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Feeling the Vibes!

In this activity, students get the opportunity to see and hear sound waves by experimenting with tuning forks.

A tuning fork is a two-pronged fork with the prongs (tines) formed from a U-shaped bar of metal (usually steel). It reaches a specific constant pitch when you set it vibrating by striking it against a surface or with an object. The pitch that a particular tuning fork generates depends on the length of the two prongs. Its main use is as a standard of pitch to tune other musical instruments.

One reason for using a fork shape is that the handle vibrates up and down as the prongs move apart and together. The handle motion is small, which means you can hold the fork by the handle without damping the vibration, but it's strong enough that the handle can transmit the vibration to another object. A resonator (a type of a hollow rectangular box) is often used. The object amplifies (increases) the sound of the fork.

In this demo, the student's arm acts as a resonator to direct path of the vibration to the ear, amplifying the sound of the fork. The molecules in the student's bones vibrate. Sound doesn't only pass through air, it can also pass through solids!

Objectives

  • Describe how sound is produced.

  • Describe what pitch is and how it varies.

Materials

  • Per Class:
    tuning forks
    rubber mallet or the rubber bottom of a shoe
    large bowl of water

    If using this as an activity, provide the materials above for each pair of students.

Key Questions

  • Part 1:If this is a 128 Hz tuning fork, how many vibrations per second does it make?
  • If this is a 128 Hz tuning fork, how fast will the student’s nose vibrate if we touch it with the fork?
  • Do you think that vibrations can pass through materials (media) other than air?
  • What did the tuning fork’s vibration go through in order to reach the person’s ear?
  • Part 2:
    Can sound travel through liquids as well as solids?
  • In which medium does sound travel the fastest: gas, liquid, or solid?
  • Draw the molecules in a gas, liquid, and solid.
  • Brainstorm with a partner to hypothesize which medium will cause the sound waves to travel fastest.
  • Refer to the movement of air particles in the activity Sound = Vibration, Vibration, Vibration) to form your theory.

What To Do

Part 1: Feeling the vibes

  1. Choose a tuning fork (e.g. 128 Hz).
  2. Strike the tuning fork with a rubber mallet or with the rubber sole of a shoe. Can the students see or hear the vibrations? Probably not.
  3. Choose a volunteer.
  4. Strike the tuning fork and firmly place the handle of it on the cartilage at the end of the student’s nose. Ask the student what it feels and sounds like.
  5. ​How could the student hear the sound more loudly (without hitting the tuning fork harder)?

  1. Ask the student to push his index finger against his ear (as though he’s plugging his ear)
  2. While his finger pushing against his ear, get the student to hold the tuning fork with the other hand.
  3. Strike the tuning fork and tell the student to place the end of its handle against his elbow. Get the student to nod when they hear the sound.

Part 2: Seeing the vibes

  1. Fill the bowl with water and be sure that the water is still.
  2. Invite a volunteer to lean in close to the water’s surface to “check for the presence of vibrations.”
  3. Strike the tuning fork and stick it into the bowl. The vibrations in the water will make waves that splash the student’s face.
  4. Invite students to gather around the bowl to see the vibrations up close (but not too close!).

Extensions

  • Vibrate your hand back and forth as quickly as you can in 10 seconds while your partner counts the number of times you vibrated it. Divide the number by 10 to calculate how many times per second you can vibrate your hand.
  • Compare it to the tuning fork’s frequency and explain why vibrating your hand does not produce a sound.
  • Turn the demo into an activity by pairing up students with a tuning fork.