Chapter 14Sound

AP Physics BLecture Notes

14-01 Producing a Sound Wave

14-02 Characteristics of Sound Waves

14-03 The Speed of Sound

14-05 Spherical and Plane Waves

14-06 The Doppler Effect

14-07 Interference of Sound Waves

14-08 Standing Waves

Sound

Sections

14-10 Standing Waves in Air Columns

14-11 Beats

Producing a Sound Wave

Characteristics of Sound

Loudness: related to intensity of the sound wave Pitch: related to frequency Audible range: about 20 Hz to 20,000 Hz Ultrasound: above 20,000 Hz Infrasound: below 20 Hz

Loudness: related to intensity of the sound wave Pitch: related to frequency Audible range: about 20 Hz to 20,000 Hz Ultrasound: above 20,000 Hz Infrasound: below 20 Hz

Phy 2054 Lecture Notes 12

Characteristics of Sound Waves

Chapter 14 Sound

The pitch of a musical note is characterized by the sound’s

(A) amplitude

(B) frequency

(C) wavelength

(D) speed

Sound can travel through any kind of matter, but not through a vacuum.

The speed of sound is different in different materials; in general, it is slowest in gases, faster in liquids, and fastest in solids.

The speed depends somewhat on temperature, especially for gases.

Sound can travel through any kind of matter, but not through a vacuum.

The speed of sound is different in different materials; in general, it is slowest in gases, faster in liquids, and fastest in solids.

The speed depends somewhat on temperature, especially for gases.

The Speed of Sound

Chapter 14 Sound

Which of the following is a true statement?

(A) Sound waves are transverse pressure waves.

(B) Sound waves can not travel through a vacuum.

(C) Sound waves and light waves travel a the same speed.

(D) When the frequency of a sound wave increases its wavelength also increases.

The Speed of Sound

Speed of a wave on a string.

LmF

v T

Speed of sound in a fluid.

v

Speed of sound in a solid.

Y

v

K273

Tm/s 331v

Speed of sound in air is temperature dependent.

Chapter 14 Sound

As the temperature of the air increases, the speed of sound through the air

(A) increases.

(B) decreases.

(C) does not change.

(D) increases if the atmospheric pressure is high.

Chapter 14 Sound

Sound of a frequency f travels through air with a wavelength l. If the frequency of the sound is increased to 2f, its wavelength in air will be

2 (D)

2 (C)

(B)

2 )A(

Spherical and Plane Waves

The intensity of sound from a point source diminishes with distance:

2r

1I

2rπ4

PI

Phy 2054 Lecture Notes 12

Chapter 14 Sound

The intensity of a point source at a distance d from the source is I. The intensity of the sound at a distance 2d from the source?

2I

(D) I 4 (C) 2I (B) 4I

)A(

Chapter 14 Sound

The intensity of a sound increases when there is an increase in the sound’s

(A) frequency

(B) wavelength

(C) amplitude

(D) speed

"I love hearing that lonesome wail of the train whistle as themagnitude of the frequency of the wave changes due to theDoppler effect."

Phy 2054 Lecture Notes 12

The Doppler Effect

The Doppler Effect

The Doppler Effect for two stationary observers

The Doppler effect occurs when a source of sound is moving with respect to an observer.

Stationary truck

Moving truck

Longer wavelengthLower frequency

Shorter wavelengthHigher frequency

Phy 2054 Lecture Notes 12

The Doppler Effect

Doppler Effect

is experienced whenever there is relative motion between the sound source and an observer.

Observer SourceObserver Source

Observer SourceObserver Source

s

oso vv

vvff

Phy 2054 Lecture Notes 12

The Doppler Effect

Observer Source

Observer Source

s

oso vv

vvff

v

vvff oso

v

vvff oso

s

oso vv

vvff

s

so vvv

ff

s

so vvv

ff

Stationary Source

HigherFrequency

Stationary Observer

HigherFrequency

Phy 2054 Lecture Notes 12

The Doppler Effect

Observer Source

Observer Source

Stationary Source

LowerFrequency

Stationary Observer

LowerFrequency

s

oso vv

vvff

v

vvff oso

v

vvff oso

s

oso vv

vvff

s

so vvv

ff

s

so vvv

ff

Phy 2054 Lecture Notes 12

The Doppler Effect

s

oso vv

vvff

v

vvff oso

s

so vvv

ff

v

vvff oso

s

so vvv

ff

Observer Source

Observer Source

Observer Source

Observer Source

Phy 2054 Lecture Notes 12

The Doppler Effect

Chapter 14 Sound

A police car with its siren on departs from a stationary observer. The frequency heard by the observer

(A) is higher than the source.

(B) is lower than the source.

(C) is the same as that of the source.

(D) is equal to zero.

You are standing still. What frequency do you detect if a fire engine whose siren emits at 1550 Hz moves at a speed of 32 m/s

a) toward you?

b) away from you?

Phy 2054 Lecture Notes 12

The Doppler Effect (problem)

A bat flies toward a wall at a speed of 5.0 m/s. As it flies, the bat emits an ultrasonic sound wave with a frequency of 30.0 kHz.What frequency does the bat hear in the reflected wave?

Phy 2054 Lecture Notes 12

The Doppler Effect (Problem)

Interference of Sound Waves

Sound waves interfere in the same way that other waves do in space.

Sources of Sound: Vibrating Strings and Air Columns

Musical instruments produce sounds in various ways –

vibrating strings

vibrating membranes

vibrating metal or wood shapes

vibrating air columns

The vibrations are transmitted to the air and then to our ears.

The vibration may be started by:

plucking

striking

bowing

blowing

Musical instruments produce sounds in various ways –

vibrating strings

vibrating membranes

vibrating metal or wood shapes

vibrating air columns

The vibrations are transmitted to the air and then to our ears.

The vibration may be started by:

plucking

striking

bowing

blowing

Phy 2054 Lecture Notes 12

Standing Waves

The strings on a guitar can be effectively shortened by fingering, raising the fundamental pitch.

The pitch of a string of a given length can also be altered by using a string of different density.

The strings on a guitar can be effectively shortened by fingering, raising the fundamental pitch.

The pitch of a string of a given length can also be altered by using a string of different density.

Phy 2054 Lecture Notes 12

Standing Waves

Standing Waves

... 3, 2, 1,n L2

nvfn

First Harmonic - Fundamental

121 λL

L2v

f 1

Second Harmonic

2λL Lv

f 2 L2v2

Third Harmonic

323 λL

L2v3

f 3

λv

f λfv

Phy 2054 Lecture Notes 12

Standing Waves

Chapter 14 Sound

The standing wave on a vibrating guitar string produces a sound wave in the air. The two waves have

(A) the same wavelength.

(B) the same speed.

(C) the same frequency.

(D) the same amplitude.

A tube open at both ends has displacement antinodes, at the ends.

First Harmonic - Fundamental

121 λL

L2v

f 1

... 3, 2, 1,n L2

nvfn

L

Second Harmonic

2λL Lv

f 2 L2v2

Third Harmonic

323 λL

L2v3

f 3

Phy 2054 Lecture Notes 12

Standing Waves in Air Columns

Chapter 14 Sound

A pipe open at both ends resonates with a fundamental frequency of 150 Hz. Which one of the following frequencies will not resonate in this pipe

(A) 300 Hz

(B) 450 Hz

(C) 500 Hz

(D) 750 Hz

A uniform narrow tube 1.8 m long is open at both ends. It resonates at two successive harmonics of frequencies 275 Hz and 330 Hz. What is the speed of sound in the gas in the tube?

Phy 2054 Lecture Notes 12

Standing Waves in Air Columns (Problem)

A tube closed at one end has a displacement node at the closed end.

L First Harmonic - Fundamental

141 λL

L4v

f 1

Third Harmonic

343 λL

L4v3

f 3

Fifth Harmonic

545 λL

L4v5

f 5

... 5, 3, 1,n L4

nvfn

Phy 2054 Lecture Notes 12

Standing Waves in Air Columns

Chapter 14 Sound

A pipe open at one end and closed at the other end. resonates with a fundamental frequency of 150 Hz. Which one of the following frequencies will not resonate in this pipe

(A) 300 Hz

(B) 450 Hz

(C) 760 Hz

(D) 1050 Hz

Standing Waves in Air Columns

Beats

Beat Frequency

21beat fff

Chapter 12 Sound

The interference of two sound waves can produces a beat frequency if the waves have

(A) the same amplitude.

(B) slightly different amplitudes.

(C) the same frequency.

(D) slightly different frequencies.

Chapter 12 Sound

Two pure tones are sounded together and a particular beat frequency is heard. If the frequency of one of the tones is decreased slightly, the beat frequency

(A) will increases.

(B) will decreases.

(C) does not change.

(D) could either increase or decrease.