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Chapter 26
Sound
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Origin of Sound
 All
sounds are produced by the
vibrations of material objects
 For example in a piano, violin, or
guitar a sound wave is produced by
vibrating strings
Origin of Sound
Sound is made when something
vibrates.
The vibration disturbs
the air around it.
 This makes changes in
air pressure.
 These changes in air
pressure move through
the air as sound
waves.

Sound Waves
-
-
-
Alternating areas of high & low
pressure in the air
ALL sound is carried through matter as
sound waves
Sound waves move out in ALL
directions from a vibrating object
Nature of Waves
(Def.) – A wave is a disturbance
that transfers energy.
 Waves
– Substance or region through
which a wave is transmitted.
 Medium
of Waves – Depends on the
properties of the medium.
 Speed
Properties of Waves
pressure
 or T




horizontal axis could be:
space: representing
snapshot in time
time: representing
sequence at a particular point in space
Wavelength () is measured from crest-to-crest
– or trough-to-trough, or upswing to upswing, etc.
For traveling waves (sound, light, water), there is a speed (c)
Frequency (f) refers to how many cycles pass by per second
– measured in Hertz, or Hz: cycles per second
– associated with this is period: T = 1/f
These three are closely related:
f = c
Longitudinal vs. Transverse
Waves
Sound is a longitudinal wave, meaning that
the motion of particles is along the direction
of propagation
 Transverse waves—water waves, light—
have things moving perpendicular to the
direction of propagation

Why is Sound Longitudinal?



Waves in air can’t really be transverse, because the
atoms/molecules are not bound to each other
– can’t pull a (momentarily) neighboring molecule sideways
– only if a “rubber band” connected the molecules would
this work
– fancy way of saying this: gases can’t support shear loads
Air molecules can really only bump into one another
Imagine people in a crowded train station with hands in
pockets
– pushing into crowd would send a wave of compression
into the crowd in the direction of push (longitudinal)
– jerking people back and forth (sideways, over several
meters) would not propagate into the crowd
– but if everyone held hands (bonds), this transverse motion
would propagate into crowd
Questions
A wave in which the particles of the medium move at right
angles to the direction of the wave is a __________.
Transverse wave
High pitch is produced by sounds that have__________.
High frequency
Speed of Sound


Sound speed in air is related to the frantic motions
of molecules as they jostle and collide
– since air has a lot of empty space, the
communication that a wave is coming through
has to be carried by the motion of particles
– for air, this motion is about 500 m/s, but only
about 350 m/s directed in any particular
direction
Solids have faster sound speeds because atoms are
hooked up by “springs” (bonds)
– don’t have to rely on atoms to traverse gap
– spring compression can (and does) travel faster
than actual atom motion
Examples of Sound Speeds
Medium
air (20C)
water
gold
brick
wood
glass
steel
aluminum
sound speed (m/s)
343
1497
3240
3650
3800–4600
5100
5790
6420
Components of Sound
 Pitch
(how high or low)
 Loudness
 Timbre
(volume)
(tone color)
Vibration
-
-
Back and forth movement of molecules of
matter
For example,
Does high density material tend to cause sound
to go faster or slower?
Is low temperature cause sound to go faster or
slower?
Is highly elastic material cause sound to go faster
or slower?
The Physics of Sound
Pitch
…
is the "highness" or "lowness" of a tone.
 Pitch
corresponds to frequency.
Pitch




The vibration patterns of
some sounds are repetitive.
Vibration patterns are also
called waveforms.
Each repetition of a waveform
is called a cycle.
We can hear frequencies
between 20 hertz or cycles
(vibrations) per second (low
pitches)to 20 kilohertz, i.e.
20,000 Hz (high pitches).
Pitch Continued
Infrasonic
frequencies < 20 Hz
Ultrasonic
frequencies > 20,000 Hz
Human hearing range
frequencies between 20 Hz and 20,000
Hz



When the frequency of
a sound doubles we
say that the pitch goes
up an octave.
We can hear a range of
pitches of about ten
octaves.
Many animals can
make sounds and hear
frequencies that are
beyond what we can
hear.
Could you hear a pitch of 19 Hz?
Sound in Air
...a longitudinal
wave in air caused
by a vibrating
object.
 Demo: slinky

The Ear
Sound is carried to our ears through vibrating
air molecules.
 Our ears take in sound waves & turn them into
signals that go to our brains.
 Sound waves move through 3 parts of the ear;
outer ear, middle ear, & inner ear.

Middle Ear
The Ear
1. The outer ear gathers sound waves,
passing them through the ear canal to a
tough membrane called the eardrum
2. The vibrating eardrum passes the sound to
three tiny bones in the middle ear – the
hammer, anvil, and stirrup – which
amplify the sound wave
The Ear
3. The stirrup vibrates and transfers the
sound to a membrane in the oval window,
then on to the inner ear’s cochlea, a spiralshaped structure that contains hair cells
4. As the hair cells in the cochlea vibrate,
nerve impulses are sent through the
auditory nerve to the brain
The Ear
The sound waves
cause pressure
changes against
our ear drum
sending nerve
impulses to our
brain.
Speakers: Inverse Eardrums
 Speakers
vibrate and push on the air
– pushing out creates compression
– pulling back creates rarefaction
 Speaker must execute complex motion
according to desired waveform
Question
What type of wave would moving a rope
attached to a wall simulate?
Sound in Air
Continued
 Sound
requires a medium.
– solid, liquid or gas
– Demo: Bell in a evacuated Bell Jar
 Sound
waves have compression and
rarefaction regions.
Noise Measuring Equipment
Sound Level Meters (SLM)
Continuous on-mobile sources
Noise Dosimeters
Mobile/variable
noise sources
Questions
Would sound travel faster on a train track or in
a pool?
Compression
-
Where molecules are being pressed together as
the sound waves move through matter
For example,
- a wave travels through the springs just like sound
waves travel through the air
- the places where the springs are close together are
like compressions in the air.
 The
Rarefaction
change in
direction of a
wave as it
crossed the
boundary
between two
media in which
the wave travels
at different
speeds
Questions
Refraction occurs because waves__________.
Move at different speeds in different mediums
In rarefaction, the molecules of the medium are _____.
Spaced apart
The Physics of Sound
The Physics of Sound
Are the air molecules more spread
out in compression or
rarefraction?
Rarefraction
Media that Transmits Sound
 Most
sounds you hear
are transmitted through
air.
 Solids and liquids are
generally good
conductor of sound—
much better than air
 Sounds cannot travel
through a vacuum
Speed of Sound in Air
 Speed
of sound = 340 meters/second or
760 miles/hour
 Sound travels faster in hot, humid
climates
 Speed of sound depends on elasticity
Speed of Sound in Air
So at higher temperatures the speed of sound is
faster because of the faster moving molecules. At 0
degrees the speed in air is 331.5 m/s. This speed
increases with the temperature at about 0.6 m/s
per degree Celsius.

Speed of sound =
(331.5 + 0.6T) m/s

T = temperature in
Celsius
Speed of Sound and Refraction
Sound travels faster in warm air. Why?
Late one summer night ( T = 20
degrees Celsius), lightning is seen from
an approaching storm and five seconds
later, thunder sound is heard. How far
away is the storm?
Speed of sound = ((331.5) + 0.6(20))m/s
Speed of sound = 343.5 m/s
V = d/t
343.5 m/s = d / 5s
D = 1717.5 m
Wavelength & Frequency
-
Wavelength is the distance between one part
of a wave and the same part of the next
wave
-
Frequency is the number of waves moving
past a point in one second
Can you predict how far a storm
is if you here thunder 8 seconds
away?
340m/s x 8 s = 2720 m
Sound Intensity and
Loudness
Intensity of Sound refers to
the amplitude of the
pressure variations in the
sound wave
Loudness
The
physiological sensation
directly related to the sound
intensity
Measured in bels
(10 bels = 1 decibels)
•As the sound spreads out from its source, the
concentration of power becomes less.


As the distance
from the source
increases the
amount of power is
spread over a
greater area.
The amount of
power per square
meter is called the
intensity of the
sound.
Loudness



To create vibrations
energy is used.
The greater amount of
energy used the louder
the sound.
The strength of the
changes in air pressure
made by the vibrating
object determines
loudness.
Loudness
 A sound
of 10
decibels is or 101 or
10 times as intense
as 0 decibels.
 20 decibels is 102 or
100 times the
intensity 0 decibels.
Source of Sound
Loudness (db)
Threshold of Hearing
0
Conversation
60
Ear Damage Begins
85
Amplified Music
110
Jet Airplane at 30 meters
140
Common Sound Intensities
2
Source of Sound
Intensity (W/m )
Jet 30 m away
10
Air-raid siren, nearby
1
Disco music, amplified
10
-1
110
Riveter
10
-3
90
Busy street traffic
10
-5
70
Conversation in home
10
-6
60
Quiet radio in home
10
-8
40
Whisper
10
-10
20
Rustle of leaves
10
-11
10
Threshold of hearing
10
-12
0
2
Sound Level (db)
140
120
Hearing Protection Devices and Their
Noise Reduction Ratings
EAR Foam Plugs (NRR = 29 dB)
EAR CARBOFLEX (NRR= 20 dB)
Moldex PURAFIT Foam Plugs
(NRR=30 dB)
The human perception of intensity is known as _____.
Loudness
Sound waves are _______________ waves.
Compressional
The speed of sound depends on?
Temperature
Density
Elasticity
Reflection of a wave occurs when the wave______.
Strikes a boundary and bounces back
The pitch of a sound depends on its__________.
Frequency
Humans do not perceive sound intensity linearly.


For us to perceive a sound
as twice as loud its
intensity must be ten
times greater.
The perceived intensity
level of sound is
measured in a logarithmic
scale using a unit called
the decibel
(dB)
2
From the perspective of the logarithmic scale
the threshold of pain is
1,000,000,000,000
times as great as the threshold of
hearing.
What is the loudest setting
should set their ipod at to
maintain a healthy ear?
Timber
is the specific
property of
sound that
enables us to
determine the
difference
between a
piano and a
harp.
An extremely broad variety of
tone colors exist because most
sounds that we perceive as
pitch actually contain many
frequencies.
The predominant pitch is
called the fundamental
frequency.
Although we would perceive a string vibrating
as a whole,
it actually vibrates in a pattern that at first appears to be
erratic producing many different overtone pitches.
What results are particular tone colors or timbres of
instruments and voices.
The other frequencies which occur in a
mathematical series are called the harmonic
or overtone series.
When C1 is the fundamental the following
pitches represent its first fifteen successive
overtones.
Forced Vibrations
 …the
setting up of vibrations in an
object by a vibrating force.
 Examples of Forced Vibration:
– A tuning fork touching a wood
surface
– Sounding boards for stringed
instruments
– Matching tuning fork boxes
Natural Frequency
 …the
frequency at which an elastic
object naturally tends to vibrate.
 At this frequency, a minimum energy is
required to produce a forced vibration.
 The natural frequency of a body
depends on its elasticity and its shape.
What factor besides shape
affects natural frequency?
Natural Frequency
Examples
 Dropping Aluminum
Rods
 Ringing Small and Large Bells
 Xylophone
 Rubbing a Wine Glass
 Mass on a Spring
38. What is the speed of a wave if the frequency is 300 Hz
and the wavelength is 150m?
S = λf
S = (150)(300) = 45000 m/s
39. What is the wavelength of a 350 Hz wave traveling at
500m/s?
S = λf
λ = 1.4 meters
40. How many cycles per second would be characteristic of a
30 m wave traveling at 300 m/s?
S = λf
10 cycles/sec (Hz)
All Shapes of Waveforms



Different Instruments have
different waveforms
– a: glockenspiel
– b: soft piano
– c: loud piano
– d: trumpet
Our ears are sensitive to the
detailed shape of
waveforms!
More waveforms:
– e: french horn
– f: clarinet
– g: violin
Section 3 – Music
A.Music – sounds that are deliberately used
in a regular pattern
1. Natural frequency – frequency at which
the material vibrates
2. Resonance – the ability of a medium to
vibrate by absorbing energy at its own
natural frequency
Resonance
 …is
the result of forced vibrations in a
body when the applied frequency
matches the natural frequency of the
body.
 The resulting vibration has a high
amplitude and can destroy the body
that is vibrating.
Examples of Resonance
 breaking
a wine glass using sound
 mass on a spring at resonance
 a singing rod caused by forced
vibration
 a tuning fork exciting a guitar string
 In 1940, the Tacoma Narrows Bridge
was destroyed by wind-generated
resonance.
1. Frequency times wavelength equals_________.
2. What is the number of complete wave cycles per unit time?
3. A ____________ is a region in the medium in which
molecules are crowded together.
4. The ability of an object to vibrate by absorbing energy at its
natural frequency is called _________.
Sound Interference
 Overlapping
crests of a wave will
result in an increased amplitude.
 Overlapping
a crest and a trough
results in a decrease in amplitude.
Beats
 Beats
- the periodic variation in
loudness of two sounds played together
 The
beat frequency is equal to the
difference in the frequency of the two
sounds.
Acoustics...
 ...the
study of sound properties.
 When
a sound wave strikes a surface it
can be.…
(a) reflected.
(b) transmitted.
(c) absorbed.
(d) all of these.
Reflection of Sound

e.g. an echo

Reverberation - re-echoed sound, multiple
reflections of sound waves from walls

Compare reflections from a hard wall with
that from a carpet wall.
Demo: Whip
Refraction of Sound

Refraction - the bending of a wave

Sound waves bend toward cooler air.
Desert and Lake Example
Radio Broadcasts

AM - Amplitude Modulation
» 535 kHz to 1605 kHz

FM - Frequency Modulation
» 88 MHz to 108 MHz

Modulation - an impression of the sound
wave on a higher frequency radio waves
Modulating Radio Waves
© 2000 Microsoft Clip Gallery

Modulation - variation of amplitude or
frequency when waves are broadcast
– AM – amplitude modulation
» Carries audio for T.V. Broadcasts
» Longer wavelength so can bend around hills
– FM – frequency modulation
» Carries video for T.V. Broadcasts
Sonar
-
-
An instrument that uses reflected sound
waves to find underwater objects
For example,
Humans use sonar
to locate or map
objects
Animals use sonar or echo location to find their
prey; these sounds have such a high pitch or
frequency that the human ear cannot hear
Bibliography
http://science.pppst.com/sound.html
 http://www.tzemach.org/music/powerpoint/physics_of_sound.ppt
 http://www.physics.ucsd.edu/~tmurphy/phys8/lectures/10_sound.ppt
 http://cmsweb2.loudoun.k12.va.us/539208161203730/lib/539208161203730/S
ound.ppt
 http://lessons.ctaponline.org/~dpower/waves.ppt
 http://www.greenville.k12.sc.us/eastside/simmons/docs/ps/Module%207.ppt
 http://www.purdue.edu/physicalfacilities/safety/presentations/NoiseExposure/
Noise.ppt
 http://www.st-and.demon.co.uk/AudioMisc/asymmetry/asym.htm
 schools.birdvilleschools.net/178720814112933850/lib/.../Chapter_12.ppt
 kisdwebs.katyisd.org/.../Waves%20and%20sound%20powerpoint.ppt
http://hypertextbook.com/physics/waves/sound/
