What causes sound?

© Boardworks Ltd 2003

What causes sound?

Take a tuning fork and strike it against a block of wood, what do you observe?

The tuning fork vibrates and you hear a sound.

All sounds are caused by vibrations.


Music

What vibrates when you sing?

Your voice-box.

What vibrates when you play a violin?

The strings


Sound waves are vibrations and so need a substance to travel through.

With air inside, the sound can be heard.

With nothing inside [a vacuum], the sound can’t be heard.

The Bell-jar experiment

What happens when the air is removed from the bell-jar?


Sound : How fast does it travel?

You need a quiet open space at least 100m long to perform this investigation.

> 100m

00:0000

START

00:0034

STOP

1) When you see the cymbals crash, press START.

2) When you hear the cymbals crash, press STOP.

Write your results in a table like this:


Try Distance[m]

Time[s]

Speed[m/s]

1. 100 0.34 294

2.

3.

4.

Calculate your average speed of sound :

Av. = [try 1 + try 2 + try 3 + try 4] 4. What errors could have crept into your results?



Sound waves need particles in order to travel.The substance that the sound travels through affects the

speed of sound greatly.

330 350

1450

5000

6000

0

1000

2000

3000

4000

5000

6000

7000

Air at 0 C Air at 30 C Water Concrete Steel

Material

Sp e

e d o

f sou

nd [m

/ s]



Travelling sound

Sound travels by particles vibrating. To understand this better you need to remember what the particles look like in a solid, liquid and a gas:

solid liquid gas

In which state are the particles closest together?

In which state are the particles furthest apart?

solid

gas

Which state does sound travel fastest through? Why?

Sound travels fastest through solids because the particles are closer together than in a liquid and a gas, so the vibrations are more easily passed from particle to particle.


Sound : How fast does it travel? Most of us have seen thunder

storms - which comes first, the thunder or lightning?

The lightning gets to our eyes before the thunder reaches our ears.

1) Thunder & lightning are made at the same time so we deduce that light travels much faster than sound.

2) In fact light travels so fast that:

the time between seeing the flash and hearing the bang = time taken for sound to travel.


Reflection : echoes

You should remember that sound is produced by a vibration and travels as a longitudinal wave

………..and that sound travels at different speeds through different substances [or media]

Sound waves reflect off hard, smooth surfaces to produce echoes


Using echoes

What do we call reflected sound? an echo

Which surfaces are the best at reflecting sound:

HARD or SOFT ?How are echoes reduced in cinemas and theatres?

By using soft materials on the walls such as curtains.

Name two animals that use echoes?

Bats and Dolphins


Reflection : Echoes

Stand at least 100m from a large, straight wall

Measure the distance from you to the wall

Use a starting pistol [or clapper board] to make a sound

Measure the time taken between firing the pistol and hearing the echo Remember, this is ‘two

way travel time’ [twtt]

STARTSTOP

150m


The sound takes 0.92s to travel 300m Remember the formula for speed?

SPEED = DISTANCE TIME v = 300 0.92

v = 326 m/s

For the Higher Tier paper you will need to be able to change the subject of the formula

Repeat this several times to obtain an average

Reflection : Echoes


Reflection : questions

Distance [m]

Time [s]

Speed [m/s]

Small aeroplane 600 5

Jet fighter 900 2

Meteorite 10,000 0.35

Cheetah 50 2.5

Which of these travel faster than the speed of sound in air?

120

450

28571

20


Studying sound

Signal generators can produce signals over a range of frequencies and of varying amplitudes.

Loudspeakers convert the signal from the signal generator into sound waves.

The oscilloscope allows us to study the frequency and loudness of a sound.


Pitch (or frequency)

A high pitch sound. A low pitch sound.

The shorter/longer the wavelength of the wave on the trace; the lower/higher the frequency of the sound.

The more waves you can see, the higher the pitch/frequency.


Oscilloscope traces

Which trace represents the highest pitched sound?

A B

‘A’ is the highest pitched sound because it has the shortest wavelength/most number of waves visible.


Loudness

A quiet sound. A louder sound.

The larger/smaller the amplitude of the wave on the trace, the louder/quieter the sound.

The bigger the waves you can see, the louder the sound.


Which trace represents the loudest sound?

A B‘B’ Is the loudest sound because it has the largest amplitude.The larger the amplitude the more energy a wave has.The more energy it has, the louder the sound.

Oscilloscope traces


Wave animation


Sound : What can I hear?

Increase the frequency of the signal provided by a signal generator whilst keeping the volume the same.

The lowest frequency I can hear is ________ Hz

The highest frequency I can hear is ________ Hz

2020 000


Sound : Are we all the same?

You have just found your hearing range - could everyone hear exactly the same frequencies as you?

We all have slightly different hearing ranges but almost 1 in 5 people suffer some sort of hearing loss. This changes with age. A baby has a wider range than an older person.

Temporary hearing loss may be caused by ear infections and colds and hearing recovers.

Permanent hearing loss and deafness can be present at birth or occur if the ear is damaged or diseased.


Sound : Are we all the same?

Hearing is tested using an audiometer and the results are shown on an audiogram.

0

10

20

30

40

50

60

70

80

0 2000 4000 6000 8000Frequency of sound [Hz]

optimalhearing

impairedhearing

Hear

ing

Loss

[dB]


Sound : Hearing Ranges

0 1 10 100

1,000

10,000

100,000

1,000,000

Frequency [Hz]

Moths

Elephants

Mice

Humans

Pigeons

Bats

Hearing Ranges of AnimalsA

nim

al

Which animal can hear the lowest frequency?

pigeons

Which animal can hear the highest frequency?

moths


Sound : The ear

1. Sound waves are collected by the ear lobe or pinna.

2. The waves travel along the ear canal.

3. The waves make the ear drum vibrate.

4. The small bones [ossicles] amplify the vibrations.

5. The cochlea turns the vibrations into electrical signals.

6. The auditory nerve takes the signals to the brain.


Sound : The ear

1.Sound waves are collected by the ear lobe or pinna.

2.The waves travel along the ear canal.

3.The waves make the ear drum vibrate.

4.The small bones (ossicles) amplify the vibrations.

5.The cochlea turns these into electrical signals.

6.The auditory nerve takes the signals

to the brain.

3

4

5

61

2


Sound : The ear


Sound : How loud are sounds?

150

140

120

100

80

60

40

20

10

0

Permanent ear damage

Can just be heard

Aircraft overhead Personal stereo

A circular saw at 2m

Loud bell

Pin being dropped

Quiet countryside


What is noise?

A noise is any unwanted sound. What you might not consider noise, loud music for example, other people might.What are the effects of noise?

1. _________

2. _________

3. _________

4. _________Nausea

VomitingHeadaches

Deafness

How can you reduce the effects of loud noise?

1. _________________

2. _________________

3. __________________________________

Ear protectors

Putting noisy machinery in insulated rooms

Double glazing


What is the upper range of human hearing?

A. 20 Hz

B. 200 Hz

C. 2 000 Hz

D. 20 000 Hz


What causes all sounds?

A. Vibrations

B. Reflections

C. Refractions

D. Heat


Which of the following can sound not travel through?

A. Liquid

B. Vacuum

C. Solid

D. Gas


A ship releases an echo sounding and 4 seconds later receives a signal from the seabed, how deep is the sea? (speed of sound in water is 1500 m/s)

A. 6000m

B. 375m

C. 750m

D. 3000m


Which of the following is not a use of ultrasound?

A. Prenatal scanning

B. Quality control in industry

C. Cleaning delicate machinery

D. Cooking food

What causes sound?

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