SoundSound Physics 102 Chapter Fourteen Amanda Hyer

SoundSoundSoundSound

Physics 102Chapter Fourteen

Amanda Hyer

20 Hz to 20,000 Hz infrasonic ultrasonic

Compressions rarefactions

Audible Frequency Range

Ultrasonic

infrasonic

20 kHz

20 Hz

Audible

Freq

uen

cy

Elastic MediaElastic MediaElastic MediaElastic Media Air Helium water rock steel

slow

fast

speed=wavelength x frequency

Speed of SoundSpeed of SoundSpeed of SoundSpeed of Sound Speed = 331 m/s + 0.6 Tc speed = 343 m/s at 20 °C

d=v x t ~ 1 km in 3 sec or 1 mile in 5 sec

Classical calculation of Sound SpeedClassical calculation of Sound Speed

v B

Bulk Modulus for Air

Bulk Modulus for Air

PV nRT

PV PV nRT

PV nRT PV

adiabatic W U

PV 5

2nRT

PV 7

2nRT _ _ And V

5

2nR

T

Pbut

P BV

VBV PV

B52

nRTP

72

nRT

B 1.4P

PV nRT M

MA

RT

P RTMA

PMA

RT

B 1.4P

PMA

RT

v B

B 1.4P

PMA

RT

v 1.4RT

MA

v 1.4RT

MA

1.4 8.314J / K / mole 273K

0.029kg / mole331.0(m / s)

ExerciseExerciseExerciseExercise How far away is a lightening bolt that is

heard 12.0 sec after the flash is seen? Given: t=12.0 sec and v =1/3 km / sec Wanted: d Solution: Use d= v t d = (1 / 3 km/sec) (12.0 sec)= 4 km

ReflectionReflectionReflectionReflection

RefractionRefractionRefractionRefractionThe bending of sound because the speed is different in adjacent regions

RefractionRefractionRefractionRefraction

sourcefar away

RefractionRefractionRefractionRefraction

sourcefar away warm

cool

Afternoon

RefractionRefractionRefractionRefraction

sourcefar away

warm

cool

Late Evening

Energy in soundEnergy in soundEnergy in soundEnergy in sound High frequencies dissipate faster than low.

loud tuning fork versus soft tuning fork.

ResonanceResonanceResonanceResonance system that has a natural frequency like a

tight string or a tube (flute)

driven by a tiny oscillating force

Resonce occurs when the frequency of the force matches the natural frequency of the system.

InterferenceInterference When two waves overlap they form an interference pattern

which can be either a constructive interference or a destructive interference.

+ =

Constructive

+ =

Destructive

Reinforcement Cancellation

InterferenceInterferenceInterferenceInterferenceConstructive (in Phase)

Destructive (180 ° out of phase)

-2

0

2

0 20

-2

0

2

0 20

-2

0

2

0 20

InterferenceInterferenceInterferenceInterferenceDestructive (180 ° out of phase)

-2

0

2

0 20

-2

0

2

0 20

-2

0

2

0 20

BeatsBeatsBeatsBeats When two different frequencies merge beats are heard

The Beat frequency is the difference between the two frequencies.

BeatsBeatsBeatsBeats

-4

-2

0

2

4

0 10

ExerciseExerciseExerciseExercise What is the beat frequency when a sound of

256 Hz meets a sound of 260 Hz Given: F1=256 Hz and F2= 260 Hz Wanted: F beats Solution: Use F Beats = Abs(F1 - F2) F Beats = Abs(F1 - F2) = 4 Hz

ResonanceResonanceResonanceResonance system that has a natural frequency like a

tight string or a tube (flute)

driven by a tiny oscillating force

Resonce occurs when the frequency of the force matches the natural frequency of the system.

Standing Waves on a Stringf

v

f1 v1

v

2L

f2 v2

vL

2f1

f3 v3

v

23L

3f1

Open Organ pipeOpen Organ pipe

f v

f1 v1

v

2L

f2 v2

vL

2f1

f3 v3

v

23L

3f1

Organ Pipe Closed at one endf

v

f1 v1

v

4L

f3 v3

v4

3 L3f1

f5 v3

v4

5 L5f1

Am and Fm broadcastingAm and Fm broadcasting

AM Amplitude Modulation carrier wave 1040 1040 kHz

FM Frequency Modulation 101 101 M Hz

Am and Fm broadcastingAm and Fm broadcasting

Radio waves are electromagnetic waves Their speed is 300,000,000 m/s They travel through outer space

Sound is an acoustical wave The speed of sound is 340 m/s They do not travel through outer space

Am and Fm broadcastingAm and Fm broadcasting

Radio waves are electromagnetic waves Their speed is 300,000,000 m/s They travel through outer space

Sound is an acoustical wave The speed of sound is 340 m/s They do not travel through outer space

Carrier FrequencyCarrier Frequency

-4

-2

0

2

4

0

AM broadcastingAM broadcasting

-4

-2

0

2

4

0

Fm broadcastingFm broadcasting

-4

-2

0

2

4

0