# Sound and Music

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```					Ch. 17 – Sound   1
2

• Sound waves
(like all
mechanical
waves) need a
material to move
through

• Therefore, there
is no sound in
space
Sound waves are longitudinal            3

The vibration travels when
molecules bump into
neighboring molecules
Sound waves are longitudinal          4

Compressions      Rarefactions
are regions of   are regions of
high pressure    low pressure
Converting the longitudinal wave   5

into an analogous transverse wave
Frequency is                           6

perceived as pitch
High frequency   high pitch   Equal temperament
scale (Hz)
C4       262
D        297
E       330
F        349
G        392
A        440
B        494
C5       524
Range of human hearing              7

• We can only hear
sounds between
20 Hz and 20,000 Hz

• Vibrations below 20 Hz
are called infrasonic

• Above 20,000 Hz
are ultrasonic
Dog whistles
are ultrasonic
Audible frequencies            8

From Kinetic Books,
section 17.2
Sound wave speed                                       9

depends on
1. Material
• Solid  Liquid  Gas
• (fast to slow)                the disturbance (bump)
travels faster if neighboring
molecule is close to you
(even better if bonded)
2. Temperature
• hotter         faster

T
v = (331 m/s) 1 +          Eqn. 4 – only good
273    for waves IN AIR
Speed of sound in various media      10

From Kinetic Books,
section 17.4
Various speeds of sound                        11

speed of sound in various
materials, at 24ºC (room temp)
Air (0ºC)       331 m/s    740 mph
Air             345        772
helium         1000      2300
water          1500      3200
seawater       1600      3500
steel          5000     11,300

1
772 mph = mile/sec
5
5 second rule for
thunder and lightning!
Sound Intensity              12

Sound intensity: The sound power
per unit area
Power    Watts
Intensity =               
Area    m 
2
13

• As a sound wave travels, it typically spreads out.
You perceive the loudness of the sound of a
loudspeaker at an outdoor concert differently at a
distance of one meter than you do at 100 meters
• The intensity of the sound diminishes with
distance
Sound Intensity v. Loudness                                14

Sound Intensity is objective
(a physical characteristic of sound that can
be measured in Watts/m2)

Sound Level (loudness) is subjective
(a physiological sensation that depends on
two things, but is measured in decibels)
a) observer
Not everyone will agree that a 80 dB sound is “loud”

b) frequency
A 3500-Hz sound at 80 dB sounds about twice as
loud to most people as a 125-Hz sound at 80 dB;
humans are more sensitive to the 3500-Hz
frequencies
Sound Level, b                                         15

Sound level: A scale for measuring
the perceived intensity
of sounds
I   where I = intensity
b = 10 log           I o = reference level intensity = 1 1012   W
Io                                                     m2

Recall: log (a/b) = log (a) – log (b)
Loudness measured in                                    16

decibels
1. decibels
• “deci” (intensity is scaled by factors of 10)
• “bel” (in honor of Alexander Graham Bell)

2. A sound level of 0 dB is the lowest threshold of hearing

•10 dB = 10 times more intense than 0 dB
•20 dB = 100 times more intense
•30 dB = 1,000 times more intense
•60 db = 1,000,000 times more intense
How loud is “loud”?                              17

Source of sound         Sound level (dB)
Nearby jet airplane           140
Jackhammer                    130
Rock concert or siren         120
Vacuum cleaner                 70
Normal conversation            50
Mosquito buzzing               40
Whisper                        30
Rustle of leaves               10
Threshold of hearing            0

10,000,000 times
more power per m2

50 dB                                   120 dB
the Doppler effect                       18

• The Doppler effect is the perceived change in
frequency due to motion of the source and/or the
listener (why do we emphasize “perceived”?
• It is NOT the change in volume
19
It’s a PERCEIVED                          20

change in pitch
• If a source of sound is moving toward you, you
hear a higher frequency than when it is at rest
• If a source of sound is moving away from you,
you hear a lower frequency than when it is at rest
21

• This effect is named
for the Austrian
physicist who first
analyzed it,
Christian Doppler
(1803-1853)
• Doppler’s research
concerned light
from stars, but his
principles apply to
sound also
Doppler effect → radar guns                      22

• When radar waves bounce off a moving object,
the FREQUENCY of the reflected radar
changes by an amount that depends on how
fast the object is moving
• The detector senses the amount of frequency
shift and translates this into a speed
23
Once you see the cop,
he’s already got you!
• This is now done
with lasers (light
waves) which are
extremely accurate
• They really can tell if
you’ve been going
just a couple of
m.p.h. over the limit!
Doppler effect                                                  24

Moving Source                   Moving Listener

   v                                    v  v Listener 
f Listener   = f Source 
vv

   f Listener    = f Source                 
    Source                                    v        
… source moving away from listener       … listener moving toward source
… source moving towards listener         … listener moving away from source

If both listener and                                 v  v Listener   
f Listener   = f Source 
 vv


source are moving:                                          Source    
Doppler problem 1                           25

QUESTION
The 16,000-Hz whine of
the jet engines of an
aircraft moving with
speed 200 m/s is heard
at what frequency by the
pilot of a second craft
trying to overtake the first at a speed of 250 m/s?
Take the speed of sound to be 320 m/s.
 320 m/s + 250 m/s 
f=                   16,000 Hz = 17,538.46 Hz
 320 m/s + 200 m/s 
When an object is                        26

at the speed of sound

• It travels as fast as its own sound waves
• The sound waves pile up on one
another in front of the moving object
27

Bell X-1

• The first man to break the “sound barrier” in
an aircraft was Chuck Yeager
• October 14, 1947
• Read the book or see this story in the movie
“The Right Stuff”
28

Bow wave = V-shaped disturbance made when
object moves faster than wave speed

Shock wave = cone-shaped disturbance made
when object moves faster than speed of sound
Object exceeds the                       29

speed of its sound waves

• Sound waves form a Mach cone
• Surface of the cone is the shock wave
• Angle of cone narrows at greater speeds
30
Does object have to make 31
sound to break sound barrier?
• NO. Once an object
moves faster than the
speed of sound, it will
make sound (“sonic
boom”)
• Examples:
• the crack of a whip
or a snapped towel
• Fired bullet
32
Misconception
• The sonic boom is not produced only when the
object exceeds the speed of sound
• The shock wave (and resulting cracking sound)
sweeps continuously behind object

“Wow! A
“What was that?!”    sonic boom!”
“Huh?”
Exploration                              33

1. How does the human vocal system produce
such amazing sounds?
• Feel your throat while making the following
sounds
• make soft sounds, loud sounds
• sing a high note, a low note
• say aaah, say eeeee

Value of f1 depends on v (speed of wave on
string or in tube) and l (length of string or tube)

```
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 views: 33 posted: 7/27/2012 language: pages: 33