characteristics of sound

sound and matter

Sound waves are mechanical waves, so they can travel only though matter and not through empty space. This was demonstrated in the 1600s by a scientist named Robert Boyle. Boyle placed a ticking clock in a sealed glass jar. The clock could be heard ticking through the air and glass of the jar. Then Boyle pumped the air out of the jar. The clock was still running, but the ticking could no longer be heard. Thats because the sound couldnt travel away from the clock without air particles to pass the sound energy along. You can see an online demonstration of the same experimentwith a modern twistat this URL: (4:06). MEDIA Click image to the left or use the URL below. URL: Sound waves can travel through many different kinds of matter. Most of the sounds we hear travel through air, but sounds can also travel through liquids such as water and solids such as glass and metal. If you swim underwater or even submerge your ears in bathwater any sounds you hear have traveled to your ears through water. You can tell that sounds travel through glass and other solids because you can hear loud outdoor sounds such as sirens through closed windows and doors.

properties of sound

Sound has certain characteristic properties because of the way sound energy travels in waves. Properties of sound include speed, loudness, and pitch.

sound waves

Why does a tree make sound when it crashes to the ground? How does the sound reach peoples ears if they happen to be in the forest? And in general, how do sounds get started, and how do they travel? Keep reading to find out.

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how sounds begin

All sounds begin with vibrating matter. It could be the ground vibrating when a tree comes crashing down. Or it could be guitar strings vibrating when they are plucked. You can see a guitar string vibrating in Figure 20.2. The vibrating string repeatedly pushes against the air particles next to it. The pressure of the vibrating string causes these air particles to vibrate. The air particles alternately push together and spread apart. This starts waves of vibrations that travel through the air in all directions away from the strings. The vibrations pass through the air as longitudinal waves, with individual air particles vibrating back and forth in the same direction that the waves travel. You can see an animation of sound waves moving through air at this URL:

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pitch

A marching band is parading down the street. You can hear it coming from several blocks away. When the different instruments finally pass by you, their distinctive sounds can be heard. The tiny piccolos trill their bird-like high notes, and the big tubas rumble out their booming bass notes (see Figure 20.5). Clearly, some sounds are higher or lower than others. But do you know why? How high or low a sound seems to a listener is its pitch. Pitch, in turn, depends on the frequency of sound waves. Recall that the frequency of waves is the number of waves that pass a fixed point in a given amount of time. High-pitched sounds, like the sounds of a piccolo, have high-frequency waves. Low-pitched sounds, like the sounds of a tuba, have low-frequency waves. For a video demonstration of frequency and pitch, go to this URL: (3:20). MEDIA Click image to the left or use the URL below. URL: To explore an interactive animation of sound wave frequency, go to this URL: The frequency of sound waves is measured in hertz (Hz), or the number of waves that pass a fixed point in a second. Human beings can normally hear sounds with a frequency between about 20 Hz and 20,000 Hz. Sounds with frequencies below 20 hertz are called infrasound. Sounds with frequencies above 20,000 hertz are called ultrasound. Some other animals can hear sounds in the ultrasound range. For example, dogs can hear sounds with frequencies as high as 50,000 Hz. You may have seen special whistles that dogs but not people can hear. The whistles produce a sound with a frequency too high for the human ear to detect. Other animals can hear even higher-frequency sounds. Bats, for example, can hear sounds with frequencies higher than 100,000 Hz.

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doppler effect

Look at the police car in Figure 20.6. The sound waves from its siren travel outward in all directions. Because the car is racing forward (toward the right), the sound waves get bunched up in front of the car and spread out behind it. As the car approaches the person on the right (position B), the sound waves get closer and closer together. In other words, they have a higher frequency. This makes the siren sound higher in pitch. After the car speeds by the person on the left (position A), the sound waves get more and more spread out, so they have a lower frequency. This makes the siren sound lower in pitch. A change in the frequency of sound waves, relative to a stationary listener, when the source of the sound waves is moving is called the Doppler effect. Youve probably experienced the Doppler effect yourself. The next time a vehicle with a siren races by, listen for the change in pitch. For an online animation of the Doppler effect, go to the URL below.

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speed of sound

The speed of sound is the distance that sound waves travel in a given amount of time. You probably already know that sound travels more slowly than light. Thats why you usually see the flash of lightning before you hear the boom of thunder. However, the speed of sound isnt constant. It varies depending on the medium of the sound waves. Table 20.1 lists the speed of sound in several different media. Generally, sound waves travel fastest through solids and slowest through gases. Thats because the particles of solids are close together and can quickly pass the energy of vibrations to nearby particles. You can explore the speed of sound in different media at this URL: Medium (20C) Air Water Wood Glass Aluminum Speed of Sound Waves (m/s) 343 1437 3850 4540 6320 The speed of sound also depends on the temperature of the medium. For a given medium such as air, sound has a slower speed at lower temperatures. You can compare the speed of sound in air at different temperatures in Table transfer the energy of the sound waves. The amount of water vapor in the air affects the speed of sound as well. Do you think sound travels faster or slower when the air contains more water vapor? (Hint: Compare the speed of sound in water and air in Table 20.1.) Temperature of Air 0C 20C 100C Speed of Sound (m/s) 331 343 386 KQED: Speed of Sound Along with cable cars and seagulls, the Golden Gate Bridge foghorn is one of San Franciscos most iconic sounds. But did you know that if you hear that foghorn off in the distance, you can calculate how many miles you are from the bridge? Using the Speed of Sound exhibit at the Outdoor Exploratorium at Fort Mason, Shawn Lani shows us how sound perception is affected by distance. For more information on the speed of sound, see http://science.kqed. MEDIA Click image to the left or use the URL below. URL:

loudness

A friend whispers to you in class in a voice so soft that you have to lean very close to hear what hes saying. Later that day, your friend shouts to you across the football field. Now his voice is loud enough for you to hear him clearly even though hes many meters away. Obviously, sounds can vary in loudness. Loudness refers to how loud or soft a sound seems to a listener. The loudness of sound is determined, in turn, by the intensity of sound. Intensity is a measure of the amount of energy in sound waves. The unit of intensity is the decibel (dB). You can see typical decibel levels of several different sounds in Figure 20.3. As decibel levels get higher, sound waves have greater intensity and sounds are louder. For every 10-decibel increase in the intensity of sound, loudness is 10 times greater. Therefore, a 30-decibel "quiet" room is 10 times louder than a 20-decibel whisper, and a 40- decibel light rainfall is 100 times louder than a 20-decibel whisper. How much louder than a 20-decibel whisper is the 60-decibel sound of a vacuum cleaner? The intensity of sound waves determines the loudness of sounds, but what determines intensity? Intensity is a function of two factors: the amplitude of the sound waves and how far they have traveled from the source of the sound. Remember that sound waves start at a source of vibrations and spread out from the source in all directions. The farther the sound waves travel away from the source, the more spread out their energy becomes. This is illustrated in Figure 20.4. The decrease in intensity with distance from a sound source explains why even loud sounds fade away as you move farther from the source. It also explains why low-amplitude sounds can be heard only over short distances. For a video demonstration of the amplitude and loudness of sounds, go to this URL: interactive animation at this URL:

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instructional diagrams

No diagram descriptions associated with this lesson

questions

Through which medium do sounds waves travel most slowly?

-->  a. air

b. wood

c. glass

d. aluminum

Assume that sound A has a decibel level of 10 and sound B has a decibel level of 30. How many times louder is sound B than sound A?

a. 3

b. 10

c. 20

-->  d. 100

What determines the intensity of sound?

a. amplitude of sound waves

b. frequency of sound waves

c. distance from the sound source

-->  d. two of the above

Compared with a low-pitched sound, a high-pitched sound has sound waves with

a. greater intensity.

-->  b. higher frequency.

c. greater amplitude.

d. longer wavelength.

Human beings can normally hear sounds with a frequency between about

a. 10 and 10,000 Hz.

-->  b. 20 and 20,000 Hz.

c. 20 and 140 Hz.

d. 10 and 120 Hz.

The speed of sound in air at 20 C is

-->  a. 343 m/s.

b. 1437 m/s.

c. 3850 m/s.

d. 4540 m/s

The Doppler effect occurs when the sound source

-->  a. is moving relative to the listener.

b. produces sound waves with a frequency above 10,000 Hz.

c. starts producing lower frequency sound waves.

d. starts producing greater amplitude sound waves.

All sounds begin with vibrations in matter.

-->  a. true

b. false

Sound waves generally travel most quickly through gases.

a. true

-->  b. false

Sounds can travel through air and water but not through solids.

a. true

-->  b. false

Sound waves travel more quickly in warm air than cold air.

-->  a. true

b. false

The amount of water vapor in the air affects the speed of sound through air.

-->  a. true

b. false

Sounds that are too high in frequency for humans to hear are called infrasound.

a. true

-->  b. false

As distance from a sound source increases, the area covered by the sound waves decreases.

a. true

-->  b. false

As the decibel level of sounds gets higher, the pitch of the sounds always gets higher.

a. true

-->  b. false

The intensity of sound waves is the same regardless of distance from the sound source.

a. true

-->  b. false

Some animals can hear sounds with frequencies as high as 100,000 Hz.

-->  a. true

b. false

how loud or soft a sound seems to a listener

-->  a. loudness

b. infrasound

c. sound

d. decibel

e. intensity

f. ultrasound

g. pitch

sounds with frequencies above 20,000 hertz

a. loudness

b. infrasound

c. sound

d. decibel

e. intensity

-->  f. ultrasound

g. pitch

unit of sound intensity

a. loudness

b. infrasound

c. sound

-->  d. decibel

e. intensity

f. ultrasound

g. pitch

how high or low a sound seems to a listener

a. loudness

b. infrasound

c. sound

d. decibel

e. intensity

f. ultrasound

-->  g. pitch

transfer of energy from a vibrating object in waves that travel through matter

a. loudness

b. infrasound

-->  c. sound

d. decibel

e. intensity

f. ultrasound

g. pitch

sounds with frequencies below 20 hertz

a. loudness

-->  b. infrasound

c. sound

d. decibel

e. intensity

f. ultrasound

g. pitch

measure of the amount of energy in sound waves

a. loudness

b. infrasound

c. sound

d. decibel

-->  e. intensity

f. ultrasound

g. pitch

diagram questions

No diagram questions associated with this lesson