# waves_sound1151

Urban and Civil

Nov 29, 2013 (4 years and 5 months ago)

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Wave

Transfers Energy Without
Transferring Matter

Wave

A wave can be described as a
disturbance that travels through a
medium from one location to
another location.

There are three types of waves:

Mechanical waves

require a material
medium to travel (air, water, ropes).

Electromagnetic waves

do not require a
medium to travel (light, radio).

Matter waves

are produced by electrons
and particles.

Mechanical Waves

Transverse waves

cause the medium to
move perpendicular to the direction of
the wave.

Longitudinal waves

cause the medium to
move parallel to the direction of the
wave.

Surface waves

are both transverse
waves and longitudinal waves mixed in
one medium. (Such as water waves)

Torsional waves

produce a twisting
motion through the medium

such as the
ones which caused the collapse of the
Tacoma Narrows Bridge.

Tacoma Narrows Bridge

Torsional Oscillation

Mechanical Universe Video

Transverse & Longitudinal
Waves

In a transverse wave, the particles of the
medium oscillate perpendicular to the
direction of wave travel.

In a longitudinal wave, the particles of the
medium oscillate along the direction of
wave travel.

3 Types of Mechanical
Waves

Wave Tutorial Links

http://library.thinkquest.org/10796/ch8/
ch8.htm

http://www.physicsclassroom.com/Class/w
aves/wavestoc.html

Longitudinal Tuning Fork Wave

Vibrating tines
produce an
alternating pattern
of high pressure and
low pressure regions.

This pattern travels
away from the fork.

Compression

high
pressure

Rarefaction

low
pressure

Period: T

The
PERIOD

of a
wave is the time
for a particle of
the medium to
complete one
oscillation.

The SI unit for
period is the
second.

Frequency: f

The
FREQUENCY

of a
wave is the number of
cycles per unit time.

The unit is Hertz (Hz)
which is a cycle per
second.

FREQUENCY

is also
the reciprocal of the
period.

1
f
T

1
T
f

Amplitude: A

The
AMPLITUDE

of a wave is the maximum
distance of a particle from the equilibrium
position.

The SI unit for amplitude is meter

Wavelength:
l (
lambda
)

The
WAVELENGTH
of
a wave is the length
of one complete
cycle.

It is the distance
between two
consecutive “in
phase” points.

In phase

points are
those that are
moving in step with
each other.

Wave Applets

Wavelength, Amplitude, Phase

Frequency, Wavelength, Speed

Longitudinal Wave

Transverse Wave

Superposition Principle 1

Superposition Principle 2

Wave Equation

The speed of a wave
is equal to the
product of the wave’s
frequency and
wavelength.

v: wave speed

f: frequency

l

: wavelength

v f
l

Speed of Wave on String

F
v
m
L

Sound Waves

The origin of any sound is a vibrating
object

Usually the frequency of the sound is the
same as that of the vibrating object

Frequency Range:

Sound: 20 Hz

20,000 Hz

Ultrasound: >20,000 Hz

Infrasound: < 20 Hz

Forced Vibration & Resonance

forced vibration

example
--

strike tuning
fork and hold the stem against the table

sounding board
--

used to amplify sound in
music boxes and all string

resonance
--

when the frequency of
forced vibrations matches the object's
natural frequency, a dramatic increase in
amplitude occurs

Speed of Sound

Aluminum

6420

Granite

6000

Steel

5960

Pyrex glass

5640

Copper

5010

Plastic

2680

Fresh water (20
º
C)

1482

Fresh water (0
º
C)

1402

Hydrogen (0
º
C)

1284

Helium (0
º
C)

965

Air (20
º
C)

343

Air (0
º
C)

331

Material

Speed (m/s)

Speed of Sound in Air

depends on wind conditions, temperature,
and humidity

does NOT depend on loudness or
frequency of the sound

all sounds travel at the same speed in the
same medium in dry air at 0
°
C ~ 331 m/s
(1200 km/h) (740 mi/h)

Sound travels faster through warm air
than cold air.

In air, v
sound

= 331.4 m/s + (0.6 m/s/C
o
)*T
C

Distance to Lightning

Light travels at
3 x 10
8
m/s in air

Sound travels at
about 330 m/s in
air at 0
o
C

It takes about 5 seconds for the sound (the
thunder) to travel 1 mile.

Count the seconds between the flash and the
sound, divide by 5, and you have the
approximate distance in miles to the
lightning.

Pitch & Loudness

Pitch

frequency

Double frequency

go up an octave

Loudness

amplitude

Units

W/m
2

*
Energy Power
Intensity
time area area
P
I
A
 

2
Energy Amplitude

Human Ear

Decibel Scale

incredibly sensitive

can hear everything
from fingertip brushing
lightly over fabric to a
loud jet engine

sound of jet engine is
about
10
12

times more
powerful than smallest
audible sound

a big difference!

decibel scale

--

smallest
audible sound is 0 dB

A sound 10 times more
powerful is 10 dB

A sound 100 times more
powerful than near total
silence is 20 dB

Sound Intensities

Loudest sound produced in laboratory

10
9

Saturn V

rocket at 50 m

10
8

Rupture of the eardrum

10
4

Jet engine at 50 m

10

Threshold of pain

1

Rock concert

10

1

Jackhammer at 1 m

10

3

Heavy street traffic

10

5

Conversation at 1 m

10

6

Classroom

10

7

Whisper at 1 m

10

10

Normal breathing

10

11

Threshold of hearing

10

12

(W/m
2
)

Intensity Level

Logarithmic Scale

Dimensionless

I
0

= 10
-
12

W/m
2

0
10log
I
I

 

 
 
Decibel Levels

Near total silence
-

0 dB

A whisper
-

15 dB

Normal conversation
-

60 dB

A lawnmower
-

90 dB

A car horn
-

110 dB

A rock concert or a jet engine
-

120 dB

A gunshot or firecracker
-

140 dB

Doppler Effect

Doppler Effect Lesson

Doppler Effect

Moving Source Moving Observer

General Expression

1
1
f f
u
v
 
 

 
 
(

1
u
f f
v

 
1
1
o
s
u
v
f f
u
v
 

 

 
 
 
Superposition Principle

Wave
interference

occurs when two or more
waves act simultaneously on a medium.

Whenever two or more waves pass through
each other, the resulting disturbance at a
given point in the medium may usually be
found by adding the individual displacements
that each wave would have caused. (
Principle
of Superposition
)

Constructive Interference

Constructive
interference occurs
when the waves are
trying to displace the
medium in the same
direction.

Destructive Interference

When these two waves are completely
overlapping, there will be complete
destructive interference.

Destructive interference occurs when the
waves are trying to displace the medium in
opposite directions.

Pulse/Wave Reflection

Fixed/Free End Reflection of Sine Wave

Free End Reflection

Fixed End Reflection

Interference between incident and
reflected pulse in a fixed end reflection

Standing Waves

For certain
frequencies, the
interference of the
incident and
reflected waves
results in a
standing wave
pattern.

Fundamental Frequency

and Harmonics

2
2
fundamental
fundamental
L
v
f
L
l

Standing Waves in a Tube

Closed on one end:

4
4
fundamental
fundamental
L
v
f
L
l

Open on both ends:

2
2
fundamental
fundamental
L
v
f
L
l

Waves Moving in and Out of Phase

When the 2 waves
are in phase, the
resulting
disturbance has a
maximum
amplitude.

When the 2 waves
are out of phase,
the resulting
disturbance has a
minimum amplitude.

Beats

Waves of slightly
different
frequencies form a
pattern of
alternating
maximum and
minimum amplitude.

The packets of
maximum amplitude
are called beats.

Noise Canceling

tiny microphones, one on each
earpiece, detect ambient noise
before it gets to your ears.

noise
-
cancellation circuitry
inverts the captured signal,
turning the noise's sound wave
upside down.

noise
-
cancellation system adds
the sonic opposite of the
external noise to whatever
you're listening to

eliminating most of the pollution
and leaving you with just your
music.

Standing Waves

http://phet.colorado.edu

Fundamental & Harmonics