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
o For example, sound waves can cause a drinking glass to vibrate
o 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
o This means that if there are no molecules, such as in a vacuum, then the sound
can’t travel through it
• The range of frequencies a human can hear is 20 Hz to 20 000 Hz
Compression & Rarefaction
• Longitudinal waves consist of compression and rarefactions:
o A compression is a region of higher density i.e. a place where the molecules are
bunched together
o A rarefaction is a region of lower density i.e. a place where the molecules are
spread out
Compressions and Rarefactions of Air in a Column
 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
o 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
Compressions and Rarefactions of Sound Reflecting from a Solid

When sound waves hit a solid, the fluctuating pressure causes the solid to vibrate

Investigating Sound in a Vacuum

Sound Waves in a Vacuum
• Sound waves are longitudinal waves
o All longitudinal waves require a medium through which to travel
• A vacuum is a region of space that does not contain air (or any other matter)
 o This means that, in a vacuum, there is no medium for sound waves
o So sound waves cannot travel in a vacuum
Using a Bell Jar
• This can be easily demonstrated using a piece of equipment called a bell jar
o This is a glass container from which air can be pumped out, creating a vacuum (or
nearly a vacuum)
• A sound-emitting object is used, such as a battery-operated ringing bell or alarm
• This is placed in a bell jar, which still contains air
o The ringing bell can be heard despite the bell jar's glass walls
• However, as the air begins being pumped out, the volume of the sound heard starts
decreasing
• When the air is completely removed from the bell jar, the ringing bell cannot be heard at
all
Sound in a Bell Jar Demonstration

In the absence of air, sound waves are unable to travel and leave the bell jar

Properties of Sound Waves

Pitch & Loudness
• The frequency of a sound wave is related to its pitch
o Sounds with a high pitch have a high frequency (or short wavelength)
o Sounds with a low pitch have a low frequency (or long wavelength)
 •

o Sounds with a large amplitude have a high volume

o Sounds with a small amplitude have a low volumeThe amplitude of a sound
wave is related to its volume
Graphs of Different Amplitudes & Frequencies

The amplitude of a wave determines the volume of the sound and the frequency determines the
pitch

Different Sound Sources

• An oscilloscope is a device that can be used to study a rapidly changing signal, such as:
o A sound wave
o An alternating current
Oscilloscope

Oscilloscopes have lots of dials and buttons, but their main purpose is to display and measure
changing signals like sound waves and alternating current
 • When a microphone is connected to an oscilloscope, the (longitudinal) sound wave is
displayed as though it were a transverse wave on the screen
• The time base (like the 'x-axis') is used to measure the time period of the wave
A Soundwave Depicted as a Transverse Wave on an Oscilloscope

A sound wave is displayed as though it were a transverse wave on the screen of the
oscilloscope. The time base can be used to measure a full time period of the wave cycle
• The height of the wave (measured from the centre of the screen) is related to
the amplitude of the sound
• The number of entire waves that appear on the screen is related to the frequency of the
wave
o If the frequency of the sound wave increases, more waves are displayed on
screen

Reflection of Sound Waves

Echoes
• Sound waves reflect off hard surfaces
o The reflection of a sound wave is called an echo
• Echo sounding can be used to measure depth or to detect objects underwater
o A sound wave can be transmitted from the surface of the water
o The sound wave is reflected off the bottom of the ocean
• The time it takes for the sound wave to return is used to calculate the depth of the water
 o This is the distance to the ocean floor plus the distance for the wave to return
o The distance the wave travels is twice the depth of the ocean
Ship using Radar

Echo sounding is used to determine water depth

Investigating the Reflection of Sound Waves
Using Echoes to Measure the Speed of Sound

Measuring the speed of sound using echoes

1. A person stands about 50 m away from a wall (or cliff) using a trundle wheel to measure
this distance
2. The person claps two wooden blocks together and listens for the echo
3. A second person has a stopwatch and starts timing when they hear one of the claps and
stops timing when they hear the echo
4. The process is then repeated 20 times and an average time calculated
5. The distance travelled by the sound between each clap and echo will be (2 × 50) m
6. The speed of sound can be calculated from this distance and the time using the equation:
 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
o The apparatus for each experiment is given in bold
Method 1: Measuring Sound Between Two Points
Measuring the Speed of Sound Using a Loud Noise

Measuring the speed of sound directly between two points

1. Two people stand a distance of around 100 m apart
2. The distance between them is measured using a trundle wheel
3. One person has two wooden blocks, which they bang together above their head
4. The second person has a stopwatch which they start when they see the first person
banging the blocks together and stops when they hear the sound
5. This is then repeated several times and an average value is taken for the time
6. The speed of sound can then be calculated using the equation:

Method 2: Using an Oscilloscope

Measuring the Speed of Sound Using Microphones
Measuring the speed of sound using an oscilloscope
1. Two microphones are connected to an oscilloscope and placed about 5 m apart using
a tape measure to measure the distance
2. The oscilloscope is set up so that it triggers when the first microphone detects a sound,
and the time base is adjusted so that the sound arriving at both microphones can be seen
on the screen
3. Two wooden blocks are used to make a large clap next to the first microphone
4. The oscilloscope is then used to determine the time at which the clap reaches each
microphone and the time difference between them
5. This is repeated several times and an average time difference calculated
6. The speed can then be calculated using the equation:

Measuring Wave Speed in Water

• Ripples on water surfaces are used to model transverse waves
o The speed of these water waves can be measured
Ripples on Water
Creating ripples in water
1. Choose a calm flat water surface such as a lake or a swimming pool
2. Two people stand a few metres apart using a tape measure to measure this distance
3. One person counts down from three and then disturbs the water surface (using their hand,
for example) to create a ripple
4. The second person then starts a stopwatch to time how long it takes for the first ripple to
get to them
5. The experiment is then repeated 10 times and an average value for the time is calculated
6. The average time and distance can then be used to calculate the wave speed using the
equation:

Speed of Sound in Air

• Sound waves travel at a speed of about 340 m/s in air at room temperature
o 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
• Sound travels at different speeds in different mediums:
o Sound travels fastest in solids
o Sound travels slowest in gases
• Some typical speeds of sound in solids, liquids and gases are:
 o Solids 5000 m/s

o Liquids 1500 m/s

o Gases 350 m/s

Ultrasound
• Humans can hear sounds between about 20 Hz and 20 000 Hz in frequency (although this
range decreases with age)
Infrasound & Ultrasound

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
Uses of Ultrasound
• When ultrasound reaches a boundary between two media, some of the waves are partially
reflected
• The remainder of the waves continue through the material and are transmitted
• Ultrasound transducers are able to:
o Emit ultrasound
o Receive ultrasound
• The time taken for the reflections to reach a detector can be used to determine how far
away a boundary is
o This is because ultrasound travels at different speeds through different media
• This is by using the speed, distance, time equation
 • Where:
o v = speed in metres per second (m/s)
o s = distance in metres (m)
o t = time in seconds (s)
• This allows ultrasound waves to be used for both medical and industrial imaging
Ultrasound in Medicine
• In medicine, ultrasound can be used:
o To construct images of a foetus in the womb
o To generate 2D images of organs and other internal structures (as long as they
are not surrounded by bone)
o 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 reflected back to the transducer by boundaries between tissues
in the path of the beam
o For example, the boundary between fluid and soft tissue or tissue and bone
• 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 echo’s 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
Foetal Imaging Using Ultrasound
 Ultrasound can be used to construct an image of a foetus in the womb
Ultrasound in Industry
• In industry, ultrasound can be used to:
o Check for cracks inside metal objects
o Generate images beneath surfaces
• A crack in a metal block will cause some waves to reflect earlier than the rest, so will
show up as pulses on an oscilloscope trace
o 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
Cracked Surface Detection Using Ultrasound

Ultrasound is partially reflected at boundaries, so in a bolt with no internal cracks, there

should only be two pulses (at the start and end of the bolt)
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.
Answer:
Step 1: List the known quantities
•

o Speed of ultrasound, v = 6000 m/s

o 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