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# CIE IGCSE Physics
# 3.4 Sound
# Contents
3.4.1 Sound Waves
3.4.2 Measuring the Speed of Sound
3.4.3 E ects of Sound Waves
3.4.4 Ultrasound
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# 3.4.1 Sound Waves Describing Sound
Sound waves are produced by vibrating sources When a sound wave comes into contact with a solid, those vibrations can be transferred to the solid For example, sound waves can cause a drinking glass to vibrate If the glass vibrates too much the movement causes the glass to shatter
Sound waves are longitudinal: the molecules vibrate in the same direction as the energy transfer
Sound waves require a medium to travel through This means that if there are no molecules, such as in a vacuum, then the sound cant travel through it
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# Compression & Rarefaction
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Longitudinal waves consist of compression and rarefactions :A compression is a region of higher density i.e. a place where the molecules are bunched together A rarefaction is a region of lower density i.e. a place where the molecules are spread out
Sound is a longitudinal wave consisting of compressions and rarefactions - these are areas where the pressure of the air varies with the wave
These compressions and rarefactions cause changes in pressure, which vary in time with the wave Therefore, sound is a type of pressure wave
When the waves hit a solid, the variations in pressure cause the surface of the solid to vibrate in sync with the sound wave
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When sound waves hit a solid, the uctuating pressure causes the solid to vibrate
## Exam T ip
When describing compressions and rarefactions, make sure to use the correct terms. It is best to refer to them as regions of high and low densities of particles instead of the particles are more 'bunched up' or 'far apart', as this is too vague and not very scienti c!
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# 3.4.2 Measuring the Speed of Sound Speed of Sound in Air
Sound waves travel at a speed of about 340 m/s in air at room temperature The higher the air temperature, the greater the speed of sound The speed of sound in air varies from 330 350 m/s
# Speed of Sound in Materials
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Sound travels at di erent speeds in di erent mediums: Sound travels fastest in solids
Sound travels slowest in gases
The table below shows some typical speeds of sound in solids, liquids and gases:
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# Measuring the Speed of Sound
There are several experiments that can be carried out to determine the speed of sound Three methods are described below The apparatus for each experiment is given in bold Method 1: Measuring Sound Between T wo Points
Measuring the speed of sound directly between two points
. Two people stand a distance of around 100 m apart
. The distance between them is measured using a trundle wheel
. One person has two wooden blocks , which they bang together above their head
. The second person has a stopwatch which they start when they see the rst person banging the blocks together and stops when they hear the sound
. This is then repeated several times and an average value is taken for the time
. The speed of sound can then be calculated using the equation:
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Method 2: Using Echoes
Measuring the speed of sound using echoes
. A person stands about 50 m away from a wall (or cli ) using a trundle wheel to measure this distance
. The person claps two wooden blocks together and listens for the echo
. A second person has a stopwatch and starts timing when they hear one of the claps and stops timing when they hear the echo
. The process is then repeated 20 times and an average time calculated
. The distance travelled by the sound between each clap and echo will be (2 50) m
. The speed of sound can be calculated from this distance and the time using the equation:
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Method 3: Using an Oscilloscope
Measuring the speed of sound using an oscilloscope
. Two microphones are connected to an oscilloscope and placed about 5 m apart using a tape measure to measure the distance
. The oscilloscope is set up so that it triggers when the rst microphone detects a sound, and the time base is adjusted so that the sound arriving at both microphones can be seen on the screen
. Two wooden blocks are used to make a large clap next to the rst microphone
. The oscilloscope is then used to determine the time at which the clap reaches each microphone and the time di erence between them
. This is repeated several times and an average time di erence calculated
. The speed can then be calculated using the equation:
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Measuring Wave Speed in Wat er
Ripples on water surfaces are used to model transverse waves The speed of these water waves can be measured
Creating ripples in water
. Choose a calm at water surface such as a lake or a swimming pool
. Two people stand a few metres apart using a tape measure to measure this distance
. One person counts down from three and then disturbs the water surface (using their hand, for example) to create a ripple
. The second person then starts a stopwatch to time how long it takes for the rst ripple to get to them
. The experiment is then repeated 10 times and an average value for the time is calculated
. The average time and distance can then be used to calculate the wave speed using the equation:
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## Exam T ip
When you are answering questions about methods to measure waves, the question could ask you to comment on the accuracy of the measurements In the case of measuring the speed of sound: Method 3 is the most accurate because the timing is done automatically Method 1 is the least accurate because the time interval is very short Whilst this may not be too important when giving a method, you should be able to explain why each method is accurate or inaccurate and suggest ways of making them better (use bigger distances) For example, if a manual stopwatch is being used there could be variation in the time measured which can be up to 0.2 seconds due to a person's reaction time The time interval could be as little as 0.3 seconds for sound travelling in air This means that the variation due to the stopwatch readings has a big in uence on the results and they may not be reliable
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# 3.4.3 E ects of Sound Waves Pitch & Loudness
The frequency of a sound wave is related to its pitch
Sounds with a high pitch have a high frequency (or short wavelength) Sounds with a low pitch have a low frequency (or long wavelength) The amplitude of a sound wave is related to its volume
Sounds with a large amplitude have a high volume Sounds with a small amplitude have a low volume
Pitch and amplitude of sound
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# Echoes
Sound waves re ect o hard surfaces The re ection of a sound wave is called an echo
Echo sounding can be used to measure depth or to detect objects underwater A sound wave can be transmitted from the surface of the water The sound wave is re ected o the bottom of the ocean The time it takes for the sound wave to return is used to calculate the depth of the water The distance the wave travels is twice the depth of the ocean This is the distance to the ocean oor plus the distance for the wave to return
Echo sounding is used to determine water depth
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# 3.4.4 Ultrasound Ultrasound
Humans can hear sounds between about 20 Hz and 20 000 Hz in frequency (although this range decreases with age)
Humans can hear sounds between 20 and 20 000 Hz
Ultrasound is the name given to sound waves with a frequency greater than 20 000 Hz
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# Uses of Ultrasound
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When ultrasound reaches a boundary between two media, some of the waves are partially re ected The remainder of the waves continue through the material and are transmitted Ultrasound transducers are able to:
Emit ultrasound
Receive ultrasound The time taken for the re ections to reach a detector can be used to determine how far away a boundary is This is because ultrasound travels at di erent speeds through di erent media This is by using the speed, distance, time equation
Where: v = speed in metres per second (m/s) s = distance in metres (m) t = time in seconds (s) This allows ultrasound waves to be used for both medical and industrial imaging
Ult rasound in Medicine
In medicine, ultrasound can be used: To construct images of a foetus in the womb To generate 2D images of organs and other internal structures (as long as they are not
surrounded by bone) As a medical treatment such as removing kidney stones An ultrasound detector is made up of a transducer that produces and detects a beam of ultrasound waves into the body The ultrasound waves are re ected back to the transducer by boundaries between tissues in the path of the beam For example, the boundary between uid and soft tissue or tissue and bone
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When these echoes hit the transducer, they generate electrical signals that are sent to the ultrasound scanner Using the speed of sound and the time of each echos return, the detector calculates the distance from the transducer to the tissue boundary By taking a series of ultrasound measurements, sweeping across an area, the time measurements may be used to build up an image
Unlike many other medical imaging techniques, ultrasound is non-invasive and is believed to be
harmless
Ultrasound can be used to construct an image of a foetus in the womb
Ult rasound in Indust ry
In industry, ultrasound can be used to: Check for cracks inside metal objects Generate images beneath surfaces
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A crack in a metal block will cause some waves to re ect earlier than the rest, so will show up as pulses on an oscilloscope trace Each pulse represents each time the wave crosses a boundary The speed of the waves is constant, so measuring the time between emission and detection can allow the distance from the source to be calculated
Ultrasound is partially re ected at boundaries, so in a bolt with no internal cracks, there should only be two pulses (at the start and end of the bolt)
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## Worked example
In the diagram above, a very high-frequency sound wave is used to check for internal cracks in a large steel bolt. The oscilloscope trace shows that the bolt does have an internal crack. Each division on the oscilloscope represents a time of 0.000002 s. The speed of sound through steel is 6000 m/s. Calculate the distance, in cm, from the head of the bolt to the internal crack.
Step 1: List the known quantities
Speed of ultrasound,
v = 6000 m/s Time taken,
t = 5 0.000002 = 0.00001 s
Step 2: Write down the equation relating speed, distance and time
distance,
d =
v
t
Step 3: Calculate the distance
d = 6000 0.00001 = 0.06 m
Step 4: Convert the distance to cm
d = 6 cm
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