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Bullfrog Caller

In this activity, students use their knowledge of vibrations and sound to mimic the sound of a bullfrog.

The “Cuica” (pronounced “kweeka”) is a Brazilian percussion instrument. Inside the Cuica, a wet cloth is rubbed up and down along a bamboo stick attached to the inside of a calf-skin drum. The rubbing motion produces the sound and the pitch is increased or decreased by changing the pressure on the calf-skin.

To see a close up of the Cuica in action, check out this Cuica video.

Sound is made when something vibrates. When using this bullfrog caller, the friction between the string and your fingers produces the vibration, which is amplified (made bigger) by the plastic container.

There are three ways to change the pitch of a vibrating string. 

  1. Pitch can be adjusted by varying the length of the string. A longer string will result in a lower pitch, while a shorter string will result in a higher pitch.
  2. Pitch can be adjusted by varying the tension of the string. A string with less tension (looser) will result in a lower pitch, while a string with greater tension (tighter) will result in a higher pitch.
  3. Pitch can be adjusted by varying the density (thickness/diameter) of the string. A thinner string will result in a higher pitch, while a thicker string will result in a lower pitch, all else being equal (think of guitar strings).


  • Describe the properties of sound.


  • Per Instrument:
    a yogurt container (or large plastic cup)
    thick absorbent string, like butcher’s twine
    2 inch nail

Key Questions

  • Does rubbing a piece of string with wet fingers normally make such a sound?
  • Where does the initial vibration come from?
  • What is the role of the cup?

What To Do

  1. Poke a hole in the middle of the bottom of the yogurt container carefully using a nail. Do this slowly because the container may crack. (They can still be used if they’re cracked, it’ll just be harder to hold the string in place).
  2. Feed the string through the hole. Tie the paperclip and string inside the cup with a double knot. This ensures that the end of the string will not pull through the hole.
  3. Make the string damp by dipping it in water.
  4. Holding the yogurt container in one hand, pinch the damp string with the other. While pinching, pull your fingers along the string away from the cup. The string should slide through your fingers and make a croaking sort of noise.

Teacher Tips:

  • Prepare plastic cups in advance by punching holes in the bottom of them.
  • The string needs to be thoroughly wet to work, so be sure to press string under the water surface to submerge it; do NOT just dip the string in and out of the water.
  • Younger students may not know how to tie a knot. A demonstration on how to tie a knot would be helpful for a younger group.
  • Emphasize to the class that bullfrog callers are NOT to be used right against someone’s ear.


  • How could you change the pitch to attract bigger or smaller frogs?
  • Brainstorm some ideas with a classmate
  • Some ways to change the pitch involve varying the length, the tension, or the thickness of the string. Which would be easiest to change?
  • Try substituting different kinds of string to see the effect.
  • Try using containers of different sizes or materials to produce different amplifications.
  • Which ones make the sound louder? Softer?