CBSE Class 9 Science Practical Skills – Sound
Basic Building Concepts
The important experience in our everyday life is the sensory experience, for this we are equipped with ears for the detection of sound.
Basically, all types of sounds which we experience are created by vibrating objects and propagate through a material medium as a wave, i.e. a medium transports the mechanical energy created by the vibrating particle from one point to another without shifting the material particles of the medium.
For human beings, the range of hearing is limited to frequencies 20Hz to 20kHz, whereas a dog can perceive vibration higher than 20 kHz, i.e. upto 45000 Hz.
The continuous disturbance which travels through a medium, transporting the energy from one point of the medium (i.e., the point where the disturbance is created) to the other point without being shifted the material is called a wave.
Nature of Sound Wave
Sound waves are the longitudinal waves because the vibration of the particle in a medium is parallel to the direction of propagation of the wave. As a result compression and rarefaction regions are formed. These involve changes in volume and density of the medium.
Compression: The regions where the particles of the medium become closer during vibration, i.e. high pressure regions are called compressions.
Rarefaction: The regions where the particles of the medium are farther apart from each other, i.e. low pressure regions are called rarefactions.
Characteristics of Wave Motion
- Only the disturbance created by the source in a medium travels through the medium. Medium does not move as a whole.
- Only energy is transported from one point to another point in a medium.
- There is no transfer of the matter through the medium.
- Particles of the medium do not travel during the propagation of the wave from one point to another.
Characteristics of the Medium for the Propagation of the Wave
- Medium should be elastic to store the potential energy.
- Medium should possess the inertia or mass to store the kinetic energy.
- Medium should have uniform density.
Due to these properties, medium can offer an opposition to change in volume. All the media, solids, liquids and gases possess these properties. Therefore, longitudinal waves can be propagated through all of them.
Propagation of Sound Waves in Air
Sound is emitted by the vibrating source. A vibrating source produces sound waves in the surrounding medium. As the wave moves, the air particles which are in contact with the vibrating source are first displaced from their equilibrium position, exert a force on the neighbours causing them to be displaced from their equilibrium position and come back to their original position. This process is continued. As a result, high air pressure regions called compressions and low air pressure regions called rarefactions are formed in the air, thereby producing a sensation of hearing in our ears.
Speed of Sound
The speed of sound depends on the nature of the medium through which the waves pass. It is proportional to the square root of the
- ratio of the elasticity of the medium to its density
- temperature of the medium
Presence of moisture increases the velocity of sound in air because the density of dry air is greater than the density of moist air.
At 20 °C temperature, the speed of sound in air is approximately 343 m/s, in water, it is 1482 m/s and in steel, it is 5960 m/s. The speed of sound in air at 0 °C is 332 m/s.
Behaviour of Sound Wave
At the interface of the two media, sound waves show four possible behaviours:
- Reflection(back into the same medium but in the different direction when it falls obliquely and obey the law of reflection same as in light).
- Refraction(moves into the second medium).
- Diffraction(bending around the obstacle without crossing the interface).
- Transmission(crossing the interface into the new material or obstacle).
Reflection of Sound
Whenever a sound wave gets reflected from an open end, there will not be any change in the disturbance propagated in a medium, i.e. compression/rarefaction will reflect and return as a compression/rarefaction. If the waves, get reflected from a fixed or rigid support end, there will be a change in the disturbance, i.e. compression will reflect and return as a rarefaction while rarefaction will reflect and return as a compression.
Thus, the disturbance which returns in the same medium but in a different direction after striking an extended surface is known as reflection of sound wave. The incident angle and reflected angle are equal. Since the wavelength of sound wave is very large as compared to the wavelength of light wave. Therefore, for reflection of sound waves:
- Extended surface is needed.
- Size of obstacle should be large.
- Surface need not necessarily be smooth or polished.
- Surface may or may not be curved.
Practical Applications of Reflection of Sound Wave
The following instruments work on the principle of reflection of sound:
- Megaphones or loudspeakers have horn-shaped openings.
- Ear trumpet or hearing aid devices.
- Concave reflecting soundboards.
- Stethoscopes used by doctors.
The reflection of sound also becomes important to the design of concert halls, cinema halls and auditoriums. The walls of such building is built from materials that reflect the sound towards audience.
To verify the laws of reflection of sound.
One clock or watch with a quiet tick, two similar cylindrical hollow cardboard tubes of 30-50 cm length, rough or smooth sound reflecting surface, drawing board, white paper sheet, cardboard.
When a sound wave falls on an extended surface or obstacle of large size, it gets bounced back into the same medium but in different direction. This phenomenon is known as reflection of sound.
The laws of reflection for the sound waves are:
- The angle of incidence made by the sound waves with the normal to the reflecting surface is equal to the angle of reflection in which sound waves get reflected, i.e.∠i = ∠r.
- The incident sound wave, reflected wave and the normal at the point of incidence, all lie in the same plane.
- Fix a white plane paper on drawing board with the help of drawing pin at the four corners of white paper.
- Draw a line XY along one edge of the paper. At any point P on this line, draw a normal PN.
- Using protractor, make an incident angle of 30° with the normal PN and mark it as ∠APN.
- Place a reflecting surface, such as stiff cardboard/plywood/glass slab/mirror on a line XY in upright position with the help of stand/modeling clay.
- Place another cardboard along normal act as a screen. This does not allow the direct hearing of the ticking sound of the clock.
- Arrange two cylindrical hollow cardboard tube in such a way that their open ends come close together as shown in the figure.
- Keep a clock near the open end of the tube placed on the line AP.
- Try to listen the clear and distinct ticking sound of the clock through the other tube placed on the other side of the screen by adjusting the position of the tube. Mark this position as B. Distinct sound can be heard because sound waves get reflected from the reflecting surface.
- Lift the end B of pipe PB, vertically upward to a small height. Try to hear the sound of the table-clock now through the pipe PB.
- Lift the tubes, reflecting surface and screen. Join the points B and P and measure the angle of reflection of sound wave.
- Repeat the experiment by taking different values of angle of incidence and tabulate your observations.
|S.No.||Angle of incidence of sound wave ∠i||Angle of reflection of sound wave|
|Error ∠i – ∠r|
When the end B of the pipe PB is lifted vertically upward to a small height, the sound of the table-clock is either weakened or diminished completely.
From the observations, we conclude that
- within experimental error limit, angle of incidence is always/approximately equal to the angle of reflection.
- the incident sound wave, reflected wave and the normal at the point of incidence all lie in the same plane. Hence the law of reflection of sound is verified.
- Reflecting surface and screen should be held vertically on the sheet of paper.
- Position of the ticking clock should be along the axis of the hollow cylindrical tube placed along AP.
- Mark the position of the second tube only when heard sound intensity of the ticking clock is maximum.
- Ear, ticking clock and reflecting surface should be in the same plane.
- Utmost care should be taken while taking the measurement of angle. The measurement of angle, either it is incident or reflection must be taken between the axis of the tube and the normal.
What is the aim of the experiment given to you?
The aim of my experiment is to verify the laws of reflection of sound.
How can you explain the reflection phenomenon in sound?
When a sound wave falls on a hard surface having a large size like wood, stiff cardboard, it bounces back into the same medium but in different direction. This phenomenon is called reflection of sound wave.
Suppose when you shout inside an empty hall, you hear your own sound after a short time. Why is it happen?
It happens because our sound is reflected from the walls of empty hall.
Can you choose any hard and plane wooden surface which serve as a reflector of sound?
Yes, any hard and plane wooden surface can serve as a reflector of sound waves.
The wavelength of sound wave is very large as compared to the wavelength of light wave.
What is the use of wooden screen placed between the two tubes?
It cuts off the direct sound from watch to ear.
What conclusion would you draw when the axes of both the tubes are inclined equally to the normal?
The angle of incidence (i) is equal to the angle of reflection (r).
Which fundamental law is followed by the mechanical wave?
Laws of reflection.
Which physical phenomenon is also based on reflection of sound waves?
Phenomenon of echo.
Which type of medium is necessary for the propagation of mechanical waves?
A material medium.
What should be the properties of the medium for the propagation of sound waves?
The medium must possess three properties, viz. elasticity, inertial and minimum friction, and density of the medium.
What is the physical cause that produces the sensation of hearing?
Vibrations of the source.
Give two applications in real-life where reflection of sound plays an important role?
The ear trumpt or hearing aid, reflecting boards fixed inside the walls of large movie hall/auditorium.
In which form longitudinal wave motion travels in a medium?
It travels in the form of compression and rarefaction in the medium.
‘What are the changes involve in a medium during the propagation of longitudinal waves?
Change in volume and density of the medium.
Why the sound waves cannot travel through air in the form of transverse waves?
Because crest and trough cannot be sustained in air.
NCERT Lab Manual Questions
While performing this experiment, why do we prefer to use pipes of larger length but of smaller diameter?
In this experiment, it is assumed that sound source is directional. Practically, due to the presence of unwanted sound in our surroundings, sound coming from source will not remain unidirectional. Therefore, it is advised to use the pipes of larger length but of smaller diameter.
How the experiment of reflection of sound is different from the experiment on laws of reflection of light?
For the reflection of sound wave, the surface may be polished or rough while the reflecting surface for light wave should be smooth and well polished.
Which sheet will you choose as sound reflecting surface for this experiment;
(a) A smooth wooden board or (b) A thermocol sheet? Why?
A smooth wooden board will be chosen as sound reflecting surface because reflection from this surface will be more. Thermocol sheet acts as a good absorber of sound.
Suppose the whole experimental set-up of this experiment is submerged in water. What changes do you expect in your observations?
The speed of sound in water at 20 °C is 1482 ms-1 while in air it is 343 ms-1 at 20 °C. Therefore, when the whole experimental set-up is submerged in water, the reflected sound will be heard faster as compared to air.
Why do we require a low amplitude sound source in this experiment?
If we take the high amplitude sound source, then we may hear the sound from the source to our ear directly and not through the reflecting ray pipe. Therefore, it is required to have a low7 amplitude sound source in this experiment.
What alterations can be made in the pipes to make the reflected sound more distinct and clear?
In order to absorb all unwanted sound rays from our surroundings and to make the reflected sound more distinct and clear, the inner surfaces of two pipes should be painted black and kept rough. A rough paper is a good absorber of sound so when we use plastic pipes, a layer of newspapers may be inserted into these pipes.
Practical Based Questions
Multiple Choice Questions/VSA (1 Mark)
Which of the following is the correct experimental set-up for verification of laws of reflection of sound? [CBSE 2010]
In the experiment of verification of reflection of sound, the incident sound is directed along
(a) axis of tube
(b) normal to the axis of tube
(c) at an angle of 30° from the axis of the tube
(d) at an angle of 45° from the axis of the tube.
While verifying the laws of reflection of sound, out of following precautions which one should not be followed? [CBSE 2011]
(a) The table top should be horizontal.
(b) Length of pipes should be sufficiently long.
(c) The reflecting surface should be smooth and hard.
(d) Ear should not be placed close to the pipe.
While doing experiment on verifying the law of reflection of sound, four students measured the angles ∠i and ∠r as shown in the diagram below. The correct measurement of the angle of incidence and angle of reflection has been done by student [CBSE 2011] [CBSE 2010, CBSE Sample Paper]
A student while verifying the laws of reflection of sound measured the angle between the incident sound wave and reflected sound wave as 130°. The angle of reflection is [CBSE 2012]
The sound of a stopclock received in the ear while doing the experiment of ‘Reflection of sound’ should be [CBSE 2011][NCT 2009]
(a) the direct sound from the clock.
(b) the sound reflected from the ceiling of the laboratory.
(c) the sound coming through the pipe after reflection from the reflecting surface.
(d) both (a) and (b).
A student performs the experiment of ‘reflection of sound’. He/she records following values of the angle of incidence and angle of reflection. [CBSE 2011]
|S.No||Set 1||Set 2||Set 3||Set 4||Set 5|
|Angle of incidence →||3o°||40°||45°||50°||60°|
|Angle of reflection →||60°||50°||45°||30°||60°|
The set correctly recorded by him/her are
(a) Set 1 and set 3 respectively.
(b) Set 1 and set 5 respectively.
(c) Set 2 and set 4 respectively.
(d) Set 3 and set 5 respectively.
In order to hear an echo of a sound wave, the minimum distance between the sound producing body and the reflecting surface should be (at normal temperature)
(a) 20 m
(b) 17.2 m
(c) 21.6 m
(d) 24.0 m
When the sound wave propagates, which physical quantity changes in the medium?
(a) Only density
(b) Only pressure
(c) Both density and pressure
(d) Only frequency.
For doing the experiment on verifying the laws of reflection of sound, a student sets up his apparatus as shown. The experiment is more likely to be performed successfully if the screen shown is a [CBSE Sample Paper II]
(a) well polished plane mirror
(b) wooden board with many holes in it
(c) a foam padded board
(d) a sheet of pure white cloth.
If the reflecting surface and the source of sound both are in the plane of the table, the position of the ear of the observer should be
(a) on the same plane
(b) on the plane at an angle of 30° to the table.
(c) on the plane at an angle of 90° below the table
(d) perpendicularly above the source.
Bats search out prey and fly in dark night by
(a) emitting and detecting reflection of ultrasonic wave
(b) emitting and detecting retraction of ultrasonic wave
(c) emitting and detecting reflection of infrasonic wave
(d) all of the above.
Ramjeet, while verifying the laws of reflection of sound, measured the angle between the incident sound wave and reflected sound wave to be 130°. The angle of incidence is [NCT 2008]
(a) Data insufficient
The angle of reflection in the figure depicted is
(c) can be 60° or 30°
(d) all options are wrong.
The given figure represents the graph between the angle of incidence and angle of reflection for a sound wave. From this graph, we can conclude that
(а) Angle of incidence decreases with the increase in angle of reflection
(b) Angle of incidence increases with the decrease in angle of reflection
(c) Angle of incidence is always equal to angle of reflection
(d) Angle of incidence sometimes increases and sometimes decreases, depending upon the loudness of sound.
Two students while performing experiment on verification of laws of reflection of sound are provided with the following choices:
- Using a narrow tube or wide tube
- Using a faint source of sound or strong source of sound.
The best results would be obtained by the student using the combination of:
(a) narrow tube and faint source of sound
(b) narrow tube and strong source of sound
(c) wide tube and faint source of sound
(d) wide tube and strong source of sound.
Short Answer Questions (2 Marks)
In the experiment, to verify the laws of reflection of sound, the tube facing the clock makes an angle equal to 40° with the reflecting surface. What will be the position of the second tube with respect to the normal, at which the ear will get the best reflected sound?
For perfect reflection, ∠i = ∠r
Here, 40° + ∠i = 90°
⇒ ∠i = 90° – 40° = 50° = ∠r
Hence, second tube should be placed at angle of 50° with the normal.
Why is it important to have a smooth reflecting surface in the experiment, to verify the laws of reflection of sound?
When sound wave is incident on any surface, some part of it is reflected and some part gets absorbed by that surface. For a smooth reflecting surface, the reflection will be more. Hence the sound that you hear through the pipe depends on the nature of the reflecting surface and thus, it is important to have a smooth reflecting surface.
How can you show experimentally in this experiment that the reflected ray lies in the same plane of the incident ray?
When the pipe, through which the sound is being heard, is lifted vertically to a small height, the sound from the low-amplitude source such as table-clock or mobile phone is either weakened or diminished completely. It shows that the reflected ray lies in the same plane of the incident ray.
While performing the experiment, to verify the laws of reflection of sound, it is advisable to put the fan off. Why?
In this experiment, it is required to hear very low- amplitude sound waves, so peaceful atmosphere is the necessary condition for performing the experiment. To hear such sounds clearly, it is further advised to put the fan off.
Why are curved sound boards and sound absorbent materials used in halls and auditoriums? On which phenomenon of sound it is based?
Curved sound boards spread sound uniformly to all places of the hall and auditorium. Draperies and other sound absorbing materials reduce the persistence of sound in hall so that original sound is clearly heard. It is based on the multiple reflections of sound waves from the surface.