast the end of the ruler vibrates as the length of the vibrating end of the ruler decreases.

Sound Longitudinal Waves

Results

As the length of the vibrating end of the ruler decreases, sound changes.

Why?

Sound is a vibration that travels through a medium. Sound waves are waves produced as a result of the vibration of a material. Sound originates from a vibrating object that forces the medium it passes through to vibrate. The ruler is an example of a vibrating sound source. When struck, the ruler can be observed moving back and forth at a particular frequency (the number of vibrations per second). The vibrations of the ruler cause the air molecules around the ruler to move back and forth at the same frequency, creating areas of compression (where the molecules are pushed together) and rarefaction (where the molecules are spread apart). Waves, such as sound waves, that have areas of compression and rarefaction are called longitudinal waves.

The movement of the air around the vibrating ruler transfers the sound energy through the air so that vibrating air enters your ears and hits against your eardrums, causing them to vibrate. The frequency of the vibration of the eardrums is interpreted by your brain as a specific sound called pitch. As the length of the ruler decreases, its frequency increases. Thus the frequency of the vibrating ruler is inversely proportional to its length. The pitch of the sound gets higher as the frequency increases.

Try New Approaches

What effect, if any, does the density of the vibrating material have on the pitch of the sound produced? Repeat the experiment using an object of comparable size but made of denser material, such as a wooden ruler.

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