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Basics of Sound & Waves

  • Sound
  • Waves
  • Frequency / Pitch
  • Amplitude / Volume
  • Activity: Water Waves
  • Activity: Slinky Compression Waves
  • Activity: Rope Model Waves
  • Materials
  • Downloadable Files

Sound is vibration that travels through a medium (air, liquid, solid) that can be detected by an animal. Vibrations have characteristic frequencies. The sound made by a ruler as it is snapped against a surface will have a frequency that reflects the number of times the end of the ruler vibrates up and down.

ruler_desk

Animals can hear different ranges of vibrational frequencies. Dogs can hear higher frequency sounds than humans. Dolphins can not hear the lower tones that humans can detect, however their high range stretches far beyond human and canine abilities.
table

Sound waves are described as mechanical compression waves. A conceptual demonstration of how compression can propagate is to stretch a spring "slinky" toy, and then tap one end. Observe the communication between coils from one end to the next.

General features that are used to describe all kinds of waves.

wave

wavelogoSurfers are familiar with the period of the wave, the time between peak heights out on the ocean, and know that it impacts the shape of the wave. The height of the wave is known as the amplitude.

The frequency of a sound wave is what determines the pitch. The lower tones of a tuba are from low frequency sound waves that emerge from the instrument.

pitch

Common ways to lower the pitch of an instrument or sound:
i) Increase the length of tube
ii) Decrease the tension on a string
iii) Increase the length of a vibrating pipe, piece of wood or metal.

Higher pitch sounds have higher frequency. The flute below will generally have higher frequency tones than the tuba.

frequencyhigh

Common ways to raise the pitch of an instrument or sound:
i) Decrease the length of tube
ii) Increase the tension on a string
iii) Decrease the length of a vibrating pipe, piece of wood or metal.

The volume of sound is indicated by the amplitude in a sound wave. The two sound waves illustrated below have the same frequency, and therefore the same pitch. However, one tone is louder than the other.
volume

Activity: Water Waves

Sound travels in waves.  We’ll examine waves in water to understand some of the properties of sound.

Concepts - Identify wave crests (high points) and troughs (low points). High and low frequency.

  1. Fill the glass pan with 1" of water. If available, place under a document camera projector and project onto white screen.
  2. When the water is completely still, tap the water at one edge of the pan with a pointed object like a pen or pencil.
  3. Observe the circles that spread from the point of contact. Note how the spacing of the circles changes when the water is tapped at faster (high frequency) and slower (low frequency) rates.

wave dish

 

 

Activity: Compression Waves

For this activity, you need a slinky (or other spring toy) and an object to tap it, preferably something with some density. We used a heavy spoon or even a stapler. Some wooden spoons were too light to see all aspects of the behavior.

    slinky
  1. Have a friend hold one end of the slinky still.
  2. Stretch the slinky so that there is a little tension in the spring (about 10 inches long for a full sized slinky), resting on a flat surface.
  3. Strike the slinky on one end with a the heavy spoon.
  4. Watch the motion of the strike travel from one end of the slinky to the other.

Extra: If your friend holds their end very still, and you strike your end of the slinky with enough energy, you may see the reflection of the wave.

This video demonstrates the propagation of a compression wave.

 

Air is compressed when the spoon strikes the milk jug. The wave travels through the jug, is focused at the mouth of the jug and then to the candle. The wave carries the sound but also blows out the candle. This compression wave is invisible because it travels through air. For a description, see Movement of Sound.

 

 

 

 

Activity: Rope Waves

  1. Have a partner hold one end of the rope completely still on a smooth surface, like a floor or carpet.
  2. Wave your end of the rope back and forth. Look for the "S" patterns of waves in the rope.
  3. Wave you end of the rope wider but at the same rate. You may need to adjust the tension on the rope. How does this change your wave?
  4. Wave your end of the rope faster. How does this change the wave?

Use terms including wavelength, amplitude and frequency to describe the waves you create. Can you generate a low frequency high amplitude wave? How about high frequency low amplitude?

     

Materials

  • glass/pyrex lasagna pan (or larger)
  • overhead projector (optional)
  • pencil or other cylindrical object
  • spring toy (slinky)
  • heavy rope about 8-12 ft long or even more fun is a long spring that can be used to demonstrate reflections and standing wave harmonics ($15, catalog SPR-1 at Education Innovations)

 

Downloadable Files

Handouts and descriptions of hands on activities at a science night. (pdf)

Handout worksheet for an in-class activity. (pdf) (answers)