17.2 Sound Waves: In Halliday and Resnick: Longitudinal waves are sound waves!
Waves and Sound...Sound waves carry this energy through the air. Sounds travel away from the source...
Transcript of Waves and Sound...Sound waves carry this energy through the air. Sounds travel away from the source...
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
Page 3
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.
Fourth Level Science Forces, Electricity & Waves: Vibrations & Waves
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.
Fourth Level Science Forces, Electricity & Waves: Vibrations & Waves
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.
Fourth Level Science Forces, Electricity & Waves: Vibrations & Waves
Activity 2 – Longitudinal & Transverse Waves
Page 6
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
Fourth Level Science Forces, Electricity & Waves: Vibrations & Waves
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.
Fourth Level Science Forces, Electricity & Waves: Vibrations & Waves
Activity 3 – Amplitude & Frequency
Page 8
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
Fourth Level Science Forces, Electricity & Waves: Vibrations & Waves
Activity 3 – Amplitude & Frequency (continued)
Page 9
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.
Fourth Level Science Forces, Electricity & Waves: Vibrations & Waves
Activity 3 – Amplitude & Frequency (continued)
Page 10
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.
Fourth Level Science Forces, Electricity & Waves: Vibrations & Waves
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
Fourth Level Science Forces, Electricity & Waves: Vibrations & Waves
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
Fourth Level Science Forces, Electricity & Waves: Vibrations & Waves
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.
Fourth Level Science Forces, Electricity & Waves: Vibrations & Waves
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).
Fourth Level Science Forces, Electricity & Waves: Vibrations & Waves
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
Fourth Level Science Forces, Electricity & Waves: Vibrations & Waves
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?
Fourth Level Science Forces, Electricity & Waves: Vibrations & Waves
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.
Fourth Level Science Forces, Electricity & Waves: Vibrations & Waves
Activity 7 – Ultrasound
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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.
Fourth Level Science Forces, Electricity & Waves: Vibrations & Waves
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.