Key concepts
Physics
Sound wave
Frequency
Pitch
Hertz

Introduction
Can you name the bestselling musical instrument in the world? If you said…harmonica, you're right! The harmonica was patented in 1821 by a 16-year-old German boy. Since then it's become the top selling instrument in the world and a household item in many places. It's easy to take instruments (and the music they make) for granted, but creating beautiful noise is not just an art—it's also a science! In this activity you will design and explore your own harmonicalike instrument made from household items. Time to tune up!

Background
The sounds we hear every day are sound waves traveling through the air and reaching our ears. Much like an ocean wave, sound waves are generated by the vibration or movement of an object in a medium. In the case of the ocean the medium is water, whereas sound waves most commonly reach us by traveling through the air. The sound wave originates from the vibrating object, such as a vocal chord, and travels through the medium (such as air) causing all of the air particles to vibrate at the frequency of the vocal chord. The frequency of this movement is commonly measured in Hertz (Hz), where one Hertz equals one vibration per second.

To help understand this, imagine two people holding a rope between them. If one person gently shakes the rope up and down one time per second, a wave will travel through the rope with a frequency of one Hz. If the person increases their speed so they are moving the rope up and down two times per second, a wave will travel through the rope at a frequency of two Hz.

Sound travels through the air in a similar manner. A violinist runs a bow over the strings of the violin, causing the strings to vibrate. The vibrating string bumps against the air particles all around it. These air particles subsequently bump against air particles next to them and so forth, so the wave travels from air particle to air particle—all at the same frequency as the vibration of the violin string.

In this activity you will create your own sonorous instrument and explore the kinds of sound waves it generates.

Materials

• Two large craft sticks (at least six inches long)
• Two wide rubber bands (#64 size works well)
• One plastic drinking straw
• Four small rubber bands
• A ruler
• Scissors
• A piece of paper
• A pen or pencil

Preparation

• Stretch the wide rubber band over one of the craft sticks lengthwise.
• Use your scissors to cut four pieces of straw, each one to one-and-a-half inches long.
• Place one of the straw pieces under the rubber band perpendicular to the craft stick. Move this straw so that it is about two inches from the end of the craft stick. This is Straw 1.
• Moving away from the end of the craft stick place another straw piece on top of the rubber band next to Straw 1. This is Straw 2.
• Place the third straw piece next to Straw 2, under the rubber band. This is Straw 3.
• Place the last straw piece next to Straw 3, on top of the rubber band. This is Straw 4.
• Straw 1 and Straw 4 should be closest to the ends of the craft stick, whereas Straws 2 and 3 should be in the middle.
• With an adult to help you hold the straws in place, put the second craft stick on top of the of the first one, creating a sandwich with the straws in between the craft sticks.
• Secure this sandwich by wrapping a small rubber band approximately one-half inch from each end of the sticks. The ends of the sticks should be pinched together with a small space between them created by the straws.
• Use your paper and pencil to draw a table with two columns and five rows. Label the first column "distance between middle straws" and fill in each space below with: "2.5 inches," "2 inches," "1.5 inches" and "1 inch." Label the second column "tone of sound."

Procedure

• To start move Straws 1 and 4 as close to the ends of the craft stick as possible. You can move the straws by gently sliding them back and forth, being careful not to pull them out of the sandwich!
• Slide Straws 2 and 3 away from each other, so that there is a distance of 2.5 inches between them.
• Hold your instrument as though it is a sandwich, with one hand gently holding each end, and the open part facing toward you. Make sure that the craft stick with the rubber band is on the bottom of your sandwich, and try to keep your fingers on the small rubber band. Make sure you aren't pressing down on the wide rubber band.
• Blow through the opening between the craft sticks as though it is a harmonica (don't blow through the straws!). What sound do you hear? Do you feel anything in your hands as you blow through your instrument? Do the craft sticks vibrate or feel different when you play a sound?
• Move Straws 2 and 3 closer together, so that there is a distance of 2 inches between them.
• Again, blow through your instrument. Notice the sound and the feeling in your hands. Does this sound different than the first time you played it? If so, in what way? Does the vibration of the instrument feel any different?
• Again move Straws 2 and 3 closer together, so that there is a distance of 1.5 inches between them.
• Blow through your instrument. Notice the sound and the feeling in your hands. Does this sound different than the other times you played it? If so, in what way? Does the vibration of the instrument feel any different?
• Finally move Straws 2 and 3 closer together again, so that there is a distance of 1 inch between them.
• Once again blow through your instrument. Notice the sound and the feeling in your hands. Does this sound different than the other times you played it? If so, in what way? Does the vibration of the instrument feel any different?
• Return Straws 2 and 3 to their original position, with 2.5 inches between them. Repeat the above steps—this time recording your observations about the tone of the sound in your table. To help you compare the sounds, rate the lowest pitch sound as a 1, and the highest pitch sound as a 4.
• Consider the results in your table. Do you notice any patterns in the tone of the sound? Did the tone change as you moved the straws? If so, did moving the straws closer to each other make the tone higher or lower?
• Flip your instrument over so that the wide rubber band is on top. Hold the instrument so that your fingers are pressing down on the wide rubber band. Blow through your instrument as you have for previous steps. Does holding the rubber band change anything about your instrument? How would you explain any differences you observe?
• Flip the instrument back over so that the rubber band is on the bottom again. Remove all of the straws, except for Straw 1. Try blowing through your instrument. Does this sound different than the other times you played it? If so, in what way? Does the vibration of the instrument feel any different?
• Try moving the remaining straw and observing how this affects the tone of the sound. Can you change the tone of the sound by moving the straw?
• Extra: Test larger and/or smaller craft sticks and materials other than the straws. How many different instruments can you make?

Observations and results
The sound made by your instrument is actually the sound created by the large rubber band vibrating as you blow through it, much as a violin string vibrates when played by a violinist. As you blew through your instrument, you might have noticed that you could feel the vibrating rubber band through the craft stick. In addition when you flipped the instrument over and pressed down on the rubber band you probably found that you could not produce any sound when you blew through it. From this you can observe that the sound is created by the rubber band, and when you prevent the rubber band from moving you can't produce any sound.

In addition in this activity you should have noticed that you could change the tone of the sound by moving the straws. When you moved Straws 2 and 3 closer together the vibrating section of the rubber band got shorter. As a result the tone of the sound got higher. The shorter rubber band vibrates more quickly, and our ears pick up these faster frequencies as a higher-pitch sound. The thinner strings on a violin make a higher pitch sound because they can vibrate more quickly than the thicker strings. Similarly men tend to have longer vocal chords than women and therefore their voices are generally lower than women's voices.

When you removed all but one straw you should have found that the sound of the instrument became lower. With only one straw the rubber band was longer, and the sound it made when it vibrated was lower. As you moved the remaining straw you could change the tone as the rubber band became shorter and longer.

More to explore
Singing Wine Glasses, from Science Buddies
Talk through a String Telephone, from Scientific American
Blowing Bottle Tops: Making Music with Bottles, from Science Buddies
Science Activities for All Ages! from Science Buddies

This activity brought to you in partnership with Science Buddies