SOUND WAVES
Sound is produced by any object that vibrates mechanically.
An object that is not vibrating cannot produce sound. Sound is therefore, classified as a wave
because like all other waves it originates from vibrations.
Transmission of sound: sound from a tuning fork
 An undisturbed tuning fork does not produce any sound. When a tuning fork is hit, its
prongs vibrate outwards and inwards continuously.
 As the prongs of the tuning fork move outwards, they give air a push. This compresses the
air and thus the push gets passed on to the neighboring particles.
 This region of compressed particles is called a compression. It is a region of high pressure.
 As the prongs move inwards, a space is left where there are few air particles. This region
is called a rarefaction. It is a region of low pressure.
 As the tuning fork goes on vibrating, waves consisting of alternating compressions and
rarefactions spread in air as shown above. The direction of motion of sound waves is the
same as that of air particles [the transmitting medium], hence sound is classified as a
longitudinal wave.
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 Since sound waves consists of a repeating pattern of high pressure (compressions) and low
pressure (rarefactions) regions moving through the medium it is sometimes referred to as
a pressure wave.
The range of audible frequency
 We have seen that sound waves are produced when an object vibrates.
 The number of vibrations per second is called the frequency of vibrations (measured in
Hz).
 Different sounds have different frequencies.
 The human hear can only detect sounds of frequencies between 20 Hz and 20 000 Hz (20
KHz). These limits are known as limits of audibility. Humans cannot detect sounds that
have frequencies outside this range.
 Sound with frequencies less than 20 Hz is called infrasound and sound with a frequency
above 20 000 Hz is known as ultrasound.
Sound and the medium
We know that sound travels through air because the air particles can move. The question now is:
will sound travel through a region where there are no particles like a vacuum.
Experiment: To investigate if sound travels through a vacuum.
1) Suspend an electric bell from wires passing through the cork in the bell jar as shown below.
2) Pass a tube through the cork and connect it to a vacuum pump.
3) Before starting the pump, close the switch to the electric bell so that the bell starts ringing.
4) Start pumping the air out of the bell jar using a vacuum pump while the bell is still in operation.
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Observation
As the air is removed from the bell jar by the vacuum pump the sound becomes fainter and fainter
until it is not heard although the bell can still be seen to be working.
If air is readmitted into the bell jar, the sound becomes louder once more.
Conclusion
Sound does not travel through a vacuum but does so in a place with matter.
Comparison of the velocity of sound in different states of matter.
Factors that affect the speed of sound in different states of matter
Sound can travel through solids, liquids and gases. This is because all the three states of matter are
made up of particles.
The speed of sound in the three states of matter is affected by two main factors and these are
elasticity and density.
Elasticity
In elastic materials, the atoms are relatively close together. Therefore, they respond quickly to each
other's motions and transmit energy with little loss. Sound travels about fifteen times faster in steel
than air, and about four times faster in water than in air.
Density
In materials in the same phase of matter, the speed of sound is lower in the denser material.
Because the denser medium has greater mass in a given volume, it has more inertia. Its
particles
do
not
move
as
quickly
as
those
of
the
less
dense
material.
The speed of sound in dense metals such as Lead and Gold is much less than the speed
of sound in Steel or Aluminium. Lead and Gold are also less elastic - another reason why
the speed of sound is slower in these metals.
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Factors that affect the speed of sound in air
Two factors affect the speed of sound in air and these are temperature and humidity.
Temperature
Changes in temperature affect the speed of sound in air (or indeed in any gas). The gas laws
indicate that the ratio (pressure/density) is proportional to the kelvin temperature, T. Thus, the
speed of sound is proportional to T. It is easier to break the sound barrier at high altitudes because
the temperature is lower.
Humidity
Changes in humidity also affect the speed of sound. The density of water vapour is less than the
density of dry air at the same pressure. At night, when the humidity tends to rise, sounds travel
faster. Sounds can be heard more clearly on a quiet misty night. This is partly also for the increased
humidity, and partly because under such conditions, there is usually a temperature inversion which
tend to refract the sounds so that they do not escape.
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Determination of the speed of sound in air.
The two methods used to determine the speed of sound in air are:
1. Using a sound and light-emitting object like a pistol.
2. Using the clap and echo method.
To determine the speed of sound using a sound and light-emitting object.
Requirements: a pistol containing blank rounds, stopwatch, measuring tape and two individuals.
Procedure:
1. One individual stands at one end of a plane playing field with a starting pistol and another
individual stands at the other end of the playing field with a stopwatch. The distance
between the two individuals should be more than one kilometer.
2. The individual with a pistol signals to the other one on the other side of the field that he is
about to fire and fires.
3. As soon as the the flash from the pistol is seen, the person with a stopwatch starts it and
stops it when he hears the sound of the pistol. The stopwatch therefore measures the time
taken for sound to travel from the gun to the observer.
Note: this method has the disadvantage of being influenced by the wind and the observer’s
reaction time (the time it takes for the person starting and stopping the stopwatch to do so).
4. To counteract the effect of wind the experimenters exchange positions and repeat the
procedure.
5. The speed of sound is finally calculated using the formula
Speed of sound =
𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑡𝑟𝑎𝑣𝑒𝑙𝑙𝑒𝑑
𝑡𝑖𝑚𝑒 𝑡𝑎𝑘𝑒𝑛
Examples: velocity of sound in air using a sound and light-emitting object.
1. An observer watching a fireworks display sees the light from an explosion and hears the
sound 2 s later. How far was the explosion from the observer? [The velocity of sound in
air is 360 m/s.]
Solution
Speed = 360 m/s.
t = 2 s.
V =?
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Velocity of sound =
360 m/s =
𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑡𝑟𝑎𝑣𝑒𝑙𝑙𝑒𝑑
𝑡𝑖𝑚𝑒 𝑡𝑎𝑘𝑒𝑛
𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒
2𝑠
Distance = 360 m × 2
= 720 m.
∴ the explosion was 720 m away from the observer.
2. A man sees steam start to come from a factory whistle and 3 s later he hears the sound. The
velocity of sound in air is 360 m/s. calculate the distance from the man to the whistle.
Solution
Velocity of sound =
360 m/s =
𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑡𝑟𝑎𝑣𝑒𝑙𝑙𝑒𝑑
𝑡𝑖𝑚𝑒 𝑡𝑎𝑘𝑒𝑛
𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒
2𝑠
Distance = 360 m × 3
= 1080 m.
Exercise
1. (a) Describe how you would find experimentally a value for the speed of sound in air using
thunder flash fireworks as the source of sound.
(b) State the principle source of error in the experiment and show how they could be produced.
(c) An explosion is made at the end of steel tube 1.6 Km long, and two sounds are heard at the
other end with an interval of 4.4 seconds between them. If the speed of sound in air is 340 m/s,
calculate the speed of sound in steel.
2. The speed of light in air is 3.0 × 108 m/s. the speed of sound in air is 0.340 Km/s. An observer
is 5.0 Km away from a lightning discharge.
(a) Calculate the travel time, to the observer of
(i) Light from the lightning flash.
(ii) Sound from the thunder.
(b) What is the time interval between the observer seeing the lightning and hearing the
thunder?
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Determining the speed of sound using echo method
-It is shown by experiment that if a person standing at a distance of more than 17m away from a
wall clap his or her hands, an echo will be heard. No echo is heard when the distance of separation
between the source of sound and the listener is less than 17m.
Definition; An echo is a reflected sound.
Expt: To measure the speed of sound using echoes.
Materials: measuring tape, a wall, two blocks of wood, and two people (one with a stopwatch and
the other one with two blocks of wood).
Procedure
1. Measure a distance of 100m at right angles to a large wall.
2. One person with two blocks of wood hits the two blocks of wood to produce a sharp clapping
sound. The person continues hitting the two blocks of wood at regular intervals until the echo
concides with the next clap of the wooden blocks.
3. When this has been achieved the other person with a stopwatch starts counting claps at zero as
he/she starts a stopwatch. Let him/her count the number of claps and stops the stopwatch at 50 or
100 claps.
4. Calculate the speed of sound using the formula
Speed =
𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑡𝑟𝑎𝑣𝑒𝑙𝑙𝑒𝑑 [𝑖.𝑒 2 ×𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑓𝑟𝑜𝑚 𝑎 𝑤𝑎𝑙𝑙]
𝑡𝑖𝑚𝑒 𝑡𝑎𝑘𝑒𝑛
Note; when using the echo method in measuring the speed of sound in air;
i.
The distance travelled by the sound to the reflecting wall and back to the listener is doubled
[i.e. is multiplied by 2]. For example, suppose a person stands 50m in front of a wall and
gives a single clap. When the echo is heard the sound has travelled 100m [i.e. 50m × 2].
ii.
To obtain the time taken to hear one clap, the stopwatch reading must be divided by the
number of clap counted. i.e.
𝑠𝑡𝑜𝑝𝑤𝑎𝑡𝑐ℎ 𝑟𝑒𝑎𝑑𝑖𝑛𝑔
Time taken for one clap = 𝑛𝑢𝑚𝑏𝑒𝑟 𝑜𝑓 𝑐𝑙𝑎𝑝𝑠 𝑐𝑜𝑢𝑛𝑡𝑒𝑑
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Examples
1) A student standing 200m from a high wall claps two pieces of wood together. He makes 100
claps within a time of 121.2s. What is the velocity of sound?
Distance travelled = 2 × 200m = 400m
Time taken for one clap =
121.2𝑠
100
= 1.212s
Speed = ?
Velocity =
Velocity =
𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑡𝑟𝑎𝑣𝑒𝑙𝑙𝑒𝑑
𝑡𝑖𝑚𝑒 𝑡𝑎𝑘𝑒𝑛
400𝑚
1.212𝑠
= 330m/s
Exercise
1) Describe an experiment you would carry out to determine the speed of sound in air using
the echo method. Make a list of apparatus you would use.
2) A student stands at a distance of 400m from a wall. The student claps two pieces of wood.
After the first claps the student claps whenever an echo is heard from the wall. Another
student starts a stopwatch at the first clap and stops it after the 20th clap. The stopwatch
records a time of 50s. Find the speed of sound. Ans. 304m/s
3) The echo of the blast of the horn of a ship near a coast with high cliffs was heard after one
and a half seconds. About how far was the ship from the cliffs? [Speed of sound in
330m/s].
4) A man standing 100m from a high wall blow a whistle. He hears the echo 1.1s later.
Calculate the velocity of sound from these observations.
5) The figure below shows a metal which is 2.4 m long being struck by a sharp blow at one
end using a light hammer. The time interval between the impact of the hammer and the
arrival of the sound wave at the other end is measured electronically.
DIGRAM EXERCISE BOOK Q5
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(a) Four measurements of the time interval are 0.44ms, 0.50ms, 0.52ms and 0.47ms.
(i) Determine the average value for the four measurements.
(ii) Calculate the speed for the value of sound in the rod.
(b) State the approximate value for the speed of sound in air.
Characteristics of sound waves
The sound we hear differ from each other in a number of ways. Three of these number of ways are
loudness, pitch and quality.
Loudness of sound
Loudness refers to the intensity of sound as perceived by our sense of hearing. The intensity of a
sound depends on the amount of movement producing it. The amplitude of the vibration
determines how loud the sound will be. The higher the amplitude of vibration the louder will be
the sound and vice versa. Thus loudness of sound depends on the amplitude of vibrations.
Pitch of sound
Pitch is how high or low sound is characterized by frequency producing it. Pitch indicates whether
sound is high or low. The pitch of a sound is the number of vibrations produced in one second
from a source of sound.
Pitch depends on the frequency: the higher the frequency the higher the pitch and the lower the
frequency the lower the frequency.
Intensity of sound
Intensity of sound is the power (rate of transfer of energy) of a sound wave unit area. It is a measure
of strength of sound per unit area in space.
Quality of sound
Pupils’ assignment: Quality of sound
1. What is meant by quality of sound?
2. Discuss the factors that affect the quality of sound.
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Ultrasonic waves (Ultrasound)
Recall that the normal human ear is capable of detecting sound within a frequency range of 20Hz
to 20 000Hz.
Ultrasound is sound with frequencies higher than 20 000Hz. These are sounds above the range of
normal human hearing.
Applications of ultrasound
1. In pre-natal scanning
In this process, ultrasonic waves are directed into the body using an ultrasonic device. The
sound waves echo back to the receiver, and are translated into an image by the computer. Thus,
doctors can watch the movement of the foetus on the screen of a computer.
2. In cleaning objects
Delicate objects such as jewellery, Lenses and surgical instruments are cleaned using
ultrasound. The item to be cleaned is placed in a liquid and the ultrasonic pulses vibrate the
dirt and impurities off the item.
Dentists and dental hygienists use ultrasound to clean hardened plague from the patient’s teeth.
3. To detect hairline fractures in metals
When ultrasound is sent to a metal flawless metals will return only one echo for each ultrasound
pulse, but two or more if a metal has fractures.
4. Sonar
Bats use an important use of reflected sound. Bats live and are most active in total darkness,
yet they can reach the desired target with pinpoint accuracy (and they never bump into the
walls of the curve!) How do they manage this?
Bats send high-frequency sound (ultrasonic) waves as they fly. The waves bounce off objects
such as walls or insects and reflect back to the bat. Bats use echoes to determine the location
of their food and to navigate around the black interior of their curve.
In this way, bats are similar to many ships and submarines. Ships also use sound waves to
navigate and to locate objects in the dark depths of the ocean.
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Suppose a ship cargo sinks in the middle of the ocean, the water is far too deep to send divers
down in search of the lost merchandise. High-frequency ultrasonic waves are used in a system
called sound navigation and ranging or sonar.
Sonar devices are often used in commercial fishing to locate large schools of fish.
Exercise
1. Explain, briefly how ultrasound is used in prenatal scanning.
2. The captain of a fishing board uses an echo sounder to determine the depth of a shoal of
fish below the boat. The captain finds that the reflected waves return after 0.15s and 0.20s.
The captain believes the first reflection to be from the fish, while the second is from the
ocean floor. If the speed of sound in water is 1440m/s, determine:
(a) the depth at which the fish are.
(b) the distance to the ocean floor from the ship.
Noise pollution
Some sounds called noises, ca be annoying. Noises can damage the ears, cause tiredness, loss of
concentration and temporary or permanent deafness.
Room acoustics
Acoustics is the science and technology of sound and its effect on people. Acoustic is also
concerned with the quality of sound heard inside buildings. It plays an important role in the design
of music halls, auditoriums, churches, libraries and industries.
When sound is generated in a room by a sound source, a listener who is seated in a room not only
receives the direct sound from the source but also receives multiple reflections of sound waves
from the ceiling, floor, walls and so on. In other words, the sound emitted from the source does
not cease immediately after it is directly received but is lengthened out owing to the received by
continuous reflections from successive portions of ceiling, floor walls and so on.
Thus, the original wave on breaking up wanders about for some time in the room. This is called
prolongation of sound waves or reverberation.
Definition
Reverberation is the persistence or prolongation of audible sound waves after a source has ceased
to emit sound.
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If reverberation of sound persists for a long duration, the sound in the room will be confused. The
acoustic conditions of the room are said to be bad owing to the generation of echoes in the room.
On the other hand the reverberation should not be too small so as to give a dead effect when no
sound is heard at all. Generally, reverberation of a sound note should take about 0.2 s to die out.
Minimizing sound pollution inside buildings
Acoustics in buildings
Sound reflects from all surfaces-the walls, ceiling, floor, furniture and people in a room. Designers
of interiors of buildings, whether office buildings, factories, or auditoriums, need an understanding
of the reflective properties of sound. If the walls of a room are too reflective the sound becomes
confused. This is due to multiple reflections called reverberation. On the other hand if the surfaces
are too absorbent, the sound levels would be too low and the hall would sound low and lifeless.
The acoustic quality of a room is affected by

the size and shape of a room.

the ability of the building materials to absorb unwanted sound.

the way the room reflects sound.
Care has to be taken about the nature and arrangement of absorptive materials in a hall to provide
the best conditions for the growth, decay and steady distribution of any reflected sounds. This leads
to satisfactory hearing and appreciation of whatever function or program taking place. In the design
of auditoriums and concert halls, a balance between reverberation and absorption is desired.
The walls of concert halls are often designed with grooves so that the sound waves are diffused.
In this way a person in the audience receives a small amount of reflected sound from several parts
of the wall rather than a larger amount of sound coming from one part of the wall.
Soundproofing.
Most buildings are designed in the following ways to reduce sound:

Use of thick walls such as concrete and bricks.

Careful ceiling around doors and openings stops sound from leaking through cracks.

Thick curtains and carpets help absorb reflected sound on floors and walls.
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General measures to minimize sound pollution

Planting more number of trees around public places will help in sound absorption
and reduce the deleterious effects.

People working in industries or noisy areas need to wear earplugs, ear muffs or noise
helmets to protect from occupational noise hazardous.
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