Chapter 13 - sound - PowerPoint Presentation

Slide1

Chapter 13 - soundSlide2

13.1 – Sound Waves

Objectives

* Explain how sound waves are produced

* Relate frequency to pitch

* Compare the speed of sound in various media

* Relate plane waves to spherical waves

* Recognize the Doppler effect, and determine the direction

of a frequency shift when there is relative motion between a

s

ource and an observer.Slide3

Where sound waves come from

Sound starts with a vibrating object

When an object vibrates, it sets the air molecules near it in motion

As a vibrating object moves to the right, the molecules on the right forced

closer together. This becomes a region of higher molecular density and

pressure. This is an area of compression.

As the vibrating object moves to the left, the molecules in the region to the right now spread father apart, causing lower molecular density and pressure. This is an area of

rarefaction.Slide4

Sound waves are longitudinal

Longitudinal Waves and Particle Motion

What is the definition of a longitudinal wave?

A wave in which the direction of the vibrating particles

a

re parallel to the direction of wave travelSlide5

Frequency and pitch

Review: What is frequency?

Frequency is the number of cycles (or vibrations or oscillations, etc.)

per unit time. The units for cycles per second, or Hertz (Hz).

Audible

sound waves for the average human are between 20 and 20,000 Hz.

Infrasonic sound waves are those with frequencies below 20 Hz

Ultrasonic

sound waves are those with frequencies above 20,000 Hz Slide6

Frequency and pitch, cont.

Pitch

is how high or low we perceive a sound wave to be.

The higher the frequency of a sound wave, the higher pitch

we perceive it to be.

Frequency is an objective, measureable quantity, while pitch is simply a perception….Slide7

Sound waves and images

Sound waves with very short wavelengths (i.e., ultrasonic waves) can be used to create visible images. Sound waves are partially reflected when they reach a boundary between materials of two different densities. Ultrasonic waves, with their very short wavelengths, are easily reflected off small objects.Slide8

Sound waves and images

Another example of using sound waves to form images is called

echolocation (or sonar)

. Dolphins and bats can send out high frequency sound wave pulses that are reflected back. The reflected waves allow the dolphin or bat or to form an image of the object that caused the wave reflection.Slide9

Speed of sound

The speed of sound depends on the medium it is traveling through.

Sound can travel through solids, liquids or gases. Because sounds waves travel via particle vibration, the speed of sound depends on how fast a medium can transfer its motion from one particle to another.

So, which medium would you expect sound to travel

through faster….a solid or a gas?Slide10

Speed of sound, cont.

The speed of sound also is dependent on the temperature

o

f the medium, especially for gases. As air gets warmer, the air molecules move around more and collide more frequently.Therefore, sound vibrations moving from particle to particle

can happen faster in warmer air.

For liquids and solids, temperature does not make much of a difference because the molecules are so close together anyway.

V = 331 + 0.6(T) v is in m/s and T is in

o

CSlide11

How sound waves travel

We’ve seen pictures and animations of longitudinal waves, and they all appear to be one-dimensional. But sound waves propagate in 3 dimensions.

The areas of compression are called

w

avefronts

.

The distance between

c

onsecutive

wavefronts

is a wavelength.

The direction of wave travel is spherically

outward (shown by red arrows).Slide12

The Doppler effect

The pitch of the car horn

g

ets higher as the car

g

ets closer to us, and gets lower as the car gets

farther away….

But pitch is related to

f

requency, and the

f

requency of the car horn

i

sn’t changing, so how

d

oes this work?Slide13

The Doppler effect, cont.

Remember, pitch is the “perception” of frequency. The relative motion of the car makes this perception change. As the car approaches you, the

wavefronts

from the horn reach you more frequently because the source of the sound is moving toward you. As the source of the sound moves away from you, you perceive the pitch to be lower because the

wavefronts don’t reach you as frequently.Slide14

Doppler effect equation

f

o

= fs

(

)

f

o

= frequency the observer hears

v

o

= velocity of observer

v

s

= velocity of source

f

s

= normal frequency of the source sound (in air)

v

= normal speed of sound in air (343 m/s)Slide15

Using the Doppler equation

If the

OBSERVER

is stationary:Then vo

= 0fo

decreases as source goes away from observer.For fo to

decrease, the denominator on the right side has to increase.

To increase denominator, use

(v

+

v

s

).

f

o

increases

as source

gets closer to

observer

.

For f

o

to

increase,

the denominator on the right side has to

decrease

.

To

decrease

denominator, use (v

-

v

s

).

f

o

= f

s

(

)

 Slide16

Using the Doppler equation, cont.

If the

SOURCE

is stationary:Then vs

= 0fo

decreases as observer goes away from the source.For fo to

decrease, the numerator on the right side has to decrease.

To decrease numerator, use

(v

- v

o

).

f

o

increases

as observer

gets closer to the

source.

For f

o

to

increase

, the numerator on the right side has to

increase

.

To

increase

numerator, use (v

+

v

o

).

f

o

= f

s

(

)

 Slide17

Doppler effect example problem

A train with horn blaring passes a station going 50 m/s.

If the people standing on the platform at the station

hear the frequency as 384 Hz after the train passes, what is the frequency of the train horn?

Ans: 440 Hz

f

o = fs

(

)

 Slide18

13.2 – Sound intensity and resonance

O

bjectives

* Calculate the intensity of sound waves

* Relate intensity, decibel level, and perceived loudness

* Explain why resonance occursSlide19

Sound intensity

Intensity

is the rate of energy flow through a unit area

perpendicular to the direction of wave motion.

Intensity =

Because power, P, is defined as the rate of energy transfer,

w

e can also describe intensity in terms of power.

Intensity =

 Slide20

Sound intensity, cont.

Intensity =

Units for power are?

Units for area are?

So, units for intensity are?

Since sound propagates outward in all directions equally, the

area affected by the intensity is the surface area of a sphere (4

r

2

).

Intensity =

Where r is the distance

from the sound sourceSlide21

Sound intensity, cont.

So, the farther you get

a

way from the source of

a

sound, the less intensethe sound because theenergy of the sound iss

pread out over a largerarea.Slide22

Sound intensity, example

What is the intensity of sound waves produced by a

t

rumpet at a distance of 3.2m if the power output of the trumpet is 0.20W? Assume the sound waves arespherical.

Ans

: 1.6 x 10-3 W/m2Slide23

Intensity and frequencySlide24

Relative intensity – decibel level

Decibel level

– is the relative intensity of a sound, determined by relating the intensity of a sound wave to the intensity at the threshold of hearing.

Units are decibels (dB)Slide25

Intensity, decibels and loudness

For each

10 dB

increase in the decibel level of a sound, a sound will be approximately twice as

loud.

For each 10 dB increase in the decibel level of a sound,The

intensity level of the sound is multiplied by 10.Slide26

Intensity, decibels and loudness example

When the decibel level of traffic noise goes from

40 dB to 60 dB, how much louder does the traffic

seem? How much greater is the sound intensity?

Ans: 4 times as loud, intensity increases by a factor of 100Slide27

resonance

If the driving pendulum is

s

et in motion, all the other

p

endulums will be “forced”into motion as well. But only

one of them will oscillateat the same frequency asthe driving pendulum.

This is the pendulum with

t

he same “natural frequency”

as the driving pendulum.Slide28

Resonance, cont.

When the “forced vibration” matches the pendulum’s natural frequency, then the amplitude of the frequency will be much larger, and the system is in

resonance.Slide29

Resonance and self-destructionSlide30

Quick Review – 13.1 and 13.2

Pitch versus frequency – musical notes

Velocity of sound in air based on air temperature

Intensity / decibel / pain chart

Doppler effect example calculation

Intensity/decibel example calculationSlide31

Musical Notes

Pitch and FrequencySlide32

Temperature effect

Velocity of sound in air

v

= (331 + 0.6T)

You’ll need this formula for CH13 lab !!!!!

What is the velocity of sound in air at 21

oC (70oF)

What is the velocity of sound in air at 38

o

C (100

o

F)

343.6 m/s

353.8 m/sSlide33
Slide34

Doppler Effect Example

An ambulance races toward the scene of

an accident at 35 m/s with its siren blaring

at a frequency of 2000Hz. People in their cars pull over and stop as the ambulance approaches. At what frequency do they

hear the siren as the ambulance approaches them? At what frequency do they hear it after it passes? (Assume v = 343 m/s)

fo

as approaching: 2227 Hz

f

o

after passing: 1815 Hz

f

o

= f

s

(

)

 Slide35

Intensity / decibel / loudness

You’re sitting in the front row of a

Smashin

’ Pumpkins concert,decibel level 110dB. When you get home, your mom makes you listen to the music at a much lower level, 70dB.

How much less intense is the music at home than at the concert?

How much quieter does the music seem to you at home?

4 steps of 10dB, each a factor of 10, so 10

4

or10,000 times less intense

4 steps of 10dB, each half as loud, (½*½*½*½) or 1/16

th

as loudSlide36

13.3 Harmonics

Standing waves

1

st

harmonic (f

1) = “fundamental frequency”

2nd harmonic (f

2

) = 2 * f

1

3

rd

harmonic (f

3

) = 3 * f

1

n

th harmonic (f

n

) = n * f

1Slide37

Harmonics, cont.

For any fixed length (L), each

h

armonic represents ½ wavelength

So for the 4

th harmonic:L = 4 (½

)L = 2 = ½ L

Harmonics and WavelengthSlide38

Harmonics, cont.

Standing waves on a vibrating string:

f

n =

f

n

= frequency of the nth harmonic

n

= harmonic number

v

= velocity of the wave on the string

L = length of the vibrating stringSlide39

Waves on a string, example

A string on a toy guitar is 34.5cm long.

What is the wavelength of its first harmonic?

When the string is plucked, the speed of waves on the string is 410 m/s. What are the frequencies

of the first three harmonics?Slide40

Standing waves in an air columnSlide41

Standing waves in an air column

For pipes OPEN at both ends:

f

n

= (nv) / 2L

fn

= frequency of the nth harmonicn = harmonic numberv = velocity of sound in the pipeL = length of the vibrating air columnSlide42

Standing waves in an air column

For pipes CLOSED at one end:

f

n

= (nv) / 4L

fn

= frequency of the nth harmonicn = harmonic numberv = velocity of sound in the pipeL = length of the vibrating air columnSlide43
Slide44

Open pipe example

What are the first three harmonics in a 2.45m long open pipe?

Assume that the speed of sound trough the pipe is 345 m/s.Slide45

Standing waves

on a string

Standing waves

in an open pipeSlide46

Standing waves

in a pipe closed

at one end Slide47
Slide48

Lab instructions

DO NOT TAP/BANG THE TUNING FORKS

ON ANYTHING HARD!!!

You must share the tuning forks

Clean up your lab station when finished!Ok to leave the cylinder,

tube, tuning forks and mallet at the lab

table

HAND IN YOUR LAB REPORT BEFORE YOU LEAVE!!!