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SoundSound

TheThe• ProductionProduction• TransmissionTransmission• DetectionDetection• ReflectionReflection• MeasurementMeasurement• Pitch, Loudness & QualityPitch, Loudness & Quality

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

Lesson 1Lesson 1

• Lesson Objectives (Production, Transmission)

– To be able to describe how sound is produce

– To understand the nature of sound

Nature of SoundNature 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?


• 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)


• 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

Other uses of sound? Hmm…Other uses of sound? Hmm…


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)

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.

RECALL

What do you think…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?

Transmission of SoundTransmission 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.

• What does that mean? Sound requires a

medium to be transmitted.

Vacuum pump

Bell Jar

Alarm clock or Bell


• 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.

Speed of SoundSpeed of Sound

Gases Liquids (25oC) 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

Lucite 2680

Lead 1322

Rubber 1600

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.



– 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

T



– 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.

Questions?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?

Lesson 2Lesson 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)

Do you know…Do you know…

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

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

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

Using the Cathode Ray OscilloscopeUsing 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 that…the Cathode Ray Oscilloscope (CRO)…

What you will see on the CRO…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.

Time

Sound SpectrumSound 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.

Medical Diagnosis?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

Something Extra…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.

Lunch Time EntertainmentLunch Time Entertainment


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?

Lesson 3Lesson 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

Do you know…Do you know…

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

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

What is echo?What is echo?

• What is echo?– It is the sound heard after the reflection of sound

from a hard, flat surface.

What happens with multiple ‘echoes’?What happens with multiple ‘echoes’?

• Note the ‘rays’ (path) taken by the sound wave…

Multiple Echoes…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.

Measuring the Speed of SoundMeasuring the Speed of Sound

Direct method:1. Students A and B are positioned at a known

distance, s apart.

2. Student A fires a starting pistol

3. 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?


Direct method Calculation:

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.

• 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.


Indirect method (Using the echo):

1. Place the 2 wooden blocks at a known distance, s away from the wall

2. Make a sharp clapping sound by knocking the 2 blocks of wood together

3. Repeat the sound at regular intervals to coincide with the echoes

4. 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?


Indirect method (Using the echo):

2 wooden blocks

Large wall

Direction of incident sound

Direction of reflected sound (echo)

Distance, s

• 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

ExampleExample

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

ExampleExample

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 ,

Wavelength of the wave, = 330 / 600 = 0.55m

ExampleExample

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 2 330 660m

Challenge yourself…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 ship’s horn is heard twice with a time lapse of 4 s. Calculate the distance s of the ship from the island.

sCliff

3000 m

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


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’?

Feedback?Feedback?

Amplifier

Lesson 4Lesson 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.

Why do a violin and a piano sound different?

What distinguishes one sound from What distinguishes one sound from another?another?What distinguishes one sound from What distinguishes one sound from another?another?

Why do a rooster and a monkey sound different?

The 3 characteristics of sound

• Loudness

• Pitch

• Quality

LoudnessLoudness

Compare the waveforms below:

What is the difference between them?

The amplitude of vibration.

So how do we vary the amplitude?

LoudnessLoudness

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.

Louder sound

The greater the energy from the source(More energy transfer)

Larger amplitude of vibration

Common sounds and their estimated loudness

SourceIntensity

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:

The faintest sound a human ear can hear is called the

Decibel (dB)

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.

PitchPitch

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.


What determines the pitch of sound?

The frequency of vibration.

Higher frequency

!

So how do we vary the pitch? E.g. guitar string

PitchPitch

Each vibrating object has its own frequency.

HumanHuman

?

Piano & GuitarPiano & Guitar

Higher pitch means? Higher or lower frequency?Pitch is raised 1 octave with every doubling of frequency!!

256 Hz 512Hz => pitch is raised by 1 octave

Applet

PitchPitch

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

QualityQuality


What determines the quality of sound?

The waveform.

QualityQuality


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?

What we learnt today …

Soft LoudLoudness

Quality or Timbre

Pitch

depends on the amplitude of vibration

depends on frequency

depends on waveform

Low High

Mixed

Characteristics of sound

Clearer

SummarySummary

What we have covered…What we have covered…

Sound

Production

Transmission

Detection

Reflection

Measuring its speed

Pitch, Loudness and Quality

Vibrating sources

Requires a medium

Audibility

Echoes

Direct and indirect method

Different waveforms

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