slowest in gases, faster in liquids and fastest in solids. Sounds are loudest near the source. Sound waves are produced by a vibrating object. Figure 1 shows how sound waves move out from a sound source through air. Notice that closest to the sound source, the compressed areas are very dark, but fade in color with distance from the speaker. The diagram shows that the spacing between the dark areas doesn't change. The equal spacing in the diagram illustrates that the frequency of sound doesn't change with distance, only the amplitude or energy of the wave decreases. Thus, the sound would be the loudest at position A, and softest at position C. This relationship is true with all sound waves. Also, the energy of the sound would travel farther through a solid than in air.
See for Yourself
Materials
2 metal spoons
string, 3 yards (1.8 m)
helper
What to Do
1 Tie the string around the handle of one of the spoons so the two ends of the string are about the same length.
2 Wrap the ends of the string around your index fingers. Let the string and spoon hang in a V shape.
3 Extend your arms in front of you as shown in Figure 2 so the spoon hangs freely without touching anything.FIG 2
4 Ask your helper to use the second spoon to gently tap the hanging spoon (Figure 3). Make note of the sound produced by the spoon.FIG 3
5 Stick your index fingers in your ears and lean forward so the hanging spoon dangles freely. Check to make sure the string is not touching any object.
6 Again, ask your helper to tap the spoon as before. Compare the two sounds.
What Happened?
Everything that vibrates sends out sound waves through the air in all directions. The frequency or pitch of the sound was the same each time the spoon was tapped. But, the amplitude, or loudness, of the sound was greater with your fingers and the string pressed into your ears. This is because when traveling through air, the sound waves move slower than when they travel through a solid. Thus, more sound waves reached your ears when they traveled up the string to your ears. Also, the string provided a direct route for the sound from the vibrating spoon to your ears.
8 The Effect of Mass on Sound
Mass is a measure of the amount of matter in an object. For example, some drinking glasses are thicker than others, thus they are more massive. In reference to sound, the more massive a vibrating object, the lower the frequency, and hence the pitch. The reverse is true if the mass decreases with a result of a higher frequency and higher pitch.
In this activity, water will be added to glasses to change their mass. Tapping on a glass with water will produce a sound with a certain frequency. It will be seen that increasing the mass of a glass and then tapping on it will produce a sound with a lower frequency. Altering the amount of water in the glass can produce different musical notes.
See for Yourself
Materials
6 drinking glasses of equal size and shape (the thinner the glass walls, the better)
water
food coloring
pencil
What to Do
1 Pour different amounts of water in five of the glasses. Leave one glass empty.
2 Using food coloring, make the water in each glass a different color. Note: The coloring doesn't change the sound that will be produced. It merely helps to associate each glass with the sound it produces.
3 Gently tap each glass with the wooden end of a pencil.
4 Compare the sounds made by each glass with water to the sound produced by the empty glass. Determine which produces the highest and the lowest frequency.
FIG 1
An experimental challenge is to alter the amount of water in each glass to produce the notes to play a simple song.
What Happened?
The sound produced by tapping each glass was different depending on the amount of water it contained. The sound heard also depended on the sound produced by the glass; thus, an empty glass was used that vibrated the fastest when tapped, so it had the highest frequency and the note was high. A glass with water in it vibrated slower when tapped because the glass and water together had a greater mass. The greater the mass, the slower the vibrations of the glass when tapped. Slower vibrations mean a lower frequency of sound waves. The high and low musical notes produced are related to frequency. High notes are produced by high frequencies and low notes by low frequencies. Adjusting the level of water in a glass can result in notes that go up or down the musical scale.
9 Natural Frequency
Natural frequency is the frequency at which a material vibrates when hit, plucked, strummed, or somehow set into motion. When a tuning fork is struck, it vibrates at a specific frequency: its natural frequency. As a tuning fork vibrates, it causes the air around itself to vibrate, which produces the sound waves you hear. A simple motion, such as rubbing the rim of a glass, can cause it to vibrate at its natural frequency and produce sound.
See for Yourself
Materials
stemmed glass (this will work best if the glass is thin)
dish detergent
small bowl of water
What to Do
1 Remove
