Chpt 24- Sound eBook

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578 U NIT 8 WAVES, SOUND, AND LIGHT

Chapter 24 SOUND

Figure 24.1: The frequencies in three

people’s voices as they say the word

hello. Each person’s voice is made up of

a mixture of frequencies.

pitch - the perception of high or lowthat you hear at different frequencies

of sound.

Voice A

L o u d n e s s

Voice B

L o u d n e s s

Voice C

L o u d n e s s

0 2.000 4.000Frequency (Hz)



24.1 Properties of Sound

Like other waves, sound has frequency, wavelength, and speed. Because sound is part of yourdaily experience, you already know its properties—but by different names. For example, the

loudnessof sound it related to its amplitude. Read on to find out more about sound’s properties.

The frequency of sound

Frequency and

pitch

Your ears are very sensitive to the frequency of sound. The pitch of a soundis how you hear and interpret its frequency. A low-frequency sound has alow pitch, like the rumble of a big truck or a bass guitar. A high-frequencysound has a high pitch, like the scream of a whistle or siren. Humans cangenerally hear frequencies between 20 Hz and 20,000 Hz. Animals may heara wider range of frequencies, or higher or lower frequencies than humans.

Most sound hasmore than one

frequency

Almost all the sounds you hear containmany frequencies at the same time. Infact, the sound of the human voice contains thousands of differentfrequencies—all at once (Figure24.1).

The frequency

spectrum

Why is it easy to recognize one person’s voice from another’s, even whenboth are saying the same word? The reason is that people have differentmixtures of frequencies in their voices. A frequency spectrumshowsloudness on the vertical axis and frequency on the horizontal axis.Figure24.1 shows the frequency spectrum for three people sayinghello. Canyou see any difference between the graphs?

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57924.1 PROPERTIES OF SOUND

Chapter 24SOUND

Intensity and loudness of sound

Decibels The unit for the intensity or strength of a sound is thedecibel (dB). We canmeasure sound intensity with scientific instruments just like we can measure

mass with a balance. The decibel scale (shown below) is convenient to usebecause most sounds fall between 0 and 100. The amplitude of a soundincreases 10 times for every 20-decibel increase (Figure24.2).

Loudness When you experience a loud sound, you experience the effects of its intensityandfrequency. Anequal loudness curvecompares how loud you hear soundsof different frequencies (Figure24.3). As you can see, the human ear

responds differently to high and low frequencies. This curve shows that lowfrequency sounds (below 100 Hz) need to have higher decibel values for youto hear them than the same as sounds between 100 and 1,000 Hz. Notice thatthe numbers are not evenly spread out on thex-axis of this graph. This type of spacing is called alogarithmic scale. You read the graph in the same way thatyou would read an evenly spaced graph.

Aco ust ics Acoustics is the science and technology of sound. Knowledge of acoustics isused to design facilities like libraries, recording studios, and concert halls. Adesign might address how to reduce sound intensity and/or whether soundneeds to be absorbed, amplified, or even prevented from entering a room.

Figure 24.2: The decibel scale

measures amplitude (loudness).

Figure 24.3: All points on an equal

loudness curve have the same loudness.

decibel (dB) - a unit of measurefor the intensity or strength of a

sound.


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580 UNIT 8 WAVES, SOUND, AND LIGHT

Chapter 24 SOUND

Figure 24.4: The boundary betweenhearing and not hearing the plane is the

“shock wave.” The person in the middle

hears a sonic boom as the shock wave

passes over him.

Figure 24.5: The speed of sound in

various materials (helium and air at

0°C and 1 atmospheric pressure).

supersonic - a term to describespeeds faster than the speed of

sound.

MaterialSound Speed

(m/s)

Air 330

Helium 965Water 1,530

Wood (average) 2,000

Gold 3,240

Steel 5,940

The speed of sound

Sound is slower thanlight

You have may have noticed that the sound of thunder often comes manyseconds after you see lightning. Lightning is what creates thunder so they

really happen at the same time. You hear a delay because sound travels muchslower than light. The speed of sound is about 1,000 km/h (660 mph). Lighttravels at 300,000 km/s (186,000 mi/s).

Subsonic andsupersonic

Objects that move faster than sound are calledsupersonic. If you were onthe ground watching a supersonic plane fly toward you, there would besilence (Figure24.4). The sound would bebehindthe plane, racing to catchup. Some military jets fly at supersonic speeds. Passenger jets aresubsonic because they travel at speeds from 600 to 800 kilometers per hour.

Sonic booms A supersonic jet compresses the sound waves that are created as its nose cutsthrough the air. A cone-shapedshock waveforms behind the point where the

waves “pile up” at the nose of the plane. As a result, you only hear noisefrom a supersonic plane once it has passed overhead. At the boundary of hearing and not hearing the plane—the shock wave—the amplitude changesabruptly causing a very loud sound called asonic boom.

Sound in liquids andsolids

Sound travels through most liquids and solids faster than through air(Figure24.5). Sound travels about 5 times faster in water, and about 18 timesfaster in steel. This is because sound is a traveling oscillation. Like otheroscillations, sound depends on restoring forces. The forces holding steelatoms together are much stronger than the forces between the molecules inair. Stronger restoring forces increase the speed of sound.


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Chapter 24SOUND

The Doppler effect

The Doppler effect is

caused by motion

TheDoppler effect is a shift in the frequency of an oscillation caused bymotion of the source of the oscillation. If a stationary object is producing

sound, listeners on all sides will hear the same frequency. However, when theobject is in motion, the frequency will notbe the same to all listeners. Peoplemoving with the object or to the side hear the frequency as if the object wereat rest. People in front hear a higher frequency. People behind hear a lowerfrequency. The Doppler effect occurs at speedsbelowthe speed of sound.

The cause of the

Doppler effect

The Doppler effect occurs because an observer hears the frequency at whichwave crests arrive at his or her ears. For the moving sound source, observer(A) in the graphic above hears a higher frequency. This is because the object’smotion causes the crests in front to be closer together. The opposite is truebehind a moving object, where the wave crests are farther apart. Observer (C)in back hears a lower frequency because the motion of the object makes morespace between successive wave crests. The greater the speed of the object, thelarger the difference in frequency between the front and back positions.

Hearing the Doppler

effect

You hear the Doppler effect when you hear a police or fire siren comingtoward you, then going away from you. The frequency shifts up when thesiren is moving toward you. The frequency shifts down when the siren ismoving away from you.

Doppler effect - an increase ordecrease in frequency caused by the

motion of the source of an oscillation(such as sound).

Doppler Radar

Doppler radar is a way to measurethe speed of a moving object at adistance. A transmitter sends a pulse

of microwaves. The waves reflectfrom a moving object, such as a car. The frequency of the reflected waveis increased if the car is moving

toward the oncoming microwavesand decreased if the car is movingaway. The difference in frequencybetween the reflected andtransmitted wave is proportional tospeed.


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Chapter 24 SOUND

Figure 24.6: The process of digital

sound reproduction.

Recording sound

The microphone To record a sound, you must store the pattern of vibrations in a way that canbe replayed and be true to the original sound. A common way to record sound

starts with a microphone. A microphone transforms a sound wave into anelectrical signal with the same pattern of vibration (Figure24.6, top).

Analog to d igi tal

conversion

In modern digital recording, a sensitive circuit called an analog to digitalconverter measures the electrical signal 44,100 times per second. Eachmeasurement consists of a number between 0 and 65,536 corresponding tothe amplitude of the signal. One second of compact-disc-quality sound is alist of 44,100 numbers. The numbers are recorded as data on the disc.

Playback of

recorded sound

To play the sound back, the string of numbers on the CD is read by a laser andconverted into electrical signals again by a second circuit. This circuit is adigital to analog converter, and it reverses the process of the first circuit. Theplayback circuit converts the string of numbers back into an electrical signal.

The electrical signal is amplified until it is powerful enough to move the coilin a speaker and reproduce the sound (Figure24.6, bottom).

Stereo sound Most of the music you listen to has been recorded in stereo. A stereorecording is actually two recordings, one to be played from the left speaker,and the other from the right. Stereo sound seems almost “live” because itcreates slight differences between when the sound reaches your left and rightears. Sound from all sources tends to reach you this way. The slightdifferences in how sound reaches your ears lets you know where sound is

coming from. Another way to describe two sound waves that arrive at slightlydifferent times is to say they are slightly out of phase.


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Chapter 24SOUND

24.1 Section Review

1. What is the relationship between pitch and frequency?

2. If you looked at the frequency spectrums of two friends saying the worddog, would they look the same or different? Explain your answer.

3. Do two sound waves that seem equally loud always have the sameamplitude? Explain.

4. What two variables affect how loud you hear sound?

5. How do the amplitudes of a 120-decibel sound and a 100-decibel soundcompare?

6. Make a graph of the relationship between the amplitude (x-axis) anddecibel level (y-axis) of sound. Describe this relationship.

7. Would an object moving at 750 km/h be supersonic or subsonic?

8. Would an object moving at 100 miles per hour be supersonic or subsonic?Use the conversion factor 1 mile =1.6 kilometers.

9. Is it possible that a commercial passenger plane traveling at normalspeeds could produce a shock wave or a sonic boom? Why or why not?

10. Why does sound travel faster through water than through air?

11. A paramedic in an ambulance does not experience the Doppler effect of the siren. Why?

12. You hear an ambulance in your neighborhood that is traveling a fewblocks from where you are. The pitch of the siren seems to be getting

lower and lower. Is the ambulance traveling toward you or away fromyou? How do you know?

13. Research: Find out how Doppler radar is used in weather forecasting.

14. What is the role of a microphone in recording sound?

15. The process of recording music involves converting between analog anddigital information. Infer from the text what the termsanaloganddigital mean. Write a definition of these terms in your own words.

16. What about stereo sounds makes it seem like you are hearing themusicians play “live”?

Ultrasound

We cannot hear or see ultrasoundwaves, but they can pass through thehuman body. Doctors use ultrasoundimages to see “inside” patients, thesame way they use X-rays. Theultrasound image pictured above is

a heart.

Research the answers to the followingquestions.

What exactly is ultrasound?

How do the frequency andwavelength of ultrasound compare to

sounds you can hear?


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Chapter 24 SOUND

Figure 24.7: Air is made of

molecules in constant random motion,

bumping off each other and the walls of

their container.

Figure 24.8: If temperature is

constant, high pressure means more

molecules per unit volume. Low pressure

means fewer molecules per unit volume.

What Can You Hear?

Take a few minutes to sit quietly andlisten to all the sounds around you.Make a list of what you hear. Be sureto record your perceived frequency of

the sounds—whether they are high orlow frequency. When you are done,write down any observations thatsurprised you.

24.2 Sound Waves

How do we know that sound is a wave? For starters, it has both frequency and wavelength. We alsoknow sound is a wave because it does all the things other waves do. Sound can be reflected,

refracted, and absorbed. Sound also shows diffraction and interference. Resonance occurs withsound waves and is especially important for understanding how musical instruments work.

What is a sound wave?

Sound in solids andliquids

Sound is a traveling oscillation of atoms. If you push on one atom, it pusheson its neighbor. That atom pushes on the next atom, and so on. The pushcauses atoms to oscillate back and forth like tiny beads on springs. Theoscillation spreads through the connections between atoms to make a soundwave. This is how sound moves through liquids and solids.

Sound in air andgases

In air, the situation is different. Air molecules are spread far apart andinteract by colliding with each other (Figure24.7). The pressure is highestwhere atoms are closest together and lowest where they are farthest apart(Figure24.8). Imagine pushing the molecules on the left side of the picturebelow. Your push squeezes atoms together creating a layer of higherpressure. That layer pushes on the next group of atoms and causes thoseatoms to squeeze together. This pattern repeats. The result is a travelingoscillation in pressure, which is a sound wave. Sound is a longitudinalwavebecause the oscillations are along the same direction that the wave travels.

The frequency rangeof sound waves

Anything that vibrates creates sound waves, as long as there is contact withother atoms. However, not all “sounds” can be heard. Humans can hear inthe range between 20 and 20,000 Hz. Bats can hear high-frequency soundsfrom 2,000 to 110,000 Hz and elephants hear lower-frequency sounds from

16 to 12,000 Hz.


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585 24.2 SOUND WAVES

Chapter 24SOUND

Sound and air pressure

Speakers If you touch the surface of a speaker, you can feel the vibration that createsa sound wave. Figure24.9shows an illustration of a speaker as well as an

exaggerated sound wave and the oscillation of pressure. When music isplaying, the surface of the speaker moves back and forth at the samefrequencies as the sound waves. The back-and-forth motion of the speakercreates a traveling sound wave of alternating high and low pressure.

Air pressu re The change in air pressure created by a sound wave is incredibly small. An80dB sound, equivalent to a loud stereo, changes the air pressure by onlyonepart in a million. Our ears are very well structured to detect the smallchanges in pressure created by sound waves.

Frequency and

pressure change

The frequency of sound indicates how fast air pressure oscillates back and

forth. The purr of a cat, for example, might have a frequency of 50hertz. Thismeans the air pressure alternates 50times per second. The frequency of a firetruck siren may be 3,000hertz. This corresponds to 3,000 vibrations persecond in the pressure of the air.

Sound speed

depends on

temperature

In air, the energy of a sound wave is carried by moving atoms and moleculesbumping into each other. Anything that affects the motion of atoms affects thespeed of sound. Molecules move more slowly in cold air and the speed of sound decreases. For example, at 0°C, the speed of sound is 330meters persecond, but at 21°C, the speed of sound is 344 meters per second.

Sound speed andpressure

At higher air pressures, molecules become more crowded. The speed of soundincreases because collisions between atoms increase. Therefore, if thepressure goes down, the speed of sound decreases. This phenomenon affectsairplanes. A plane that is subsonic at low altitudes may become supersonic athigher altitudes where the temperature and pressure are lower.

Sound speed and

molecular weight

Lighter atoms and molecules move faster than heavier ones at the sametemperature. The speed of sound is higher in helium gas because heliumatoms are lighter (and faster) than either the oxygen (O2) or nitrogen (N2)molecules that make up air.

Figure 24.9: This is what a sound

wave might look like if you could see the

atoms. The effect of sound on air

molecules is exaggerated.

How many vibrations of air pressure

occur per second when the note A(440 Hz), is played?


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Chapter 24 SOUND

Figure 24.10: The frequency and

wavelength of sound are inversely

related. When the frequency goes up, the

wavelength goes down proportionally.

The wavelength of sound

Range of wavelengths of

sound

The wavelengths of sound in air can be compared to the size of everydayobjects ( Table24.1). As with other waves, the wavelength of a sound is

inversely related to its frequency (Figure24.10). A low-frequency, 20-hertzsound has a wavelength the size of a large classroom. At the upper range of hearing, a 20,000-hertz sound has a wavelength about the width of a finger.

Wavelengths of

sounds are

important

Differences in sound are due to differences inboth frequency and wavelength. If you want tomake a sound of a certain wavelength (orfrequency), you need to have a vibrating objectthat is similar in size to the wavelength of that

sound. So, how is a French horn able to produceso many different sounds? A French horn makessound by vibrating the air trapped in a longcoiled tube. Short tubes only fit shortwavelengths and make higher-frequency sounds.Long tubes fit longer wavelengths and makelower-frequency sounds (Figure24.10). Openingand closing the valves on a French horn allowsthe player to add and subtract different-lengthtubes, changing the frequency of the sound.

Table 24.1: Frequency and Wavelength for Some Typical Sounds

Frequency (Hz) Wavelength Typical Source

20 17 m rumble of thunder

100 3.4 m bass guitar

500 70 cm (27”) average male voice

1,000 34 cm (13”) female soprano voice

2,000 17 cm (6.7”) fire truck siren

5,000 7 cm (2.7”) highest note on a piano

10,000 3.4 cm (1.3”) whine of a jet turbine

20,000 1.7 cm (0.67”) highest-pitched sound you can hear


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587 24.2 SOUND WAVES

Chapter 24SOUND

Standing waves

What is a

standing wave?

You just learned that a French horn makes sounds by confining waves withintubes of different lengths. A wave that is confined in a space is called a

standing wave. It is possible to make standing waves of almost any kind,including sound, water, and even light. You can experiment with standingwaves using a vibrating string. Vibrating strings create sound on a guitar orpiano.

Harmonics A string with a standing wave is a kind of oscillator. Like all oscillators, a string hasnatural frequencies. The lowest naturalfrequency is called the fundamental . Avibrating string also has other naturalfrequencies calledharmonics. The diagram at

the left shows the first three harmonics. Youcan find the harmonic number by counting thenumber of “bumps” or places of greatestamplitude. The first harmonic has one bump,the second has two, the third has three, and soon. The place of highest amplitude on a stringis theantinode. The place where the string doesnot move is called anode.

Resonance of

sound

Spaces enclosed by boundaries can createresonancewith sound waves. Like a

French horn, a panpipe makes music when sound resonates in tubes of different lengths (Figure24.11). One end of each tube is closed and the otherend is open. Blowing across the open end of a tube creates a standing waveinside the tube. The closed end of a pipe is a closed boundary and it makes anodein the standing wave. The open end of a pipe is an open boundary to astanding wave and makes anantinode. The pipe resonates to a certainfrequency when its length is one-fourth the wavelength of that frequency. If the pipe resonates at the fundamental frequency, then the wavelength of thefundamental is four times the length of the pipe.

Figure 24.11: A panpipe is made

from tubes of different lengths. The

diagram shows the fundamental for a

standing wave of sound in a panpipe.

standing wave - a wave that isconfined in a space.

fundamental - the lowest naturalfrequency of an oscillator.

harmonic - one of many naturalfrequencies of an oscillator.


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Chapter 24 SOUND

Figure 24.12: Sound displays all

the properties of waves in its

interactions with materials and

boundaries.

reverberation - multiple echoes of sound caused by reflections of sound

building up and blending together.

Using Your Ears

Go to a concert hall, an auditorium, oreven a smaller space. Make a map of the place. Play music from onelocation. While the music is playing,walk around and identify where youhear the music well and where youhear dead spots. Add these details toyour sketch.

Interaction between sound waves and boundaries

Interactions of sound and materials

Like other waves, sound waves can be reflected by hard surfaces andrefracted as they pass from one material to another. Diffraction causes sound

waves to spread out through small openings. Carpet and soft materials canabsorb sound waves. Figure24.12illustrates these four sound interactions.

Reverberation In a good concert hall, the reflected sound and direct sound from themusicians, along with sound reflected from the walls, creates a multiple echocalledreverberation. The right amount of reverberation makes the soundseem livelier and richer. Too much reverberation and the sound gets“muddy.” Concert hall designers choose the shape and surface of the wallsand ceiling to provide the best reverberation. Some concert halls havemovable panels that can be raised or lowered from the ceiling to help shapethe sound.

Constructing a goodconcert hall

Direct sound (A) reaches thelistener along with reflectedsound (B,C) from the walls. Theshape of the room and thesurfaces of its walls must bedesigned and constructed so thatthere is some reflected sound, butnot too much.

Interference can

also affect soundquality

Reverberation also causesinterference of sound waves. When two waves interfere, the total can belouder or softer than either wave alone. The diagram above shows a musicianand an audience of one person. The sound reflected from each wall interferesas it reaches the listener. If the distances are just right, one reflected wavemight be out of phase with the other. The result is that the sound is quieter atthat spot. An acoustic engineer would call it adead spot in the hall. Deadspots are areas where destructive interference causes some of the sound tocancel with its own reflections. It is also possible to make very loud spotswhere sound interferes constructively. The best concert halls are designed tominimize both dead spots and loud spots.


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58924.2 SOUND WAVES

Chapter 24SOUND

24.2 Section Review

1. How could you increase the air pressure inside a bag containing a groupof air molecules?

2. Is sound a longitudinal or transverse wave? Justify your answer.

3. A 200-hertz sound has a wavelength about equal to the height of an adult.Would a sound with a wavelength equal to the height of a two-year-oldchild have a higher or lower frequency than 200 Hz?

4. For each situation, identify when sound would travel faster and why.

a. Outside on a winter day or outside on a summer day?

b. Through water or air?

c. When air pressure is high or low?

d. Through a piece of wood that floats in water or through a piece of steel that sinks in water?

e. Through a gas that is 90% nitrogen (N2) and 10% helium (He) orthrough a gas that is 90% helium (He) and 10% nitrogen (N2)?

5. The first five harmonics for a vibrating string are shown inFigure24.13.

a. For each harmonic, identify the number of wavelengths represented.

b. For each harmonic, identify the number of nodes and antinodes thatare present (include the ends of the string in your count).

c. Which of the five harmonics has the highest natural frequency?

d. Make a drawing that shows what the 6th harmonic would look like.

6. A panpipe is made of five pipes. The longest pipe is 25 centimeters longand the shortest is 5 centimeters long. Which of these pipe produces thehighest-frequency sound and why?

7. Would a full concert hall have different reverberation from an empty hall?Explain.

8. It is extremely difficult to play, record, and hear live music in a park orother open space. Explain why this is so. Use the word reverberation inyour answer.

9. You and your band want to record a CD in your basement. What might

you need to do to make your basement a good place for recording music?

Figure 24.13: Question 5.

How Long Does a Pipe Have to Be to

Play the Note E?

You wish to make a pipe that makes asound with a frequency of 660 hertz(the note E). Use the relationshipbetween wave speed and frequency todetermine the wavelength of this note. The pipe length needs to be one-fourth

the wavelength to make a resonance inthe fundamental mode. Assume thespeed of the sound is 340 m/s.


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Chapter 24 SOUND

Figure 24.14: The recorded wave

form from 0.02 seconds of music.

You have probablyheard of J ohann

Sebastian Bach. Heis considered one of the greatest western

composers. He wasknown for writing contrapuntal, orcounterpoint, music. An example of this style occurs when you sing Row,

Row, Row Your Boat as a round. Find

out more about counterpoint andBach’s music. Then, listen to Bach’sThe Art of Fugue. Write about yourimpressions of this music.

24.3 Sound, Perception, and Music

Sound is everywhere in our environment. We use sound to communicate and we listen to sound forinformation about what is going on around us. Our ears and brain are constantly receiving and

processing sound. In this section, you will learn about how wehear a sound wave and how the earand brain construct meaning from sound. This section will also introduce some of the sciencebehind music. Musical sound is a rich language of rhythm and frequency, developed overthousands of years of human culture.

The perception and interpretation of sound

Constructing

meaning from

patterns

As you read this paragraph, you subconsciously recognize individual letters.However, themeaning of the paragraph is not in the letters themselves. Themeaning is in thepatternsof how the letters make words and the words make

sentences. The brain does a similar thing with sound. A single frequency of sound is like one letter. It does not have much meaning. The meaning insound comes from patterns of many frequencies changing together.

Ears hear many

frequencies

at once

When you hear a sound, the nerves in your ear respond to more than 15,000different frequencies at once. This is like having an alphabet with 15,000letters! The brain interprets all 15,000 different frequency signals from theear and creates a “sonic image” of the sound.

Complex sound

waves

Imagine listening to live music from a singer and a band. Your ears can easilydistinguish the voice from the instruments. How does this occur? Themicrophone records a single “wave form” of how pressure varies with time.

The recorded wave form is very complex, but it contains all the sound fromthe instruments and voice (Figure24.14).

How the brain finds

meaning

The brain makes sense of this sound because the ears separate the sound intodifferent frequencies. Your brain, receiving signals from your ears, haslearned to recognize certain patterns of how each frequency changes and getslouder and softer over time. One pattern might be a sung word. Anothermight be a musical note from a guitar. Inside your brain is a “dictionary” thatassociates a meaning with a pattern of frequency the same way an ordinarydictionary associates a meaning from a pattern of letters (a word).


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59124.3 SOUND, PERCEPTION, AND MUSIC

Chapter 24SOUND

The frequency spectrum and the sonogram

Frequency spectrum A frequency spectrum is a graph that shows the amplitudes of differentfrequencies present in a sound. Amplitude, or loudness, is represented on the

y-axis, and frequency is shown on thex-axis. Sound containing manyfrequencies has a wave form that is jagged and complicated. The wave form inthe Figure24.15 is from an acoustic guitar playing the note E. The frequencyspectrum shows that the complex sound of the guitar is made from manyfrequencies, ranging up to 10,000 Hz and beyond.

What is asonogram?

More information about a sound is available when a graph combines thevariables—frequency, amplitude, and time. A sonogram shows frequencyon the vertical axis and time on the horizontal axis. The loudness (amplitude)is shown by a color range.

Reading a sonogram The sonogram below (left) shows the wordhello lasting from 1.4 to2.2seconds. A sonogram of your voice (or anyone else’s) sayinghellowouldlook different because every voice is unique. In this example, you can see thatthere are many frequencies almost filling up the space between 0 and5,000Hz. The sonogram on the right is a simpler version of this type of graph.Which bar represents a loud sound of 100 Hz lasting from 1 to 3seconds(A, B, C, or D)?

Complex sonogram

of the word “hello”

A simple version of

a sonogram

Soft Loud

Key

F r e q u e n c y ( H z )

F r e q u e n c y ( H z )

Time (s) Time (s)

Figure 24.15: Each peak in the

spectrum represents the frequency and

amplitude of a wave that makes up the

wave form.

frequency spectrum - a graphthat shows the amplitudes of different

frequencies present in a sound.sonogram - a graph that shows thefrequency, amplitude, and time lengthfor a sound.


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Chapter 24 SOUND

Figure 24.16: The structure of the

inner ear. When the eardrum vibrates,

three small bones transmit the

vibrations to the cochlea. The vibrations

make waves inside the cochlea, which

shake hairs attached to nerves in the

spiral. Each part of the spiral is

sensitive to a different frequency.

How we hear sound

The cochlea Thecochleaprovides us with our ability to interpret sound—in other words,our sense of hearing. However, the cochlea is in the inner ear (Figure24.16).

Sound has to reach the cochlea by first entering the ear canal where itencounters the eardrum. Here, the sound waves cause the eardrum to vibrate. Then, three delicate bones of the inner ear transmit these vibrations to theside of the cochlea. In turn, fluid in the spiral channel of the cochlea vibratesand creates waves. Nerves along the channel have tiny hairs that shake whenthe fluid vibrates. Near the entrance, the channel is relatively large so thenerves respond to longer-wavelength, lower-frequency sound. The nerves atthe small end of the channel respond to shorter-wavelength, higher-frequency sound.

The semi-circular

canals

As you know, the function of our ears is hearing. But did you know that your

ears also provide you with your sense of balance? Near the cochlea in theinner ear are three semicircular canals. Like the cochlea, each canal containsfluid. The movement of this fluid in the canals indicates how the body ismoving (left–right, up–down, or forward–backward).

Human hearing In general, the combination of the eardrum, bones, and the cochlea limit therange of human hearing to between 20 hertz and 20,000 hertz. However,hearing varies greatly among different people and changes with age. Somepeople can hear sounds above 15,000 Hz and other people can’t. On average,people gradually lose high-frequency hearing with age. Most adults cannot

hear frequencies above 15,000 hertz, while children can often hear to20,000hertz.

Hearing can be

damaged by loudnoise

Hearing is affected by exposure to loud or high-frequencynoise. Listening to loud sounds for a long time can causethe hairs on the nerves in the cochlea to weaken or breakoff, causing permanent damage. Therefore, it is importantto always protect your ears by keeping the volume of noiseat a low or reasonable level. It is also important to wear earprotection if you have to stay in a loud place. In concerts,many musicians wear earplugs on-stage to protect their hearing.


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59324.3 SOUND, PERCEPTION, AND MUSIC

Chapter 24SOUND

Music

Pitch Thepitchof a sound describes how high or low we hear its frequency. Ahigher frequency sound is heard as a higher pitch. However, because pitch

depends on the human ear and brain, the way we hear a sound can be affectedby the sounds we heard before and after.

Rhythm Rhythm is a regular time pattern in a series of sounds. Here is a rhythm youcan “play” on your desk: TAP-TAP-tap-tap-TAP-TAP-tap-tap. Play “TAP”louder than you play “tap.” Rhythm can be made with sound and silence orwith different pitches. People respond naturally to rhythm. Cultures aredistinguished by their music and the special rhythms used in music.

The musical scale Music is a combination of sound and rhythm. Styles of music are vastlydifferent but all music is created from carefully chosen frequencies of sound.Most of the music you listen to is created from a pattern of frequencies called amusical scale. Each frequency in the scale is called anote. The C majormusical scale that starts on the note C (262 Hz) is shown in the diagram below.

The approximate frequencies of the notes in this scale are listed. Noticethat this scale begins and ends with C and that the higher C is twice thefrequency of the lower C. These two Cs are anoctaveapart. Anoctave is therange between any given frequency and twice that frequency. Notes that are anoctave apart in frequency share the same name because they sound similar tothe ear.

rhythm - a regular time pattern in aseries of sounds.

musical scale - a pattern of frequencies.

note - one frequency in a musicalscale.

octave - a range defined as beingbetween a single frequency value andtwice that frequency value. On amusical scale, these two notes wouldhave the same name.

Getting to Know Octaves

1. What is the frequency and name of

the note that is one octave lower thanC-262 Hz?

2. What is the name and frequency of

the note that is two octaves higher than A-440 Hz?


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Chapter 24 SOUND

beat - the oscillation between twosounds that are close in frequency.

consonance - a combination of frequencies that sound pleasant.

dissonance - a combination of frequencies that sound unpleasant.

Bats and Beats

Bats use echolocation to navigate andfind insects for food. Like a “sonicflashlight,” the bat’s voice “shines”ultrasound waves into the night. Thesound occurs as “chirps,” short bursts

of sound that rise in frequency. Whenthe sound reflects off an insect, thebat’s ears receive the echo. Since thefrequency of the chirp is alwayschanging, the echo comes back with aslightly different frequency. Thedifference between the echo and thechirp makes beats that the bat can

hear. The beat frequency isproportional to how far the insect isfrom the bat. A bat can evendetermine where the insect is bycomparing the echo it hears in the leftear with what it hears in the right ear.

Consonance, dissonance, and beats

Harmony Harmony is the study of how sounds work together to create effects desiredby the composer. From experience, you know that music can have a profound

effect on people’s moods. For example, the tense, dramatic sound track of ahorror movie is a vital part of the audience’s experience. Harmony is based onthe frequency relationships of the musical scale.

Beats When two frequencies of sound are not exactly equal in value, the loudness of the total sound seems to oscillate or beat . The diagram below illustrates howbeats occur for two waves occurring simultaneously. Thesuperpositionprinciplestates that when sound waves occur at the same time, they combineto make a complex wave. The sound (amplitude) of this wave is louder thaneither wave separately when the waves are in phasedue to constructiveinterference. When the waves areout of phase, the sound is quieter due to

destructive interference. We hear the alternation in amplitude as beats.

Consonance and

dissonance

When we hear more than one frequency of sound and the combination soundspleasant, we call it consonance. When the combination sounds unsettling,we call itdissonance. Consonance and dissonance are related to beats.When frequencies are far enough apart that there are no beats, we getconsonance. When frequencies are too close together, we hear beats that arethe cause of dissonance. In music, dissonance is often used to create tensionor drama. Consonance can be used to create feelings of balance and comfort.


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595 24.3 SOUND, PERCEPTION, AND MUSIC

Chapter 24SOUND

Making sounds

Voices The human voice is a complex sound that starts in the larynx, a small, hollowchamber at the top of your windpipe. The termvocal cords is a littlemisleading because the sound-producing structures are not really cords butfolds of expandable tissue that extend across the larynx. The sound that startsin the larynx is changed by passing through openings in the throat and mouth(Figure 24.17). Different sounds are made by changing both the vibrations inthe larynx and the shape of the openings.

The guitar The guitar has become a central instrument inpopular music. Guitars come in many typesbut share the common feature of makingsound from vibrating strings. A standardguitar has six strings that are stretched along

the neck and body of the guitar. The stringshave different weights and therefore differentnatural frequencies.

For a guitar in standard tuning, the heavieststring has a natural frequency of 82 hertz andthe lightest a frequency of 330 hertz. Eachstring is stretched by a tension force of about125 newtons (28 pounds). The combinedforce from six strings on a folk guitar is more

than 750 newtons (170pounds). The guitar istuned by changing the tension in each string. Tightening a string raises its naturalfrequency and loosening lowers it.

Each string canmake many notes

A typical guitar string is 63 centimeters long. To make different notes, thevibrating length of a single string can be shortened by holding it down againstone of many metal bars across the guitar’s neck called frets (Figure24.18).

The frequency goes up as the vibrating length of the string gets shorter. Aguitar with 20 frets and six strings can play 126 different notes, some of whichare duplicates.

Figure 24.17: Notice how the shape

of the structures in the throat and

mouth change as the human voice

creates the sounds AH, EE, EH, and

OH.

Figure 24.18: A guitarist can play a

note by playing an “open string” or he

can shorten the length of a string by

pressing down on a fret.


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Chapter 24 SOUND

Figure 24.19: The sound of the note

C (262 Hz) played on a piano and on a

guitar. Notice that the fundamental

frequencies are the same but the

harmonics have different amplitudes.

Figure 24.20: A tuning fork

produces a single frequency.

Harmonics and the sound of instruments

Same note, differentsound

The same note sounds different when played on different instruments. As anexample, suppose you listen to the note C (262 Hz) played on a guitar and thesame C (262 Hz) played on a piano. A musician would recognize both notesas being C because they have the same frequency and pitch. However, as youknow, a guitar sounds like a guitar and a piano sounds like a piano. If thefrequency of the note is the same, what gives each instrument itscharacteristic sound?

Instruments make

mixtures of

frequencies

A guitar and a piano have recognizable sounds because each note playedis not a single pure frequency. The most important frequency is still thefundamental note (C-262 Hz, for example). The variation comes from theharmonics. Remember, harmonics are frequencies that are multiples of thefundamental note. We have already learned that a string can vibrate at many

harmonics. This is true for all instruments. A single C note from a grandpiano might include 20 or more different harmonics.

Recipes for sound Consider that every instrument has its own recipefor the frequency contentof its sound. Another word for “recipe” in this context istimbre. InFigure24.19, you can see how the mix of harmonics for a guitar compares tothe mix for a piano when both instruments play the note C (262 Hz). Here,you can see that the timbre of a guitar is different from that of a piano.

Tuning and beats A tuning fork is a useful tool for tuning an instrument because it produces asingle frequency (Figure24.20). Here’s how a tuning fork is used. Let’s say

the A string on a guitar is out of tune and its natural frequency is 445 hertz. The correct frequency for A is 440 hertz. To tune the guitar, you need an Atuning fork which will produce vibrations at 440 hertz when it is struck.When you play the guitar string and listen to the tuning fork, you will hear abeat frequency of 5 beats per second, or 5 hertz. The beat frequency becomeszero when the string is tuned to the tuning fork so that both it and the guitarstring have a natural frequency of 440 hertz. The beats go away when thestring is in tune.


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597 24.3 SOUND, PERCEPTION, AND MUSIC

Chapter 24SOUND

24.3 Section Review

1. Do you hear sounds around you as one frequency at a time or as manyfrequencies at once?

2. Which of the frequencies in Figure24.21is a soft sound that lasts fiveseconds? What is the frequency of this sound?

3. What is the difference between a sonogram and a frequency spectrum?

4. If sound B has twice the amplitude of sound A, sound A is:

a. louder

b. softer

c. higher pitched

d. lower pitched

5. How does the cochlea allow us to hear both low-frequency and high-frequency sound?

6. What is the range of frequencies for human hearing?

7. If you were talking to an elderly person who was having trouble hearingyou, would it be better to talk in a deeper voice (low-frequency sound) ora higher voice (high-frequency sound)?

8. What is one way that your body knows if it is upside down or not?

9. If two sound waves have exactly the same frequency, will you hear beats?Why or why not?

10. A musician in a group plays a “wrong” note. Would this note disrupt theharmony or the rhythm of the song being played? Explain your answer.

11. The note G is 392 Hz. What is the frequency of this note one octavehigher?

12. Explain the appearance of the complex wave in Figure24.22. Inparticular, explain the areas of higher amplitude and lower amplitude.

13. Why does an A played on a violin sound different from the same noteplayed on a guitar?

14. How is the length of a string on a stringed instrument related to the lengthof a pipe on panpipe? Use the wordsfrequencyandwavelength in your

answer.





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MUSICCONNECTION

C H A P T E R 2

4

598 CHAPTER 24 SOUND

Music represents life. A particular piece of music may describe a real,

fictional or abstract scene from almost any area of human experience or

imagination. It is the musicians job to paint a picture which communicates

to the audience the scene the composer is trying to describe. I hope that

the audience will be stimulated by what I have to say (through the

language of music) and will therefore leave the concert hall feeling

entertained. If the audience is instead only wondering how a deaf musician

can play percussion then I have failed as a musician. For this reason my

deafness is not mentioned in any of the information supplied by my office

to the press or concert promoters.

Unfortunately, mydeafness makesgood headlines. Ihave learnt fromchildhood that if Irefuse to discussmy deafness withthe media theywill just make itup. The severalhundred articlesand reviewswritten about meevery year add upto a total of many thousands, only a handful accurately describe myhearing impairment. More than 90% are so inaccurate that it wouldseem impossible that I could be a musician. This essay is designed toset the record straight and allow people to enjoy the experience ofbeing entertained by an ever evolving musician rather than somefreak or miracle of nature.

Deafness is poorly understood in general. For instance, there is acommon misconception that deaf people live in a world of silence.To understand the nature of deafness, first one has to understandthe nature of hearing.

Hearing is basically a specialized form oftouch. Sound is simply vibrating air which theear picks up and converts to electrical signals,which are then interpreted by the brain. Thesense of hearing is not the only sense thatcan do this, touch can do this too. If you arestanding by the road and a large truck goes

by, do you hear or feel the vibration? The answer is both. With verylow frequency vibration the ear starts becoming inefficient and therest of the body's sense of touch starts to take over. For some reasonwe tend to make a distinction between hearing a sound and feelinga vibration, in reality they are the same thing. It is interesting to

note that in the Italian language this distinction does not exist. Theverb 'sentire' means to hear and the same verb in the reflexive form'sentirsi' means to feel. Deafness does not mean that you can't hear,only that there is something wrong with the ears. Even someonewho is totally deaf can still hear/feel sounds.

If we can all feel low frequency vibrations why can't we feel highervibrations? It is my belief that we can, it's just that as the frequencygets higher and our ears become more efficient they drown out themore subtle sense of 'feeling' the vibrations. I spent a lot of time inmy youth (with the help of my school Percussion teacher RonForbes) refining my ability to detect

vibrations. I would stand with my handsagainst the classroom wall while Ronplayed notes on the timpani (timpaniproduce a lot of vibrations). Eventually Imanaged to distinguish the rough pitchof notes by associating where on my bodyI felt the sound with the sense of perfectpitch I had before losing my hearing. Thelow sounds I feel mainly in my legs andfeet and high sounds might be particularplaces on my face, neck and chest.

HearingAn Essay by Dame Evelyn GlennieReprinted from www.evelyn.co.uk

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MUSICCONNECTION

CHA P T E R

2 4

599UNIT 8 W AVES, SOUND, AND LIGHT

It is worth pointing out atthis stage that I am nottotally deaf, I amprofoundly deaf.Profound deafness covers

a wide range ofsymptoms, although it iscommonly taken to meanthat the quality of thesound heard is notsufficient to be able tounderstand the spokenword from sound alone. With no other sound interfering, I canusually hear someone speaking although I cannot understand themwithout the additional input of lip-reading. In my case the amountof volume is reduced compared with normal hearing but moreimportantly the quality of the sound is very poor. For instance when

a phone rings I hear a kind of crackle. However, it is a distinctivetype of crackle that I associate with a phone so I know when thephone rings. This is basically the same as how normally hearingpeople detect a phone, the phone has a distinctive type of ringwhich we associate with a phone. I can in fact communicate overthe phone. I do most of the talking whilst the other person can saya few words by striking the transmitter with a pen, I hear this asclicks. I have a code that depends on the number of strikes or therhythm that I can use to communicate a handful of words.

So far we have the hearing of sounds and thefeeling of vibrations. There is one other

element to the equation, sight. We can also seeitems move and vibrate. If I see a drum head orcymbal vibrate or even see the leaves of a treemoving in the wind then subconsciously my

brain creates a corresponding sound. A common and ill informedquestion from interviewers is 'How can you be a musician when youcan't hear what you are doing?' The answer is of course that Icouldn't be a musician if I were not able to hear. Another oftenasked question is 'How do you hear what you are playing?' Thelogical answer to this is; how does anyone hear?. An electrical signalis generated in the ear and various bits of other information fromour other senses all get sent to the brain which then processes the

data to create a sound picture. The various processes involved in

hearing a sound are very complex but we all do it subconsciously sowe group all these processes together and call it simply listening. Thesame is true for me. Some of the processes or original informationmay be different but to hear sound all I do is to listen. I have nomore idea of how I hear than you do.

You will notice that more and more the answers are headingtowards areas of philosophy. Who can say that when two normallyhearing people hear a sound they hear the same sound? I wouldsuggest that everyone's hearing is different. All we can say is thatthe sound picture built up by their brain is the same, so thatoutwardly there is no difference. For me, as for all of us, I am betterat certain things with my hearing than others. I need to lip-read tounderstand speech but my awareness of the acoustics in a concertvenue is excellent. For instance, I will sometimes describe an acousticin terms of how thick the air feels.

To summarize, my hearing is something that bothers other peoplefar more than it bothers me. There are a couple of inconveniencesbut in general it doesn't affect my life much. For me, my deafness isno more important than the fact I am female with brown eyes. Sure,I sometimes have to find solutions to problems related to my hearingand music but so do all musicians. Most of us know very little abouthearing, even though we do it all the time. Likewise, I don't knowvery much about deafness, what's more I'm not particularlyinterested. I remember one occasion when uncharacteristically Ibecame upset with a reporter for constantly asking questions onlyabout my deafness. I said: 'If you want to know about deafness, youshould interview an audiologist. My speciality is music.'

In this essay I have tried to explain something which I find verydifficult to explain. Even so, no one really understands how I dowhat I do. Please enjoy the music and forget the rest.

Questions:

1. Which two of your senses can convert sound waves toelectrical signals? Which do you use more frequently?

2. How did Evelyn Glennie’s percussion teacher help her refineher ability to distinguish pitch?

3. Describe an occasion when you have been able to see

vibration caused by a sound wave.

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Chapter 24 SOUND

Chapter 24 Assessment

Vocabulary

Select the correct term to complete the sentences.

Section 24.1

1. A moving object that makes a sound will sound differently if

the object is moving toward or away from you due to the ____.

2. The unit for measuring the loudness of a sound is the ____.

3. How your ears hear and interpret a sound of a certain

frequency is called the ____.

4. ____ objects move faster than the speed of sound waves.

Section 24.2

5. You can tell which ____ a vibrating string is experiencing by

counting the nodes and antinodes.

6. A(n) ____ is a wave confined or trapped in a certain space.

7. The ____ is the lowest natural frequency of an oscillator.

8. A multiple echo in a concert hall or other room is called a(n)

____.

Section 24.3

9. A(n) ____ is a graph that shows the amplitudes of different

frequencies that make up a sound.

10. As two sounds of slightly different frequencies go in and out

of phase, ____ can be heard.

11. ____ is a regular time pattern in a series of sounds.

12. A(n) ____ is a pattern of frequencies used by musicians.

13. The range between a frequency on a musical scale and a

frequency that is twice as great is called a(n)____.

14. A graph that shows the frequency, amplitude, and time of a

sound such as a person saying a word is called a(n) ____.

15. A combination of sounds of different frequencies that sound

pleasant is called ____.

16. A combination of sounds of different frequencies that sound

unpleasant is called ____.

17. Each frequency on a musical scale is called a(n) ____.

Concepts

Section 24.1

1. Give an example of a sound with a high pitch and example of

a sound with a low pitch.

2. Explain how you can tell the difference between the voices of

two people if they are saying the same word.

3. Approximately how many decibels is each of the following

sounds.a. the cafeteria at your school at lunch

b. an alarm clock

c. a running sink faucet

4. Do all frequencies of sounds at 40 decibels seem equally loud

to your ears? Explain.

5. How fast do sound waves travel in air? How does this

compare to the speed of light waves?

6. What is a sonic boom?

consonance

decibel

standing wave

octave

fundamental

sonogram

frequency spectrum

Doppler effect

musical scale

supersonic

rhythm

dissonance

note

beat

reverberation

pitch

harmonic



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601C HAPTER 24 ASSESSMENT

Chapter 24SOUND

7. Why do sound waves travel faster in steel than in air or

water?

8. A car honking its horn moves toward you. Does the horn’s

pitch sound higher or lower than it would if the car were

parked? Explain.9. What does it mean to say a recording is in stereo?

Section 24.2

10. Draw a diagram that shows what air molecules look like

when a sound wave is traveling through the air.

11. Does sound travel faster in warm or cold air? Why?

12. Does a person’s voice sound higher or lower after inhaling

helium gas? Why?

13. How is the wavelength of a sound wave related to its

frequency?

14. Which would create sound waves with longer wavelengths, a

cat meowing or a bear growling?

15. Why does a flute produce higher-pitched sounds than a

tuba?

16. What is the difference between a node and an antinode on a

standing wave?

17. Draw a standing wave on a string with 6 nodes and

5 antinodes. Which harmonic did you draw?

18. The diagram to the right shows a harmonic of

a vibrating string.

a. Which harmonic is shown?

b. How many wavelengths does the standing

wave contain?c. What is the wavelength of the standing

wave?

19. List the four ways sound waves can interact

with materials and boundaries.

Section 24.3

20. How many different frequencies do nerves in

your ear sense at the same time when you hear a sound?

21. Which type of graph gives more information, a frequency

spectrum or a sonogram? Explain.22. What do your ears sense in addition to sounds?

23. Does the outer, larger part of the cochlea hear higher or

lower frequencies?

24. What can happen if a person listens to loud sounds for a long

time?

25. What causes the alternation of loud and soft sounds that

occur when similar frequencies are played together?

26. Which of the following guitar strings would have the highest

natural frequency?

a. A thick string that is very loose

b. A thick string that is tight

c. A thin string that is very loose

d. A thin string that is tight

27. What is the purpose of frets on a guitar?

28. How is the sound created by a tuning fork different from the

sound created by plucking a guitar string?



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Chapter 24 SOUND

Problems

Section 24.1

1. While you are at home, you hear the dishwasher with a

loudness of 40 dB and a siren outside with a loudness of

60 dB. How much greater is the amplitude of the siren’s

sound than the amplitude of the dishwasher’s sound?

2. A 100-hertz sound and a 10,000-hertz sound are heard at an

equal loudness. If the 100 Hz sound is at 40 decibels, what is

the intensity of the 10,000 Hz sound?

3. A sound wave takes 0.2 seconds to travel 306 meters. What

is the speed of sound in this material? Through which of the

materials in Figure 24.5 is the wave traveling?

Section 24.2

4. Suppose you stand in front of a tall rock wall that is

170 meters away. If you yell, how long does it take for the

echo to get back to your ears if the speed of sound is 340 m/s?

5. A sound wave has a speed of 340 m/s and a wavelength of

10 meters. What is its frequency? Would you be able to hear

this sound?

6. The range of human hearing is between 20 Hz and

20,000 Hz. If the speed of sound is 340 m/s, what is the

longest wavelength you can hear? What is the shortest?

Section 24.3

7. The note E has a frequency of 330 hertz. What is the

frequency of the E note one octave higher?

8. A note has a frequency of 988 hertz. What is the frequency of

the note one octave lower? What note is this?

Applying Your Knowledge

Section 24.1

1. People can usually hear sounds with frequencies between 20

and 20,000 hertz. Some animals can hear higher or lower

frequencies than people can. Research to find out the

hearing ranges of several different animals.

2. The Doppler effect is used to figure out whether stars are

moving toward or away from Earth. Red light has a lower

frequency than blue light. If the color of a star’s light shifts

to red, is it moving toward or away from Earth?

3. Light waves travel at 300,000 km/s. Sound waves in warm

air travel at approximately 0.34 km/s. During a

thunderstorm, a lightning bolt strikes 2 kilometers away

from you. How long does it take you to see the lightning?How long does it take you to hear the thunder?

Section 24.2

4. Science fiction movies sometimes show explosions in outer

space that make loud sounds. Explain why this is not

scientifically correct.

Section 24.3

5. The beat frequency is the frequency of the loud and soft

sounds heard when two sounds create beats. It is calculated

by subtracting the frequencies of the two different soundwaves. For example, playing 322 Hz and 324 Hz sounds will

result in a beat frequency of 2 Hz. Suppose you strike two

tuning forks and hear a beat frequency of 4 Hz. One tuning

fork has a frequency of 440 Hz. Can you determine the

frequency of the other fork? Explain.

6. How do noise cancelling headphones work? Do they work

equally well for all types of sounds? Review Chapter 23 and

do research to find the answers to these questions.


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