Waves and Sound...Sound waves carry this energy through the air. Sounds travel away from the source like ripples on a pond. The further from the source you are the less energy reaches

Dalkeith High School Level 4 Physics

Waves and Sound

By recording and analysing sound signals, I can describe how they can be manipulated and used in sound engineering.

SCN 4-11a

Fourth Level Science Forces, Electricity & Waves: Vibrations & Waves

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INSTRUCTIONS: Always put today’s date and copy carefully each HEADING. Symbols used in this booklet:

Copy

The little pencil symbol means that you copy the passage neatly into your Physics Jotter. It is important that the Copy Passages are copied accurately since the content may appear in the End of Unit Test.

Read

The little book symbol means that you must read the passage carefully so you can extract the required information and so that knowledge is gained for the test.

What to do

This little symbol means you must collect apparatus and carry out an experiment or follow instructions in an activity. Remember, apparatus may be delicate and costly and should be treated as such. Please return all apparatus to its appropriate place of storage.

Questions Answer in sentences

This little question symbol means that there are some questions to be answered as best as you can. If you are unsure of an answer, your teacher may help or you can find out the answer from other sources like a text book or internet.

More to do

The little plus sign means that if you have the time there is more work that can be done. At all times, instructions should be carefully followed. Follow your instructions. When doing practical work:

it should be carried out quietly and safely.

equipment should be returned to the correct place.


Activity 1 – What is Sound?

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Read

In order for us to hear a sound something must

make our eardrums move. You may remember

from previously in Science that all moving objects

have kinetic energy. Energy must be transferred

to our ears. Sound waves carry this energy

through the air.

Sounds travel away from the source like ripples

on a pond. The further from the source you are

the less energy reaches you. This is why if you

are too far from a sound you cannot hear it. A

person can probably only shout over a distance of

around 100m before the sound gets too quiet to

hear. When the volcanic island of Krakatoa

erupted it generated the loudest sound ever

historically reported — the massive explosion was distinctly heard as far away as the

island of Rodrigues near Mauritius (approx. 3000 miles or 4800 km). This is like us hearing

an explosion that happened in New York.

Nowadays if someone is a bit hard of hearing there are electronic hearing aids that they

can wear. These are really like miniature tannoy systems. They have a microphone

which picks up sounds and changes them into electrical signals, an amplifier which gives

energy to the signal and a loudspeaker which changes the amplified electrical signal

back into sound and sends the sound into the ear.

In the olden days before electronics people used to make use of ear trumpets to help

them hear. The ear trumpet collected sound waves and passed them down a tube into the

person’s ear. This is a bit like the satellite dish collecting radio signals and reflecting them

to the aerial.

http://en.wikipedia.org/wiki/Rodrigues_%28island%29

http://en.wikipedia.org/wiki/Mauritius


Activity 1 – What is Sound? (continued)

Page 5

Questions

a) What must happen for us to hear a sound?

b) What do sound waves carry through the air?

c) What was the loudest sound ever historically reported?

d) Jeremy is hard of hearing. Write a short note to Jeremy to explain what he could do

improve his hearing.

Now watch the animation Sound Production on page 162 of Exploring Science

Book 8 showing how different musical instruments are able to produce sound

e) Explain briefly how sound is produced using each of the musical instruments

shown.

More to do

The video Sounds on page 162 of Exploring Science Book 8 and try and identify the

origin of each sound.

When this man talks, his vocal chords vibrate to produce sound waves, which travel through the air.


Activity 2 – Longitudinal & Transverse Waves

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What to do

Follow the teacher demonstration and attempt to display the difference between

longitudinal and transverse waves.

Collect a transverse and longitudinal wave diagram and copy the following passage.

In longitudinal waves the vibration of the particles is in the same direction as the

direction of energy transport.

In transverse waves the vibration of the particles is at right angles to the direction of

energy transport.

Sound is an example of a longitudinal wave.

Light, water waves and waves on a string are examples of transverse waves.

The simple demonstration experiment below can be used to show how sound energy is

transferred through air.

Observe what happens to the candle flame when the sound emitted from the loudspeaker

is directed towards it.

Loudspeaker


Activity 2 – Longitudinal & Transverse Waves (continued)

Page 7

As the diaphragm of the speaker vibrates back and forth it disturbs the surrounding

air molecules. The air molecules then pass on the disturbance to adjacent air molecules.

In this way the originating disturbance from the speaker travels through air (the medium)

via the air molecule as a sound wave. The air molecules do not themselves travel from the

speaker to the ear rather they just vibrate to and fro.

As the air molecules move in the same direction as the wave, sound waves are therefore

longitudinal waves.

The wavelength of a sound wave is the distance between successive compressions or

rarefactions as shown in the diagram above.


Activity 3 – Amplitude & Frequency

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Read

In order for sound to be heard it requires a medium to pass through. Sound can travel

(propagate) through solids liquids and gases at different speeds. This is due to the

proximity of particles in these materials and their ability to pass on the sound vibrations.

However, sound cannot travel through a vacuum, a volume of space with no (or very

few) particles.

Scientists use microphones to detect sounds. If a microphone is attached to an

oscilloscope, you can see a representation of the sound wave produced. By looking at

the wave (or trace) on the oscilloscope screen, scientists can make comments about the

amplitude (volume) and frequency (pitch) of the sound.

Copy and complete the following diagrams


Activity 3 – Amplitude & Frequency (continued)

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What to do

Part A: Oscilloscope Traces

1. Collect an oscilloscope trace sheet.

2. Follow the instructions on the sheet and attempt to complete all diagrams in the left

hand column of blank traces using the apparatus provided.

3. Review the solutions and if required use the diagrams on the right hand column to

make any corrections.

More to do

Use the Sound Wave animation page 164 of Exploring Science Book 8 and try and

identify the correct oscilloscope trace before observing it.


Activity 3 – Amplitude & Frequency (continued)

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Read

The frequency of a sound produced can also be altered by adjusting the length of the

object which produces the sound vibrations.

What to do

Part B: Test tube pipes

1. Collect four test tubes and a test

tube rack.

2. In each test tube place a different

volume of water, leaving an air column height of 2, 4, 6 and 8 cm in each.

3. Gently blow across the top of each one, noting down in a suitable table of height of

air column (in cm) and frequency of sound (value 1 to 5 , 1 being lowest frequency ,

5 being highest)

4. Explain your results and make some valid conclusions based on them

More to do

Use the Pitch of a Guitar animation page 164 of Exploring Science Book 8 and try

and determine the pitch of sound produced before moving the guitarist’s hand.


Activity 4 – The Wave Equation

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Read

With the increase in mobile technology most of our day to day communications are

completed using waves passing through the air, wires or fibre optics. We need to

understand how to describe a wave so that we are able to talk about waves in a

meaningful way.

What to do

Collect a waves diagram and stick it into your jotter. Listen to your teacher describe the

main parts of the wave.

Terms to describe waves

Part Symbol Unit Description

wavelength m It is the distance between two successive

points on a wave in phase.

amplitude A m

It is measured from the centre line to the

crest or trough. It is a measure of how much

energy a wave carries.

frequency f Hz

This is how many waves are produced each

second. This is the same as the number of

waves that pass a point in one second.

period T s This is the time to produce one wave.

wavespeed v m/s This is the distance a wave travels in one

second.

A

A


Activity 4 – The Wave Equation (continued)

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Read the following statements

We can find the speed (v) of a wave by measuring how far the wave travels (distance, d)

in a known time interval (time, t). This uses the formula:

It is also possible to calculate the speed of a wave if we know the wavelength () and

frequency (f) of the wave. This uses the formula:

The frequency of a single wave can be expressed as the relationship between the

frequency and the time taken for one wave, the period of a wave, (T).

Similarly, the frequency of a number of waves (N) passing a given point in a fixed time (t)

can be expressed as:

Example:

A water wave has a wavelength of 50 cm. Twenty waves pass a point in 10 seconds.

Calculate:

a) The frequency of the wave.

b) The speed of the wave.

Solution: (a) Solution: (b)

st

N

f

10

0.2

?

Hzf

f

t

Nf

2

10

20

m

Hzf

v

5.0

2

?

smv

v

fv

/1

5.02


Activity 4 – The Wave Equation (continued)

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Questions

1. If the speed of sound travels at 333 m/s, how long does it take to travel 3000 m?

2. If the speed of sound in water is 1500 m/s, how far does it travel in 3 mins 45 s?

3. Sound travels at 5050 m/s in railways tracks. A train is 4 km away. How long does it

take the sound to reach a man with his ear to the tracks?

Note 1 km = 1000 m

4. A wave has a wavelength of 34 cm and a frequency of 75 Hz. Calculate its speed.

Note 1 cm = 0.01 m

5. A wave has a speed of 18 m/s ad a frequency of 9 Hz. Calculate:

a) The wavelength of the wave.

b) The period of the wave.

6. What is the wavelength of a microwave with a frequency 12 500 MHz?

Hint 1Mhz = 1000 000 Hz and the speed of radio waves is 300 000 000 m/s.

7. 96 waves arrive at a beach every minute. The speed of waves is 4.8 m/s. Calculate:

a) The frequency of the waves.

b) The wavelength of the waves.

8. A wave travels 60 m in 30 s. It has a wavelength of 5 cm. Calculate:

a) The wavespeed.

b) The frequency of the wave.

c) The period of the wave.


Activity 5 – The Speed of Sound

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Read

You may well have noticed before that sound and light

do not travel at the same speed. For instance, during a

firework display or a thunderstorm, which do you

observe first: the light or sound?

Light travels extremely fast. In fact, the speed of light

in a vacuum (3 x 108 m/s, or 300000000 m/s) is the

fastest speed anything in the universe can travel at.

Sound travels much slower through air. We can use

this difference between the speed of light and the

speed of sound to attempt to measure the speed of

sound in air using a variety of methods.

What to do Method 1: Outdoor measurements

1. You and two partners will measure out a fixed distance of 50 metres (using either a

surveyor’s tape or trundle wheel).

2. One partner will stand at one end of this distance (d) and will make a sound. The

other partner will indicate using a hand up or waving a flag the instant the sound

has been made.

3. You will measure the time (t) using a stopclock taken until you hear the sound.

4. This distance and time should be noted in a suitable table.

5. You will then repeat this procedure twice, with each partner alternating their role in

the group.

6. At the end of the practical activity, the group will calculate the average time (

taken and using the formula determine the speed of sound in air (v).


Activity 5 – The Speed of Sound (continued)

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Method 2: Indoor measurements

1. Use the apparatus already pre assembled as shown in the diagram.

2. Hit the metal plate sharply.

3. Record the time (t) into your table.

4. Repeat twice and then calculate the average time ( taken and using the formula

determine the speed of sound in air (v).

Questions

1. The approximate speed of sound in are can range between 320-340 m/s. Which of

the two methods used was most accurate?

2. What improvements could have been made to each experimental procedure to

improve its reliability?

More to do

Use the Speed of Sound animation page 172 of Exploring Science Book 8 and

use to try another method of measuring the speed of sound

Attempt distance Between microphones (m)

time in milliseconds

(ms)

speed in metres per second (m/s)

1 1.0

2 1.0

3 1.0

4 1.0

Average speed


Activity 6 – Detecting Sound & The Range of Hearing

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Read

The human ear has two functions: hearing and balance. The ear has three main parts: the

outer, middle and inner ear. The outer ear is the part you can see and opens into the ear

canal. The eardrum separates the ear canal from the middle ear. Three small bones in the

middle ear transmit sound vibrations to the inner ear. The inner ear contains the cochlea

which converts the vibrations into electrical signals. These electrical signals pass along the

nerve to the brain. The semicircular canals in the ear have nothing to do with hearing. They

are required for balance.

What to do

Collect a copy of the ear diagram and use the information from the passage above or

the Hearing video page 169 of Exploring Science Book 8 to answer the following

questions:

1. What is detected by the human ear?

2. What is the function of the three small bones in the inner ear?

3. Which part of the ear converts vibrations into electrical signals?


Activity 6 – Detecting Sound & The Range of Hearing (continued)

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Although our ears are tuned to detecting a large range of audible frequencies, there is still an

upper and lower limit to the range of human hearing. The following demonstration will help you

determine your range of hearing.

What to do

1. Your teacher will adjust the controls on the oscilloscope to a low frequency.

2. Listening intently, you should indicate by raising your hand when you begin to detect an

audible sound. This is your lower threshold of hearing.

3. Your teacher will continue to increase the frequency.

4. Keep your hand raised until you can no longer detect an audible sound. You have

reached your upper threshold of hearing.

5. Collect a Range of Hearing passage and complete using your data:

Lowest frequency I can hear = ________________ Hertz

Highest frequency I can hear = ________________ Hertz

When listening to music, you hear sounds of many frequencies. On average, humans

can detect frequencies between 20 Hertz and 20 000 Hertz. These are audible

frequencies for humans. As we get older, the upper limit gradually reduces to about

15 000Hz. Some animals can detect and communicate using frequency sounds either

side of the range of human hearing.

More to do

Watch the BBC video How animals use sound to communicate video page 162 of

Exploring Science Book 8 to find out more about the frequency ranges other animals

can use to communicate.


Activity 7 – Ultrasound

Page 18

Read

‘As blind as a bat’ is a common saying.

Bats are almost blind, but they can

locate obstacles or insects by using

very high frequency sound waves.

Humans can hear frequencies up to

20 000Hz. Higher frequencies than this

are called Ultrasounds. When these

higher frequency waves are sent out and hit an object some are reflected like an audible

echo. Bats, dolphins and whales are some mammals that use these echoes to find their

way around, evade predators, or catch food. This process is called echolocation.

Ultrasounds are used in medicine to scan unborn babies.

This is a safe way to monitor the growth and health of an

unborn baby. Ultrasounds are sent out from a transmitter

and are reflected back from the baby to a detector. A

picture or scan can then be viewed on a computer screen.

Ultrasound can also be used in medicine to destroy kidney

stones.

An industrial application of ultrasound is in fishing,

where fishermen use ultrasound at to locate shoals of

fish. Ultrasounds are sent out and reflected by the sea

bed. If there are fish in the way, the ultrasounds will

reflect from them and be detected back at the boat more

quickly.

A military application of ultrasound is in the detection of enemy submarines, either

through active or passive sonar. Active sonar is the method by which torpedoes use

ultrasound pulses to echolocate targets. Passive sonar is a method by which boats use

pulses todetect potential targets which cause the transmitted signal to be reflected

differently.


Activity 7 – Ultrasound (continued)

Page 19

Questions

1. What is ultrasound?

2. Describe a medical application of ultrasound.

3. Describe a non-medical application of ultrasound.

Read

Ultrasound numerical problems generally involve calculating the total distance travelled by

a reflected ultrasound. The same principle should be applied when considering audible

sounds being reflected, or echoed. The example below shows how to attempt these types

of problems.

Example:

A sonar pulse was sent down from a ship looking for a

shoal of fish and two pulses were reflected back, the first

after 0.85s and the second after 2.3 seconds. If the speed

of sound in water is 1500 m/s. Calculate:

(a) The depth of the water.

Solution:

The pulse that takes the longest time to be reflected is the one which reflects off the

seabed.

?

3.2

/1500

d

st

smv

md

d

tvd

3450

3.21500

This is the total distance travelled. Therefore, the distance from boat to seabed is half

i.e. 1725 m.

Page 20

Questions

1. An object is detected by ultrasound as long as it is equal to one wavelength of

the ultrasound. If the frequency of the ultrasound is 50 kHz, what is the size of

the smallest object detected given that the ultrasonic pulses travel with a

speed of 340 m/s in air.

2. A series of sonar pulses is used by fishermen to detect shoals of fish under

the water. The speed of sound in sea water is 1200 m/s.

a) An echo is received after 0.3 s. How far has the sound travelled?

b) How deep is the water?

c) A second echo is received after 120 ms. How far had the sound

travelled.

Note 1 ms = 0.001 s.

d) How deep is the shoal of fish?

3. The speed of sound in the human body is 1500 m/s and an echo from a

foetus is detected 0.1 ms after an ultrasound transmission. The frequency of

ultrasound is 250 kHz.

Note 1 kHz = 1000 Hz.

a) How many ultrasonic pulses are emitted every second?

b) What is the period of ultrasound?

c) How deep in the mother’s body is the part of the foetus which provides

the echo?

4. A man uses a ‘silent’ dog whistle to call his dog.

a) Explain why the dog can hear it but the man cannot.

b) Suggest a possible frequency for the dog whistle.

c) Another whistle emits a sound of wavelength 18 mm. Will this act as a

‘silent’ whistle? Explain your answer.

Note 1 mm = 0.001 m.

Documents

Waves and Sound...Sound waves carry this energy through the air. Sounds travel away from the source like ripples on a pond. The further from the source you are the less energy reaches