Sound
Section 1: The Nature of Sound
Here is an old riddle: If a tree falls in a forest and no one hears it, does the tree make a sound? To answer the riddle, you must decide what the word “sound” means. If sound is something that a person must hear, then the tree makes no sound. If sound can happen whether a person hears it or not, then the tree makes a sound.
Sound Waves
To a scientist, a falling tree makes a sound whether someone hears it or not. When a tree crashes down, the energy with which it strikes the ground causes a disturbance. Particles in the ground and the air begin to vibrate, or move back and forth. The vibrations create a sound wave as the energy travels through the two mediums. Sound is a disturbance that travels through a medium as a longitudinal wave.
Making Sound Waves
A sound wave begins with a vibration. Look at the mental gong shown in Figure 1. When the gong is struck, it vibrates rapidly. The vibrations disturb nearby air particles. Each time the gong moves to the right, it pushes air particles together, creating a compressions. When the gong moves to the left, the air particles bounce back and spread out, creating a rarefaction. These compressions and rarefactions travel through the air as longitudinal waves.
How Sound Travels
Like other mechanical waves, sound waves carry energy through a medium without moving the particles of the medium along. Each particle of the medium vibrates as the disturbance passes. When the disturbance reaches your ears, you hear the sound.
A common medium for sound is air. But sound can travel through solids and liquids, too. For example, when you knock on a solid wood door, the particles in the wood vibrate. The vibrations make sound waves that travel through the door. When the waves reach the other side of the door, they make sound waves in the air on the far side.
A common medium for sound is air. But sound can travel through solids and liquids, too. For example, when you knock on a solid wood door, the particles in the wood vibrate. The vibrations make sound waves that travel through the door. When the waves reach the other side of the door, they make sound waves in the air on the far side.
Interactions of Sound Waves
Sound waves interact with the surfaces they contact and with each other. Sound waves reflect off objects, different through narrow openings and around barriers, and interfere with each other.
Reflection
Sound wave may reflect when they hit a surface. A reflected sound wave is called an echo. In general, the harder and smoother the surface, the stronger the reflection. Look at Figure 2. When you clap your hands in a gym, you hear an echo because the hard surfaces – wood, brick, and metal – reflect sound directly back at you. But you don’t always hear an echo in a room. In many rooms, there are soft materials that absorb most of the sound that strikes them.
Diffraction
Have you ever wondered why you can hear your friends talking in a classroom before you walk through the doorway? You hear them because sound waves do not always travel in a straight line. Figure 3 shows how sound waves can diffract through openings such as doorways.
Soundwaves can also diffract, or bend, around corners. This is why you can hear someone who is talking in the hallway before you come around the corner. The person’s sound waves bend around the corner. Then they spread out so you can hear them even though you cannot see who is talking. Remember this the next time you want to tell a secret!
Soundwaves can also diffract, or bend, around corners. This is why you can hear someone who is talking in the hallway before you come around the corner. The person’s sound waves bend around the corner. Then they spread out so you can hear them even though you cannot see who is talking. Remember this the next time you want to tell a secret!
Interference
Sound waves may meet and interact with each other. You may recall that this interaction is called interference. The interference that occurs when sound wave meet can be constructive or destructive. In section 3, you will learn how interference affects the sound of musical instruments.
The speed of Sound
Have you ever wondered why the different sounds from musicians and singers at a concert all reach your ears at the same time? It happens because the sounds travel through air at the same speed. At room temperature, about 20° C, sound travels through air at about 343 m/s. This speed is much faster than most jet plans travel through the air!
The speed of sound is not always 343 m/s. Sound waves travel at different speeds in different mediums. Figure 4 shows the speed of sound in different mediums. The speed of sound depends on the elasticity, density, and temperature of the medium the sound travels through.
The speed of sound is not always 343 m/s. Sound waves travel at different speeds in different mediums. Figure 4 shows the speed of sound in different mediums. The speed of sound depends on the elasticity, density, and temperature of the medium the sound travels through.
Elasticity
If you stretch a rubber band and then let it go, it returns to its original shape. However, when out stretch modeling clay and then let it go, it stays stretched. Rubber bands are more elastic than modeling clay. Elasticity is the ability of a material to bounce back after being disturbed.
The elasticity of a medium depends on how well the mediums particles bounce back after being disturbed. To understand this idea, look at figure 5. In this model, the particles of a medium are linked by springs. If one particle is disturbed, it is pulled back to its original position. In an elastic medium, such as a rubber band, the particles bounce back quickly. But in a less elastic medium, the particles bounce back slowly.
The more elastic a medium, the faster stound travels in it. Sound can travel well in solids which are usually more elastic than liquids or gasses. The particles of a solid do not move very far, so they bounce back and forth quickly as the compressions and rarefactions of the sound waves pass by. Most liquids are not very elastic. Sound does not travel as well in liquids as it does in solids. Gases generally are not very elastic. Sound travels slowly in gases.
The elasticity of a medium depends on how well the mediums particles bounce back after being disturbed. To understand this idea, look at figure 5. In this model, the particles of a medium are linked by springs. If one particle is disturbed, it is pulled back to its original position. In an elastic medium, such as a rubber band, the particles bounce back quickly. But in a less elastic medium, the particles bounce back slowly.
The more elastic a medium, the faster stound travels in it. Sound can travel well in solids which are usually more elastic than liquids or gasses. The particles of a solid do not move very far, so they bounce back and forth quickly as the compressions and rarefactions of the sound waves pass by. Most liquids are not very elastic. Sound does not travel as well in liquids as it does in solids. Gases generally are not very elastic. Sound travels slowly in gases.
Density
The speed of sound also depends on the density of a medium. Density is how much matter, or mass, there is in a given amount of space, or volume. The denser the medium, the more mass it has in a given volume. Figure 6 shows two cubes that have the same volume. The brass cube is denser because it has more mass in a given volume.
In materials in the same state of matter – solid, liquid, or gas – sound travels more slowly in denser mediums. The particles of a dense material do not move as quickly as those of a less dense material. Sound travels more slowly in dense metals, such as lead or silver, than in iron or steel.
In materials in the same state of matter – solid, liquid, or gas – sound travels more slowly in denser mediums. The particles of a dense material do not move as quickly as those of a less dense material. Sound travels more slowly in dense metals, such as lead or silver, than in iron or steel.
Temperature
In a given medium, sound travels more slowly at lower temperatures than at higher temperatures. Why? At a low temperature, the particles of a medium move more slowly than at a high temperature. So, they are more difficult to move and return to their original positions more slowly. For example, at 20°C the speed of sound in air is about 343 m/s. But at 0°C, the speed of sound is about 330 m/s.
At higher altitudes, the air is colder than at lower altitudes, so sound travels more slowly at higher altitudes. On October 14, 1947, Captain Charles E. “Chuck” Yeager of the United States Air Force used this knowledge to fly faster than the speed of sound.
To fly faster than the speed of sound, Captain Yeager flew his plane to an altitude of more than 12,000 meters. Here, the air temperature (NOT FINISHED!!!)
At higher altitudes, the air is colder than at lower altitudes, so sound travels more slowly at higher altitudes. On October 14, 1947, Captain Charles E. “Chuck” Yeager of the United States Air Force used this knowledge to fly faster than the speed of sound.
To fly faster than the speed of sound, Captain Yeager flew his plane to an altitude of more than 12,000 meters. Here, the air temperature (NOT FINISHED!!!)
Section 2: Properties of Sound
Suppose you and a friend are talking on a sidewalk and a nosy truck pulls up next to you and stops, leaving its motor running. What would you do? You might talk louder, almost shout, so your fiend can hear you. You might lean closer and speak into your friend’s ear so you don’t have to raise your voice. Or you might walk away from the noisy truck so it’s not as loud.
Loudness
Loudness is an important property of sound. Loudness describes your perception of the energy of a sound. In other words, loudness describes what you hear. You probably already know a lot about loudness. For example, you know that your voice is much louder when you shout than when you speak softly. The closer you are to a sound, the louder it is. Also, a whisper in your ear can be just as loud as a shout from a block away. The loudness of a sound depends on two factors: the amount of energy it takes to make the sound and the distance from the source of the sound.
Energy of a Sound Source
In general, the greater the energy used to make a sound, the louder the sound. If you did the Discover activity, you may have noticed this. The more energy you used to pull the guitar string back, the louder the sound when you let the string go. This happened because the more energy you used to pull the string, the greater the amplitude of the string’s vibration. A string vibrating with a large amplitude produces a sound wave with a large amplitude. Recall that the greater the amplitude of a wave, the more energy the wave has. So, the larger the amplitude of the sound energy the wave has. So the larger the amplitude of the sound wave, the more energy it has and the louder it sounds.
Distance from a Sound Source
If your friend is speaking in a normal voice and you lean in closer, your friend’s voice sounds louder. Loudness increases the closer you are to a sound source. But why?
Imagine ripples spreading out in circles after you toss a pebble into a pond. In a similar way, a sound wave spreads out from its source. Close to the sound source, the sound wave covers a small area, as you can see in Figure 8. As the wave travels away from its source, it covers more area. The total energy of the wave, however, states the same whether it is close to the source or far from it. Therefore, the closer the sound wave is to its source, the more energy it has in a given area. The amount of energy a sound wave carries per second through a unit area is its intensity. A sound wave of greater intensity sounds louder. As you move away from a sound source, loudness decreases because the intensity decreases.
Imagine ripples spreading out in circles after you toss a pebble into a pond. In a similar way, a sound wave spreads out from its source. Close to the sound source, the sound wave covers a small area, as you can see in Figure 8. As the wave travels away from its source, it covers more area. The total energy of the wave, however, states the same whether it is close to the source or far from it. Therefore, the closer the sound wave is to its source, the more energy it has in a given area. The amount of energy a sound wave carries per second through a unit area is its intensity. A sound wave of greater intensity sounds louder. As you move away from a sound source, loudness decreases because the intensity decreases.
Measuring Loudness
The loudness of different sounds is compared using a unit called the decibel (dB). Figure 9 shows the loudness of some familiar sounds. The loudness of a sound you can barely hear is about 0 dB. Each 10-dB increase in loudness represents a tenfold increase in the intensity of the sound. For example, soft music at 30 dB sounds ten times louder than a 20 dB whisper. The 30 dB music is 100 times louder than the 10 dB sound of rustling leaves. Sounds louder than 100 dB can cause damage to your ears, especially if you listen to those sounds for long periods of time.
Pitch
Pitch is another property of sound you may already know a lot about. Have you ever described someone’s voice as “high pitched” or “low-pitched?” The pitch of a sound is a description of how high or low the sound seems to a person. The pitch of a sound that you hear depends on the frequency of the sound wave.
Pitch and Frequency
Sound waves with a high frequency have a high pitch. Sound waves with a low frequency have a low pitch. Frequency is measure in Hertz (Hz). For example, a frequency of 5 Hz means 50 vibrations per second. Look at Figure 10. A bass singer can produce frequencies lower than 80 Hz. A trained soprano voice can produce frequencies higher than 1,000 Hz.
Most people can hear sounds with frequencies between 20 Hz and 20,000 Hz. Sound waves with frequencies above the normal human range of hearing are called ultrasound. The prefix ultra- means “beyond”. Sounds with frequencies below the human range of hearing are called infrasound. The prefix infra- means “below”. People cannot hear either ultrasound waves or infrasound waves.
Most people can hear sounds with frequencies between 20 Hz and 20,000 Hz. Sound waves with frequencies above the normal human range of hearing are called ultrasound. The prefix ultra- means “beyond”. Sounds with frequencies below the human range of hearing are called infrasound. The prefix infra- means “below”. People cannot hear either ultrasound waves or infrasound waves.
Changing Pitch
Pitch is an important property of music because music usually uses specific pitches called notes. To sing or play a musical instrument, you must change pitch often.
When you sing, you change pitch using your vocal cords. Your vocal cords are located in your voice box, or larynx, as shown in Figure 11.When you speak or sing, air from your lungs is force up the trachea, or windpipe. Air then rushes past your vocal cords are able to vibrate more than 1,000 per second!
To sing different notes, you use muscles in your throat to stretch and relax your vocal cords. When you r vocal cords stretch, they vibrate more quickly as the air rushes by them. This creates higher-frequency sound waves that have higher pitches. When your vocal cords relax, lower-frequency sound waves with lower pitches are produced.
With musical instruments, you change pitch in different ways depending on the instrument. For example, you can change the pitch of a guitar string by turning a knob to loosen or tighten the string. A tighter guitar string produces a higher frequency, which you hear as a note with higher pitch.
When you sing, you change pitch using your vocal cords. Your vocal cords are located in your voice box, or larynx, as shown in Figure 11.When you speak or sing, air from your lungs is force up the trachea, or windpipe. Air then rushes past your vocal cords are able to vibrate more than 1,000 per second!
To sing different notes, you use muscles in your throat to stretch and relax your vocal cords. When you r vocal cords stretch, they vibrate more quickly as the air rushes by them. This creates higher-frequency sound waves that have higher pitches. When your vocal cords relax, lower-frequency sound waves with lower pitches are produced.
With musical instruments, you change pitch in different ways depending on the instrument. For example, you can change the pitch of a guitar string by turning a knob to loosen or tighten the string. A tighter guitar string produces a higher frequency, which you hear as a note with higher pitch.