The wavelength, The wavelength, λλ, is 20 m. What is the , is 20 m. What is the speed of this wave?speed of this wave?

CT 2.1.10CT 2.1.10

3Time (sec)

A) 1 m/s B) 6 m/sC) 10 m/s D) 15 m/sE) None of the above/not enough info/not sure

SoundSoundChapter 15Chapter 15

Perceiving SoundPerceiving Sound

What is sound?

Sound is a longitudinal wave which travels through the air through a series of compressions and rarefactions.

Sound WavesSound Waves

Sound travels different speeds Sound travels different speeds in different media. Sound in different media. Sound typically travels faster in a typically travels faster in a solid that a liquid and faster in solid that a liquid and faster in a liquid than a gas.a liquid than a gas.

The higher the temperature, The higher the temperature, the faster the particles of the the faster the particles of the medium will move and the medium will move and the faster the particles will carry faster the particles will carry the sound.the sound.

Medium (Co) m/s

Air (0o) 331

Air (20o) 343

Helium (0o) 972

Water (25o) 1493

Seawater (25o)

1533

Copper (25o) 3560

Iron (25o) 5130

Speed of Sound

Sound waves move through air because of Sound waves move through air because of regular variations or oscillations in air pressure regular variations or oscillations in air pressure

Perceiving SoundPerceiving Sound

Pitch – Frequency of vibrationsWe can hear frequencies from 20 Hz to

20,000 Hz are infrasonic.A high-pitched sound has a high frequency

(a screaming girl). A low-pitched sound has a low frequency (a fog-horn).

By the time you’re age 70 , you can’t hear frequencies above 8,000 Hz…That’s why older people have a hard time hearing.

Sounds above 20,000 Hz are termed ultrasonic. Some animals, such as dogs, can hear frequencies in this range in which humans cannot hear.

Sound WavesOn a longitudinal wave the area squeezed together is called the compression. The areas spread out are called the rarefaction.

The wavelength is the distance from the center of one compression to the center of the next compression.

What is the Doppler Effect?

The Doppler Effect is the apparent change in frequency detected when the sound is moving relative to the hearer.

Video-Excellent example of Doppler Effect with car horn (26 seconds)

Video-A Motorcycle does the Doppler Effect (27 seconds)

Doppler EffectDoppler Effect

The effect that explains why a moving sound The effect that explains why a moving sound will sound different when a sound is coming will sound different when a sound is coming towards you or away from you.towards you or away from you.

The frequency perceived by a detector is equal The frequency perceived by a detector is equal to the velocity of the detector relative to the to the velocity of the detector relative to the velocity of the wave, divided by the velocity of velocity of the wave, divided by the velocity of the source relative to the velocity of the wave, the source relative to the velocity of the wave, multiplied by the wave’s frequency.multiplied by the wave’s frequency.€

fd = f sv − vdv − vs

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You hear an increasing pitch of an ambulance.

Draw a picture that describes this process.Draw a picture that describes this process.

Which picture(s) describes this process?Which picture(s) describes this process?

You hear an increasing pitch of an ambulance.

vpva = 0

va = 0vp

vavp = 0

vavp = 0

1

4

2

3

5 1 & 2 6 3 & 4

7 1 & 4 8 2 & 3

Doppler Effect - Doppler Effect - ExampleExample

A trumpet player sounds C above middle C A trumpet player sounds C above middle C (524 Hz) while traveling in a convertible at (524 Hz) while traveling in a convertible at 24.6 m/s. If the car is coming toward you, 24.6 m/s. If the car is coming toward you, what frequency would you hear? Assume that what frequency would you hear? Assume that the temperature is 20the temperature is 20ooC.C.

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fd = f sv − vdv − vs

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What is sound intensity?

Sound intensity is the energy that the sound wave possesses. The greater the intensity of sound the farther the sound will travel and the louder the sound will appear.

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intensity =P

4πr2

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intensity =Power

4π( ) distance from the source( )2

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Sound intensity - Problem

What is the intensity of the sound waves produced by a trumpet at a distance of 3.2 m when the power output of the trumpet is 0.20 W? Assume that the sound waves are spherical.

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intensity =P

4πr2

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ProblemsProblems

You are in an auto traveling at 55 mph (24.6 You are in an auto traveling at 55 mph (24.6 m/s). A second auto is moving toward you a m/s). A second auto is moving toward you a the same speed. Its horn is sounding at 475 the same speed. Its horn is sounding at 475 Hz. What frequency do you hear? Use 343 Hz. What frequency do you hear? Use 343 m/s as the speed of sound.m/s as the speed of sound.

At a maximum level of loudness, the power At a maximum level of loudness, the power output of a 75-piece orchestra radiated as output of a 75-piece orchestra radiated as sound is 70.0 W. What is the intensity of sound is 70.0 W. What is the intensity of these sound waves to a listener who is sitting these sound waves to a listener who is sitting 25.0 m from the orchestra?25.0 m from the orchestra?

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fd = f sv − vdv − vs

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⎝ ⎜

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intensity =P

4πr2

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⎝ ⎜

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Perceiving SoundPerceiving Sound

Loudness - Dependent on Loudness - Dependent on amplitudeamplitudeRemember – amplitude for a longitudinal Remember – amplitude for a longitudinal

wave is the closeness of the compressionwave is the closeness of the compressionLoudness is very closely related to

intensity. Loudness is the human perception of the sound intensity. The unit for loudness is decibels.

Amplitude - SoundAmplitude - Sound

Intensity vs. DecibelIntensity vs. Decibel

Loudness in Decibels

Harmonics…. Harmonics…. Standing Waves for Standing Waves for

StringsStrings1=2L f1

Fundamental frequency

2=2/2L f2=2f1 2nd harmonic

3=2/3L f3=3f1 3rd harmonic

1=2/4Lf4=4f1 4th harmonic

1=2/5L f1=5f1 5th harmonic

Although we would perceive a string Although we would perceive a string vibrating as a whole,vibrating as a whole,

it actually vibrates in a pattern that at first it actually vibrates in a pattern that at first appears to be erratic producing many different appears to be erratic producing many different overtone pitches. What results are particular overtone pitches. What results are particular

tone colors or timbres of instruments and tone colors or timbres of instruments and voices.voices.

Harmonic Pattern?Harmonic Pattern?

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fn = nv

2L n =1,2,3...

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frequency = harmonic number x speed of waves on string

2( )length of vibrating string

Standing Waves for a PipeStanding Waves for a Pipe

Open at BOTH endsOpen at BOTH ends

Open at ONE endOpen at ONE end€

fn = nv

2L n =1,2,3...

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frequency = harmonic number x speed of waves in the pipe

2( )length of vibrating air column

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fn = nv

4L n =1,3,5...

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frequency = harmonic number x speed of waves in the pipe

4( )length of vibrating air column

Harmonic ExampleHarmonic Example

What are the first three harmonics in a 2.45 What are the first three harmonics in a 2.45 m long pipe that is open at both ends? What m long pipe that is open at both ends? What are the first three harmonics of the pipe when are the first three harmonics of the pipe when one end of the pipe is closed? Assume that one end of the pipe is closed? Assume that the speed of sounds in air is 345 m/s for both the speed of sounds in air is 345 m/s for both of these situations.of these situations.

““Timbre” (TAM-ber)Timbre” (TAM-ber)

is the specific is the specific property of property of sound that sound that enables us to enables us to determine the determine the difference difference between a between a piano and a piano and a harp.harp.

BeatsBeats

Interference of waves of slightly different Interference of waves of slightly different frequencies traveling in the same direction, frequencies traveling in the same direction, perceived as a variation in loudness.perceived as a variation in loudness.

NoiseNoise

Interference of waves of slightly different Interference of waves of slightly different frequencies traveling in the same direction, frequencies traveling in the same direction, perceived as a variation in loudness.perceived as a variation in loudness.