Sound

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Sound

• The
• Production
• Transmission
• Detection
• Reflection
• Measurement
• Pitch, Loudness Quality

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Why do we study this topic on sound?

• Sound is experienced in every moment of our
  lives. It can come in the form of music, noise or
  communication.
• Knowledge of sound is also useful in real-life
  applications.
• E.g. Estimating the depth of the sea

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Lesson 1

• Lesson Objectives (Production, Transmission)
• To be able to describe how sound is produce
• To understand the nature of sound

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Nature of Sound

• Activity 1
• Ask a friend to hold a piece of paper 10cm in
  front of your face.
• Focus your voice on the paper, and try talking
  into it.
• Slowly increase the loudness.
• (Try putting your hand against your throat and
  feel too)
• What did you notice?
• What did you and your friend feel?

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Nature of Sound

• Activity 2 (Try this at home)
• Similarly, you may stick a piece of paper in
  front of your Hi-Fi woofer.
• Watch what happens to the paper when you turn up
  the volume.
• (Watch the paper as it comes to music of heavy
  bass)

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Nature of Sound

• Did you notice this?
• The paper seems to be vibrating
• What can you gather?
• Sound is a form of energy
• Originates from a source of vibration or
  oscillation

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Other uses of sound? Hmm
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Nature of Sound
RECALL

• What is sound?
• Sound is a form of energy that is passed from one
  point to another as a wave
• Produced by vibrating sources placed in a medium
  (air, liquid or solid)
• Longitudinal wave (Eg. Tuning Fork)

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Nature of Sound
RECALL

• For sound waves in air, compressions and
  rarefactions can be thought of as changes in air
  pressure
• Other terms used to describe a longitudinal wave
  includes amplitude, frequency, period,
  wavelength, wave Speed.

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What do you think

• We know that light waves (from the sun) are able
  to travel through space (vacuum) to reach earth.
• But can sound waves travel through vacuum too?
• Given 3 medium (namely Air, Water and Iron),
  which do you think sound travels
• fastest in?
• and slowest in?
• How else can we vary the speed of sound in air?

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Transmission of Sound

• The Bell-jar Experiment
• We were able to hear the bell ring loud and
  clear.
• But when the vacuum pump is turned on, the sound
  becomes fainter and fainter.

Vacuum pump

• What does that mean?
• Sound requires a medium to be transmitted.

Bell Jar
Alarm clock or Bell
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Transmission of Sound
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Transmission of Sound

• Given 3 medium (namely Air, Water and Iron),
  which do you think sound travels
• fastest in?
• and slowest in?
• In general, molecules in a solid are packed much
  closer together than those in a liquid or gas,
  allowing sound waves to travel faster through it.

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Speed of Sound

• Some Interesting Facts
• Sound waves travel about 5 times faster in fresh
  water and 17 times faster in iron than in air!
• Not all speed of sound in solids are faster than
  in liquids.

Gases Gases Liquids (25oC) Liquids (25oC) Solid Solid
Material v (m/s) Material v (m/s) Material v (m/s)
Hydrogen (0oC) 1286 Glycerol 1904 Diamond 12000
Helium (0oC) 972 Sea Water 1533 Pyrex Glass 5640
Air (20oC) 343 Water 1493 Iron 5130
Air (0oC) 331 Mercury 1450 Aluminium 5100
Kerosene 1324 Brass 4700
Methyl Alcohol 1143 Copper 3560
Carbon Tetrachloride 926 Gold 3240
Carbon Tetrachloride 926 Lucite 2680
Lead 1322
Rubber 1600
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Transmission of Sound

• How else can we vary the speed of sound in air?
• By varying the temperature.
• The table shows how the speed of sound varies in
  different temperature.
• The speed of sound is proportional to

Temperature in oC Speed of Sound in ms-1
-10 325.4
-5 328.5
0 331.5
5 334.5
10 337.5
15 340.5
20 343.4
25 346.3
30 349.2
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Transmission of Sound

• How else can we vary the speed of sound in air?
• By varying the humidity of air.
• Humidity is a measure of the amount of water
  vapor in the air the more humid the air is, the
  faster sound waves travel in it.
• However, pressure does not change the speed of
  sound in air.

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Questions?

• Self Check
• Is sound a form of energy?
• Why do sound travel faster in solids, as compared
  air?
• Which of these (Temperature, humidity or
  pressure) affects the speed of sound in air?
• Do you think sound travels faster or slower in
  places of higher altitudes?

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Lesson 2

• Lesson Objective (Detection)
• What can we use to determine the frequency of a
  sound?
• To identify the spectrum of sound frequencies
• Ultrasound
• Audible range
• Infrasound
• Uses of Ultrasound
• (Please read through Handout on How Ultrasound
  Works)

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Do you know

• How gynaecologist are able to tell the gender of
  a unborn baby?

• Why there are some whistles which we cannot
  hear?Eg. Dog whistle

• That commercial electronic pest repeller make use
  of high frequency sounds to chase pests away?

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Using the Cathode Ray Oscilloscope

• Previously, we briefly talked about how to
  convert a longitudinal wave into a transverse
  wave
• Today, we shall look at one other apparatus that
  can do thatthe Cathode Ray Oscilloscope (CRO)

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What you will see on the CRO

• The input sound is first captured (eg. by a
  Microphone), and converted into digital signals
  based on the loudness of the sound (amplitude),
  the pitch (frequency) and the quality.
• In addition, since the waveform is plotted
  against time, we will be able to determine the
  period and frequency of the sound wave.

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Sound Spectrum

• For electromagnetic waves, we have the
  Electromagnetic Spectrum.
• For sound, we have the Spectrum of Sound
  frequencies.

Frequencies (Hz) Also Known As Remarks
1-20 Infrasound Not heard by human ears but can be felt as vibrations
20-20000 Audible Frequencies The range of frequencies is also known as range of audibility.
20000 above Ultrasound Mainly used in medical diagnosis.
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Medical Diagnosis?

• Obstetrics and Gynecology
• measuring the size of the fetus to determine the
  due date
• determining the position of the fetus to see if
  it is in the normal head down position or breech
• checking the position of the placenta to see if
  it is improperly developing over the opening to
  the uterus (cervix)
• seeing the number of fetuses in the uterus
• checking the sex of the baby (if the genital area
  can be clearly seen)
• checking the fetus's growth rate by making many
  measurements over time
• detecting ectopic pregnancy, the life-threatening
  situation in which the baby is implanted in the
  mother's Fallopian tubes instead of in the uterus
  
• determining whether there is an appropriate
  amount of amniotic fluid cushioning the baby
• monitoring the baby during specialized procedures
  - ultrasound has been helpful in seeing and
  avoiding the baby during amniocentesis (sampling
  of the amniotic fluid with a needle for genetic
  testing). Years ago, doctors use to perform this
  procedure blindly however, with accompanying use
  of ultrasound, the risks of this procedure have
  dropped dramatically.
• seeing tumors of the ovary and breast
• Cardiology
• seeing the inside of the heart to identify
  abnormal structures or functions
• measuring blood flow through the heart and major
  blood vessels
• Urology
• measuring blood flow through the kidney
• seeing kidney stones
• detecting prostate cancer early

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Something Extra

• For the past few years, people who are learning
  the art of relaxation and the art of yoga is
  rapidly increasing.
• In practising the art of yoga, there is one
  particular skill that using the word, or rather
  the sound ohm as the main key.
• But what is so significant about this sound?
• Try making the sound on your own, and elaborate
  how was the feeling after a while.

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Lunch Time Entertainment
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Questions?

• Self Check
• Is sound wave also a radio wave?
• What can we use to determine the frequency of a
  sound wave?
• What range of frequencies can the human ear hear?
• Which range of frequencies do you think the dog
  whistle operates in?
• Which area is ultrasound commonly used in?

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Lesson 3

• Lesson Objective (Reflection, Measurement)
• Understand how echo is produced
• To measure the speed of sound
• Indirect Method Using Echo
• Direct Method Without using Echo
• Other uses of echo

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Do you know

• Why ships are able to tell the depth of the ocean
  water before deciding to anchor?
• (Sonar)

• What reverb stands for? (We usually see this
  word, when we configure our PC audio card for
  games or applications.)

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What is echo?

• What is echo?
• It is the sound heard after the reflection of
  sound from a hard, flat surface.

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What happens with multiple echoes?

• Note the rays (path) taken by the sound wave

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Multiple Echoes

• Typically in an enclosed hall/gymnasium,
• There will be multiple reflections of sound from
  the many reflecting surfaces such as the ceilings
  and the walls
• The echoes generated creates an impression that a
  sound lasts for a long time
• This effect of prolonged sound due to the merging
  of many echoes is called reverberation.

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Measuring the Speed of Sound

• Direct method
• Students A and B are positioned at a known
  distance, s apart.
• Student A fires a starting pistol
• Student B, upon seeing the flash of the starting
  pistol, starts the stopwatch and stops when he
  hears the sound. The time interval is then
  recorded.

Is there any way we can improve this part of the
experiment?
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Measuring the Speed of Sound

• Direct method Calculation

• As speed (v) is defined as distance travelled per
  unit time
• Divide the distance s by the time interval
  recorded by the stop watch.
• ie. Speed, v s / t
• Repeat the experiment a few times, and take the
  average of the calculated speeds.

Note The Direct method measure the speed of
sound the same way you measure the speed of a
runner, with a stopwatch on a closed track.
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Measuring the Speed of Sound

• Indirect method (Using the echo)
• Place the 2 wooden blocks at a known distance, s
  away from the wall
• Make a sharp clapping sound by knocking the 2
  blocks of wood together
• Repeat the sound at regular intervals to coincide
  with the echoes
• Start from zero clap and start the stopwatch.
  Stop the stop watch at, say, 50 claps

Is there any way we can improve this part of the
experiment?
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Measuring the Speed of Sound

• Indirect method (Using the echo)

Large wall
Direction of incident sound
Direction of reflected sound (echo)
Distance, s
2 wooden blocks

• Find the average time for 1 clap (t)
• The speed of sound in air can be calculated by
  dividing 2s, by the average time for 1 clap.
• Average Speed 2s / t

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Example

• Q.
• Consider a storm brewing some distance away, and
  a lightning strike occurs.
• 5 seconds later the observer hears the thunder.
• Given that the speed of sound in the air is
  330m/s.
• Can you gauge how far an observer is from a storm
  using the speed of sound?
• A.
• Distance of lightning from observer
• Speed of sound Time taken
• 330 5 1650 m

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Example

• Q.
• The annoying sound from a mosquito is produced
  when it beats its wings at the average rate of
  600 wing beats per second.
• a) What is the frequency (in Hertz) of the
  sound wave?
• b) Assuming the sound wave moves with a
  velocity of
• 330 m/s, what is the wavelength of the
  wave?
• A.
• a) Frequency 600Hz
• b) Using v f l,
• Wavelength of the wave, l 330 / 600 0.55m

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Example

• Q.
• A man stands some distance away from a cliff. He
  gives a shout and hears his echo 4s later. How
  far away is he from the cliff?
• (Take speed of sound in air to be 330 ms-1)
• A.
• Time for sound to travel from man to cliff and
  from cliff to man is 4s.
• Hence, time for sound to travel from man to cliff
  is 2s.
• Therefore, distance between man and cliff is

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Challenge yourself

• Q.
• The survivor of a shipwreck lands on an island
  which is 3000 m from a vertical cliff. He sees a
  ship anchored between the island and the cliff. A
  blast from the ships horn is heard twice with a
  time lapse of 4 s. Calculate the distance s of
  the ship from the island.

3000 m
s
Cliff
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Other uses of echo

• Bats emit a high frequency sound which is
  reflected by an object in its path.
• Enables the bat to pinpoint the location of
  obstacles and avoid them

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Questions?

• Self Check
• What is an echo?
• How do we measure the speed of sound in air?
• How do you think we can adjust the surrounding to
  make echoes more prominent?
• What do you understand by the word feedback?

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Feedback?
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Lesson 4

• Lesson Objective
• Understand how the loudness and pitch of sound is
  related to amplitude and frequency of the
  waveform
• Discuss how the quality of sound is reflected on
  the waveform.

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Why do a violin and a piano sound different?
Why do a rooster and a monkey sound different?
What distinguishes one sound from another?
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The 3 characteristics of sound

• Loudness
• Pitch
• Quality

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Loudness
Compare the waveforms below
What is the difference between them?
The amplitude of vibration.
So how do we vary the amplitude?
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Loudness
The loudness of sound is dependent on the amount
of energy which is transferred to the medium. It
in turn is dependent on the amplitude of
vibrations of the object.
The greater the energy from the source
(More energy transfer)
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Common sounds and their estimated loudness
Source Intensity Level
Threshold of Hearing (TOH) 0 dB
Rustling Leaves 10 dB
Whisper 20 dB
Normal Conversation 60 dB
Busy Street Traffic 70 dB
Vacuum Cleaner 80 dB
Large Orchestra 98 dB
Walkman at Maximum Level 100 dB
Front Rows of Rock Concert 110 dB
Threshold of Ear Pain 130 dB
Military Jet Takeoff 140 dB
Instant Perforation of Eardrum 160 dB
One convenient unit of sound level
Decibel (dB)
The faintest sound a human ear can hear is called
the
Threshold of Hearing
Note Any sound above 85 dB can cause hearing
loss, and the loss is related both to the power
of the sound as well as the length of exposure.
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Pitch
Sound waves are caused when a vibrating object
introduce vibration into a medium. Vibrating
objects e.g. vocal cords,
guitar string, tines of tuning
fork, Medium e.g. air, water particles etc. How
often the particles of the medium vibrate is
referred to the frequency of a wave.
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Pitch
Compare the waveforms below
Higher frequency!
What determines the pitch of sound?
The frequency of vibration.
So how do we vary the pitch? E.g. guitar string
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Pitch
Each vibrating object has its own frequency.
Human
Piano Guitar
Higher pitch means? Higher or lower frequency?
Pitch is raised 1 octave with every doubling of
frequency!! 256 Hz ? 512Hz gt pitch is raised by
1 octave
Applet
?
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Quality
Similar musical notes (i.e. same pitch) sounds
different on different instruments (e.g. piano
and guitar). These sounds have different quality
or timbre.
Tuning fork
Piano
Trumpet
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Quality
Compare the waveforms below
What determines the quality of sound?
The waveform.
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Questions?

• Self Check
• What are the 3 characteristics of sound, that
  will determine what we hear?
• What happens to the waveform when we try to vary
  each of this characteristics?
• A sound mixer is often accompanied with an
  equalizer. How will this equalizer affect the
  output sound?

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Summary
What we learnt today
depends on the amplitude of vibration
Soft
Loud
Loudness
depends on frequency
Characteristics of sound
Low
High
Pitch
depends on waveform
Quality or Timbre
Mixed
Clearer
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What we have covered
Vibrating sources
Production
Requires a medium
Transmission
Detection
Audibility
Sound
Reflection
Echoes
Measuring its speed
Direct and indirect method
Pitch, Loudness and Quality
Different waveforms