Sound
Sound waves are longitudinal. Humans hear sounds from 20 - 20 000 Hz. Higher frequencies
are called ultrasound. Oscilloscopes trace sound waves.
Properties of sound waves and comparing speed of waves
Sound waves are longitudinal waves which means they vibrate in the same direction as the
direction of travel. They are produced by vibrating sources, such as speakers. Sound waves
can only travel through a solid, liquid or gas medium. They travel fastest in solids, then
liquids and slowest in gases. A good model for sound waves is a spring.
The sections of the sound waves where the particles are pushed together are areas of
compression. The sections of the sound waves where the particles are further apart are areas
of rarefaction.
Sound waves can be reflected to form an echo, refracted and diffracted.
Human hearing and the speed of sound
The range of human hearing
The number of waves per second is measured in Hertz (Hz). Sounds in the normal range of
human hearing are between about 20 Hz and 20,000 Hz (20 kHz), but the range becomes less
as we get older. Sounds with frequencies above about 20 kHz are called ultrasound.
The speed of sound
The speed of sound in air is about 340 m/s. This is much less than the speed of light in air
which is about 300,000,000 m/s. This explains why we see lightning before hearing thunder.
The speed of sound in water is about 1560 m/s. The speed of sound in solids varies depending
upon the solid. It is about 1600 m/s in rubber and about 5000 m/s in steel.
The speed of sound in air is often measured using the following method:
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two people (person A and person B) are placed a distance apart, eg 100 metres
person A fires a starter’s pistol
person B times the difference between seeing the flash of the gun and hearing the sound this is measured in seconds
The speed of sound can be calculated using this equation:
Ultrasound
Ultrasound is the name given to sound waves that have frequencies greater than 20,000Hz (20
kHz). This is above the normal hearing range for humans, so we cannot hear ultrasound.
Reflections
When ultrasound waves reach a boundary between two substances with different densities,
they are partly reflected back. The remainder of the ultrasound waves continue to pass
through. A detector placed near the source of the ultrasound waves is able to detect the
reflected waves. It can measure the time it takes for an ultrasound wave to leave the source,
and bounce back to the detector. The further away the boundary, the longer the time between
leaving the source and reaching the detector.
distance is measured in metres (m)
speed is measured in metres per second (metre/second, m/s)
time is measured in seconds (s)
Worked example
Sound travels through water at about 1,400 m/s. If it takes 0.5 s for a sound to reach a
boundary and reflect back to the detector, the total distance travelled is:
distance = speed × time
= 1,400 × 0.50
= 700 m
The distance to the boundary is half this, which is 350 m.
Imaging
Computers are able to create detailed images by
combining many ultrasound reflection readings.
This is used in medicine for pre-natal scanning
(checking unborn babies).
When ultrasound is used for scanning babies, the
distances and times are much smaller than those
in the example above.
3D ultrasound scan of a human foetus
Ultrasound can be used in industry for quality control procedures to check manufactured
objects, such as railway tracks and oil pipelines, for damage or defects. The diagram shows
how a piece of metal may be tested for cracks or other flaws using ultrasound:
Sonar
Sonar is used on ships and submarines to detect fish, other vessels or the sea bed. A pulse of
ultrasound is sent out from the ship. It bounces off the seabed or shoal of fish and the echo is
detected. The time taken for the wave to travel indicates the depth of the seabed or shoal of
fish.
Oscilloscopes and sound
Sound waves are longitudinal waves. Their vibrations occur in the same direction as the
direction of travel. Sound waves can only travel through a solid, liquid or gas.
When an object or substance vibrates, it produces sound. The bigger the vibrations, the greater
the amplitude and the louder the sound.
An oscilloscope is a machine that shows the wave shape of an electrical signal. When
connected to a microphone they can show the wave shapes of sounds.
Oscilloscope traces
A microphone converts sound energy into electrical energy in the form of electronic signals.
A computer or an oscilloscope can be used to display these electronic signals, which show the
same changes in amplitude and frequency as the sound waves.
When these signals are observed on the oscilloscope, the oscilloscope pattern will indicate the
same changes in amplitude and frequency which correspond to the wave's loudness and pitch
(frequency).
Volume (amplitude) – shown by the height of the waves displayed. The larger the amplitude
of the waves, the louder the sound.
Pitch (frequency) – shown by the spacing of the waves displayed. The closer together the
waves are, the higher the pitch of the sound.
The loudness of a sound is a measure of the amplitude of the wave. The greater the amplitude,
the louder the sound.
The pitch of a sound is a measure of the frequency of the wave. The higher the frequency, the
higher the pitch.
These diagrams show snapshots from oscilloscope traces of three sounds.
Diagrams 1 and 2 show two sounds with the same frequency (wave spacing) but different
amplitude (the height of the trace).
The trace on 1 comes from a sound with a smaller amplitude than on 2. Sound 1 is quieter
than sound 2.
Diagrams 2 and 3 show two sounds with the same amplitude but different frequencies (wave
spacing). The faster the vibrations, the higher the pitch (frequency) of the sound.
The trace on 3 comes from a sound with a higher frequency than the one on 2. So sounds 2
and 3 are the same volume (amplitude), but 3 has higher pitch (frequency).
These diagrams below show oscilloscope traces of three sounds:
Sounds 1 and 2
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The sound waves have the same frequency, so the sounds have the same pitch.
Sound 2 has a greater amplitude than sound 1, so sound 2 is louder.
Sounds 2 and 3
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The sound waves have the same amplitude, so the sounds have the same loudness.
Sound 3 has a greater frequency than sound 2, so sound 3 is higher pitched.
The frequency of a sound is the number of oscillations (waves) per second and is measured in
hertz (Hz). It can be calculated by:
The time period is the time taken to complete one oscillation and is measured in seconds. So
if the time period of a sound wave is 0.01 seconds, then the frequency is calculated as:
frequency = 1 ÷ 0.01
= 100 hertz
This would make a low pitched sound (low frequency) as this is towards the bottom of our
audible range.
Analogue and digital signals
Communications signals can be analogue or digital.
Analogue signals
Music and speech vary continuously in frequency and amplitude. In the same way, analogue
signals can vary in frequency, amplitude, or both. FM (Frequency Modulated) and AM
(Amplitude Modulated) are two types of radio signals. The diagram shows a typical
oscilloscope trace of an analogue signal.
Digital signals
Digital signals are a series of pulses consisting of two states: ON (1) or OFF (0). There are no
values in between. DAB (Digital Audio Broadcast) radio is transmitted as digital signals. The
diagram shows a typical oscilloscope trace of a digital signal.
Data transmission
Digital signals maintain their quality better than analogue signals.
Noise
All signals become weaker as they travel long distances. They may also pick up random extra
signals. This is called noise. It is heard as crackles and hiss on radio programmes. Noise
affects both analogue and digital signals.
Analogue signals
Noise adds extra random information to analogue signals. Each time the signal is amplified,
the noise is also amplified. Gradually, the signal becomes less and less like the original signal.
Eventually, it may be impossible to make out the music in a radio broadcast against the
background noise, for example.
Digital signals
Noise also adds extra random information to digital signals. However, this noise is usually
lower in amplitude than the amplitude of the ON states. As a result, the electronics in the
amplifiers can ignore the noise and it does not get passed along. This means that the quality of
the signal is maintained, which is one reason why television broadcasters have changed from
analogue to digital and radio broadcasters are in the process of changing.
Because digital signals can carry more information per second than analogue signals, higher
quality programmes or more channels can be broadcast. Another advantage of digital signals
is that information can be stored and processed by computers.
This slidesdemonstrates how the amount of noise affects analogue and digital signals:
Answers
1- Sound waves are longitudinal waves. They vibrate in the same direction of travel.
2- Sound waves travels fastest through solids because the particles are closest together.
3- The areas where particles are pulled further apart in sound waves are called areas of
rarefaction.
4- Sounds above the human range of hearing are called ultrasound. This is about 20,000
hertz.
5- The speed of sound in air is about 340 m/s. The speed of light is 300,000 km/s.
6- Waves travel fastest in light, then sound and finally water. So light waves are faster
than sound.
7- The correct equation for speed is speed = distance ÷ time.
8- SONAR is used in fishing boats to identify shoals of fish to catch.
9- The units of frequency are hertz.
10- Frequency is the correct scientific term for pitch. This is measured in hertz.
Answers:
1- To measure the speed of sound in air we must measure the time it takes for a sound to travel
a measured distance.
2- We can use an oscilloscope to produce an image of sound waves.
3- The pitch of a sound is the frequency of the wave.
4- Humans can hear sounds in the range 20Hz to 20 000Hz .
5- When the sound level of noise rises to higher levels, we refer to this as noise pollution.
6- Sound levels are measured on a scale known as the decibel (dB) scale.
7- Exposure to 90dB sound levels for a long time can cause permanent hearing loss.
8- The sound level of a pneumatic drill 5 metres away is 100dB.
9- Ultrasounds are high frequency vibrations beyond the range of human hearing.
10- Ultrasound is used in medicine to break up kidney stones and gall stones.
3.3.2 Sound & Oscilloscopes
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Sound & Oscilloscopes
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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
Oscilloscopes have lots of dials and buttons, but their main purpose is to display and
measure changing signals like sound waves and alternating current
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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 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
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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