Simple Instruments

Learning Objectives

In this activity, students will understand the relation between sound and the physical vibrations of objects. As well, they will begin to understand how to affect the two main properties of sound: pitch and volume.

Materials

• Glasses
• Water supply
• Metal spoon
• Shoebox
• Elastic bands of varying widths
• Pencil / pen / small cylinder of any sort
• Tape
• Scissors
• Plastic bottles

Motivation

What we experience as sound is the way that our minds perceive the vibrations of matter. Usually, we experience sound as vibrations that travel through the air and reach our ears, but sound waves can travel though any substance. Sound has two main properties: volume and pitch. Volume is how loud or soft the sound is, and it is characterized by the size of the vibration in the air. Big vibrations give loud sounds, and small vibrations give soft sounds. Pitch is a sense of how high or low the note is, and it is characterized by the speed of the vibrations in the air. Fast vibrations are interpreted as being high pitched sounds (like an alarm clock, or the notes from a flute) and slow vibrations are interpreted as being low notes (like the sound of a car engine or the notes from a tuba).

The principles of the water xylophone and the bottle organ seem similar, but actually they are quite different. When you add water to the bottom of the bottles, you are reducing the amount of air inside them. Because of the way sound waves have to move, this makes the air molecules vibrate faster, which produces a higher tone. The tone from the bottle is produced by the air vibrating, not the bottles themselves.

When you add water to the glasses, you make them heavier. When you then strike them with the spoon, they have to vibrate slower, and thus produce a lower tone. In the case of the water glass xylophone, the tone is produced by the glass vibrating, not the air inside it.

The box guitar is similar though not quite the same. In this case, the elastic bands will vibrate and produce a certain sound because they push on the air molecules as they bounce back and forth. As you press down on the shoebox and shorten the length of the elastic band, you cause the elastic band to vibrate faster, which again raises the pitch. The harder you pull on the elastic bands, the louder the note will be.

Procedure

• Water Glass Xylophone: Fill 3-7 glasses with different amounts of water. Using a metal spoon, tap each glass and make note of the sound it makes. Have the students arrange the glasses in order of highest to lowest, and see if they notice a pattern with the respect to the height of the water. Then, get them to play a melody on it.
• Shoebox Guitar: Cut a hole in the top of the shoebox. Pull 4-7 elastic bands of different widths around the length of your box, and roll a pencil underneath (this lifts up the bands and allows them to vibrate freely). Tape the pencil down to ensure that it stays where it supposed to. Pluck the rubber bands to make noise, and use your fingers to hold down part of the elastic bands to shorten the length of the part that is vibrating. By doing this, they will change the pitch.
• Bottle Organ: Take 3-5 plastic soda bottles of any size, and fill them with varying amount of water. Since most of the other instruments will be producing higher sounds, using 2 liter bottles will give some nice bass notes. To play, place the tops of the bottles against your lower lip and blow. Experiment to get the most volume for your breath. What happens to the pitch if you squeeze the bottle? What are you changing?
• After all the instruments are built and have been experimented with separately, try playing them together. Write a song.

Investigating Questions

• What is sound?
• What is pitch?
• What is volume?
• How do we change pitch in the water bottles and glasses?
• How do we change pitch with the shoebox guitars?