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Sound Section 1

© Houghton Mifflin Harcourt Publishing Company

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Section 1 Sound Waves

Section 2 Sound Intensity and Resonance

Section 3 Harmonics

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The student is expected to:TEKS

7A examine and describe oscillatory motion and

wave propagation in various types of media

7B investigate and analyze characteristics of

waves, including velocity, frequency, amplitude,

and wavelength, and calculate using the

relationship between wavespeed, frequency, and

wavelength;

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(Continued):TEKS

7C compare characteristics and behaviors of

transverse waves, including electromagnetic waves

and the electromagnetic spectrum, and

characteristics and behaviors of longitudinal waves,

including sound waves

7D investigate behaviors of waves, including

reflection, refraction, diffraction, interference,

resonance, and the Doppler effect

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What do you think?

• What is sound?• What do all of the sounds that you hear have in

common?• How do they differ?

• Can sounds travel through solids? Liquids? Gases?• Is one type of material better for transmitting sound

waves?

• When race cars or emergency vehicles pass you, the sound changes. In what way, and why?

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What is Sound?

• Sound is a longitudinal wave.• All sound waves are produced

by vibrating objects.– Tuning forks, guitar strings, vocal

cords, speakers

• The vibrating object pushes the air molecules together, forming a compression.

• It then spreads them apart, forming a rarefaction.

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Graphing Sound Waves

• The diagram shows compressions (dark) and rarefactions(white). If you measured the pressure or density of the air and plotted these against position, how would the graph appear?

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Click below to watch the Visual Concept.

Visual Concept

Sound Waves

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Characteristics of Sound

• Frequency is the number of waves per second. • You have heard of ultrasound. What is it?• Frequencies audible to humans are between

20 Hz and 20 000 Hz.– Middle C on a piano is 262 Hz.– The emergency broadcast signal is 1 000 Hz.

• Infrasound frequencies are lower than 20 Hz.• Ultrasound frequencies are greater than

20 000 Hz.

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Visual Concept

Comparing Infrasonic and Ultrasonic Sounds

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Pitch

• Pitch is the human perception of how high or low a sound appears to be.– Pitch is primarily determined by frequency.– Pitch also depends slightly on other factors.

• Higher frequencies appear to have a higher pitch when played loudly, even though the frequency does not change.

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Speed of Sound

• Sound waves travel though solids, liquids and gases.– In which would the speed generally be greatest?

Why?• Solids. Because the molecules are more closely packed, the

particles respond more rapidly to compressions.

– How might the temperature of air affect the speed of sound waves? Why?

• Higher temperature increases the speed of the waves because the particles are moving faster and colliding more often.

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Speed of Sound

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Spherical Waves• Sound propagates in three

dimensions.• The diagram shows:

– Crests or wave fronts (blue circles)

– Wavelength ()– Rays (red arrows)

• Rays indicate the direction of propagation.

• How would these wave fronts appear different if they were much farther from the source?

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Spherical Waves

• Wave fronts and rays become more nearly parallel at great distances.

• Plane waves are simply very small segments of a spherical wave a long distance from the source.

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Doppler Effect

• Why are the waves closer together on the left?– Waves are closer because the vehicle moves to the left

along with the previous wave.

• How will the sound be different for observer A and observer B?– Higher frequency

(pitch) for observer A

• Continued on the next slide….

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Doppler Effect• How would the wave

pattern change if the vehicle moved at a faster speed? How would it sound different?– At a higher speed, waves

would be even closer together and the pitch difference would be even greater.

• The Doppler effect is the observed change in frequency due to the motion of the source or observer.

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Visual Concept

Doppler Effect and Sound

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Now what do you think?

• What is sound?– What do all of the sounds that you hear have in

common?– How do they differ?

• Can sounds travel through solids? Liquids? Gases?– Is one type of material better for transmitting sound

waves?

• When race cars or emergency vehicles pass you, the sound changes. In what way, and why?

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7B investigate and analyze characteristics of waves,

including velocity, frequency, amplitude, and

wavelength, and calculate using the relationship

between wavespeed, frequency, and wavelength;




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What do you think?

• Members of rock bands generally protect their ears from the loud sounds to prevent damage to their hearing. • How do we determine the loudness of a sound?

• What quantity is loudness measuring?• What units are used?

• Name three ways you can reduce the loudness of the music heard by a person in the audience.

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Sound Intensity

• Vibrating objects do work on the air as they push against the molecules.

• Intensity is the rate of energy flow through an area.– What is “rate of energy flow” called?

E/t is called power (P).

– Since the waves spread out spherically, you must calculate the area of a sphere. How?

• A = 4r2

– So, what is the equation for intensity?

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Sound Intensity

• SI unit: W/m2

• This is an inverse square relationship.– Doubling r reduces intensity by ¼.– What happens if r is halved?

• Intensity increases by a factor of 4.

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Intensity and Decibels• An intensity scale based on human perception of

loudness is often used.• The base unit of this scale is the bel. More

commonly, the decibel (dB) is used.– 0.1 bel = 1 dB,1 bel = 10 dB, 5 bels = 50 dB, etc.– The lowest intensity humans hear is assigned a value

of zero.

• The scale is logarithmic, so each increase of 1 bel is 10 times louder.– An increase in intensity of 3 bels is 1 000 times louder.

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Classroom Practice Problems

• The intensity of the sound from an explosion is 0.10 W/m2 at a distance of 1.0 × 103 m. Find the intensity of the sound at a distance of 5.0 × 102 m, 1.0 × 102 m and 10.0 m.– Answers: 0.41 W/m2, 1.0 × 101 W/m2, 1.0 × 103 W/m2

• Find the approximate decibel equivalents of these sound intensities using Table 2.– Answers: 110 dB, 130 dB, 150 dB

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Audible Sounds

• The softest sound humans can hear is called the threshold of hearing.– Intensity = 1 10-12 W/m2 or zero dB

• The loudest sound humans can tolerate is called the threshold of pain.– Intensity = 1.0 W/m2 or 120 dB

• Human hearing depends on both the frequency and the intensity.

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Visual Concept

Human Hearing

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Forced Vibrations

• Sympathetic vibrations occur when a vibrating object forces another to vibrate as well.– A piano string vibrates the sound board.– A guitar string vibrates the bridge.

• This makes the sound louder and the vibrations die out faster.– Energy is transferred from the string to the sound

board or bridge.

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Resonance

• The red rubber band links the 4 pendulums.

• If a blue pendulum is set in motion, only the other blue pendulum will have large-amplitude vibrations.– The others will just move a small

amount.

• Since the vibrating frequencies of the blue pendulums match, they are resonant.

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Resonance

• Large amplitude vibrations produced when the frequency of the applied force matches the natural frequency of receiver– One blue pendulum was the driving force and the

other was the receiver.

• Bridges have collapsed as a result of structural resonance.– Tacoma Narrows in the wind– A freeway overpass during an earthquake

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Visual Concept

Resonance (Frequency)

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• Members of rock bands generally protect their ears from the loud sounds to prevent damage to their hearing. – How do we determine the loudness of a sound?

• What quantity is loudness measuring?• What units are used?

– Name three ways you can reduce the loudness of the music heard by a person in the audience.

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7B investigate and analyze characteristics of waves,

including velocity, frequency, amplitude, and

wavelength, and calculate using the relationship

between wavespeed, frequency, and wavelength;




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What do you think?

• A violin, a trumpet, and a clarinet all play the same note, a concert A. However, they all sound different.• What is the same about the sound?

• Are the frequencies produced the same?• Are the wave patterns the same?

• Why do the instruments sound different?

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Standing Waves on a String

• There is a node at each end because the string is fixed at the ends.

• The diagram shows three possible standing wave patterns.

• Standing waves are produced by interference as waves travel in opposite directions after plucking or bowing the string.

• The lowest frequency (one loop) is called the fundamental frequency (f1).

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Standing Waves on a String

• To the left is a snapshot of a single loop standing wave on a string of length, L.

• What is the wavelength for this wave?– Answer: = 2L

• What is the frequency?– Answer:

1 2

v vf

L

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Harmonics

• n is the number of loops or harmonic number.• v is the speed of the wave on the string.

– Depends on tension and density of the string

• L is the length of the vibrating portion of the string.• How could you change the frequency (pitch) of a string?

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Visual Concept

Fundamental Frequency

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Standing Waves in an Air Column

• Wind instruments also use standing waves.– Flutes, trumpets, pipe organs, trombones, etc.

• Some instruments have pipes open at both ends while others have one end closed.– Air is free to move at open ends so antinodes occur.– Closed ends are nodes.

• The velocity of the wave is now the velocity of sound in air (346 m/s at 25°C).

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Both Ends Open

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Closed at One End

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Wind Instruments

• Wind instruments are not as simple as organ pipes.– The shape is not always cylindrical.– The holes change the wave patterns as well.– The size of the “pipe” varies along the length.

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• One string on a toy guitar is 34.5 cm long.– What is the wavelength of the first harmonic or the

fundamental wavelength?• Answer: 69.0 cm or 0.690 m

– The string is plucked and the speed of the waves on the string is 410 m/s. What are the frequencies of the first three harmonics?

• 590 Hz, 1200 Hz, 1800 Hz• Note: The use of significant figures causes the multiples of

590 to be 1200 and 1800 because only two significant figures are present in the answer.

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• An organ pipe open at both ends is 34.5 cm long.– What is the wavelength of the first harmonic or the

fundamental wavelength?• Answer: 69.0 cm or 0.690 m

– What are the frequencies of the first three harmonics if the air temperature is 25.0°C?

• Answers: 501 Hz, 1000 Hz, 1500 Hz

– Answer the same questions if the pipe is closed at one end.

• Answers: 251 Hz, 753 Hz, 1250 Hz

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Timbre or Quality of Sound

• Instruments do not vibrate in a single mode.– Several harmonics are produced at the same time.– The particular harmonics and intensity of each vary

with different instruments.

• Timbre is the quality of the tone resulting from the combination of harmonics.

• The fundamental frequency (1st harmonic) determines the pitch.– Adding other harmonics changes the timbre.

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Timbre

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Visual Concept

Timbre

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Beats

• The diagram shows two waves of different frequencies. Sketch the superposition or sum of these waves.

• How would the combined wave sound?

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Beats

• Produced by two waves with the same intensity and different frequencies– Generally the frequencies are nearly the same.

• The sound pulses or changes from loud to soft and back.

• Beats are used to tune instruments.– If a tuner hears beats, the instruments are slightly out

of tune.

• The number of beats heard per second is the difference in the two frequencies.

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Visual Concept

Beat

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• A violin, a trumpet, and a clarinet all play the same note, a concert A. However, they all sound different.– What is the same about the sound?

• Are the frequencies produced the same?• Are the wave patterns the same?

– Why do the instruments sound different?

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