Class 9 Science Chapter 11 Sound NCERT Notes

Chapter 11 Sound Class 9 Science CBSE NCERT Notes

Sound energy is a form of energy because of which our ears are able to hear something. One cannot create sound or destroy it. But one can transform one form of energy into sound energy.

Sound is produced due to the vibration of objects. The motion of materials or objects causes vibration.

Vibration is a kind of rapid to and fro motion of an object about a central position. It is also referred to as oscillation. For example, a stretched rubber band when plucked vibrates and produces sound.

Propagation of sound

The sound produced by a vibrating object travels through a medium to a listener. The medium can be solid, liquid or gas. When an object vibrates, the particles around the medium vibrates.

The particle in contact with the vibrating object is first displaced from its equilibrium position. It then exerts a force on the adjacent particle and the adjacent particle is displaced from its position of rest.

After displacing the adjacent particle the first particle comes back to its original position. This process repeats in the medium till the sound reaches the ear.

The disturbance produced by the vibrating body travels through the medium but the particles do not move forward themselves.

A wave is a disturbance which moves through a medium by the vibration of the particles of the medium. So sound is considered as a wave. Since sound waves are produced due to the vibration of particles of the medium sound waves are called mechanical waves.

Propagation of sound through air

Air is the most common medium through which sound travels. When a vibrating object moves forward, it pushes and compresses the air in front of it forming a region of high pressure called compression (C). The compression moves away from the vibrating object.

When the vibrating object moves backward, it forms a region of low pressure called rarefaction (R). As the object moves to and fro rapidly, it produces a series of compressions and rarefaction in the air which makes the sound to propagate in the medium.

A vibrating object creating a series of compressions (C) and rarefactions (R) in the medium.

Sound needs a medium to travel

A medium is necessary for the propagation of sound waves.

The matter or substance through which sound is transmitted is called a medium. The medium can be solid, liquid or gas.

Sound cannot travel in vacuum. A true vacuum refers to the complete absence of matter. Sound waves can travel only through matter. So, sound needs a physical medium in order to travel anywhere.

Wave that requires medium to propagate is called Mechanical Wave. For example, sound wave. Sound cannot travel in the absence of a medium.

Activity to show that sound needs a material medium for its propagation

Suspend an electric bell in an air tight bell jar. Connect the bell jar to a vacuum pump. If the switch is pressed, we can hear the sound of the bell. If air is pumped out through the vacuum pump, we cannot hear the sound of the bell. This shows that sound needs a medium to travel and sound cannot travel in vacuum.

Bell jar experiment showing sound cannot travel in vacuum

Sound waves are longitudinal waves

Sound propagates in a medium as a series of compressions (C) and rarefactions (R).

In these waves the particles move back and forth parallel to the direction of propagation of the disturbance. Such waves are called longitudinal waves.

There is another kind of waves called transverse waves. In these waves the particles oscillate up and down perpendicular to the propagation of the direction of disturbance.

Activity

Stretch a slinky and push and pull it alternately at one end. If you mark a dot on the slinky, the dot moves back and forth parallel to the direction of the propagation of the disturbance.

Characteristics of a sound wave

Sound wave can be described by its frequency, amplitude and speed. Sound can be graphically represented as a wave. There is changes in the density and pressure as sound moves in a medium.

Compressions are the regions of high pressure and density where the particles are crowded and are represented by the upper portion of the curve called crest.

Rarefactions are the regions of low pressure and density where the particles are spread out and are represented by the lower portion of the curve called trough.

The distance between two consecutive compressions (crests) or two consecutive troughs is called wave length. It is represented by the symbol λ (Greek letter lamda). Its SI unit is metre (m).

Frequency of sound wave

When sound is propagated through a medium, the density of the medium oscillates between a maximum value and a minimum value. The change in the density of the medium from a maximum value to a minimum value and again to the maximum value is one oscillation. The number of oscillations per unit time is called the frequency of the sound wave.

Low pitch sound has low frequency and high pitch of sound has high frequency.

It is represented by the symbol ν (Greek letter nu).
Its SI unit is hertz (Hz).

Time period of sound wave

The time taken for the change in the density of the medium from maximum value to a minimum value and again to the maximum value is the time period of the sound wave.

Or, the time taken for one complete oscillation in the density of the medium is called the time period of the sound wave.

It is represented by the letter T.
The SI unit is second (s).
Frequency and time are represented as follows:

ν for one oscillation:
T = 1/ν
or, ν = 1/T

Amplitude of sound wave

The magnitude of the maximum disturbance in the medium on either side of the mean value is the amplitude of the sound wave.

Soft sound has small amplitude and louder sound has large amplitude.

Or, the amplitude of sound wave is the height of the crest or tough. It is represented by the letter A. The SI unit is the same as that of density or pressure.

Pitch and loudness of sound

The pitch of sound (shrillness or flatness) depends on the frequency of vibration. If the frequency is high, the sound has high pitch and if the frequency is low, the sound has low pitch.

Since the sounds are travelling at about the same speed, the one with the shorter wavelength will go by more frequently; it has a higher frequency, or pitch. In other words, it sounds higher.

The loudness of sound depends upon the amplitude of vibration.

If the amplitude is bigger, the sound is loud and if the amplitude is smaller, the sound is soft.

Speed of sound

The speed of sound is different in different media. The speed of sound is more in solids, less in liquids and least in gases.

The speed of sound also depends on the temperature of the medium. If the temperature of the medium is more, the speed of sound is more.

Relationship between Speed (v), frequency (ν) and wave length (λ)
Speed = wave length ✕ frequency
V = λ ✕ ν

Speed of sound in different media at 25 ºC

Reflection of sound

Like light, sound gets reflected at the surface of a solid or liquid and follows the laws of reflection.

The angle of incidence is equal to the angle of reflection.
The incident ray, the reflected ray and normal at the point of incidence all lie in the same plane.

Activity

Take two pipes of the same length and arrange them on a table near a wall or metal plate. Keep a clock near the open end of one pipe and try to hear the sound of the clock through the other pipe by adjusting the position of the pipe.

Now measure the angles of incidence and reflection. Then lift the second pipe and try to hear the sound. It will be seen that the angle of incidence is equal to the angle of reflection. The incident ray, the reflected ray and normal all lie in the same plane.

Echo

If we shout or clap near a reflecting surface like tall building or a mountain, we hear the same sound again. This sound which we hear is called echo. It is caused due to the reflection of sound.

To hear an echo clearly, the time interval between the original sound and the echo must be at least 0.1 s.

Since the speed of sound in air is 344 m/s, the distance travelled by sound in 0.1 s = 344 m/s ✕ 0.1 s = 34.4 m.

So to hear an echo clearly, the minimum distance of the reflecting surface should be half this distance, that is 17.2 m.

Reverberation

Echoes may be heard more than once due to repeated or multiple reflections of sound from several reflecting surfaces. This causes persistence of sound called reverberation.

In big halls or auditoriums to reduce reverberation, the roofs and walls are covered by sound absorbing materials like compressed fibre boards, rough plaster or draperies.

Uses of multiple reflection of sound

Megaphones, horns, musical instruments like trumpets, shehnais etc. are deigned to send sound by multiple reflection in a particular direction without spreading in all directions.

Doctors listen to sounds from the human body through a stethoscope. The sound of heartbeat reaches the doctor’s ears by multiple reflection.

Generally the ceilings of cinema halls and auditoriums are curved so that sound after multiple reflection reaches all parts of the hall. Sometimes a curved sound board is placed behind the stage so that sound after multiple reflection spreads evenly across the hall.

Range of Hearing

Human beings can hear sound frequencies between 20 Hz and 2000 Hz.

Sound whose frequency is less than 20 Hz is called infrasonic sound. Animals like dogs, elephants, rhinoceros, whales etc. produce and hear infrasonic sound.

Sound whose frequency is more than 2000 Hz is called ultrasonic sound. Animals like dolphins, bats, rats porpoises etc. produce and hear ultrasonic sound.

Bats use reflection of ultrasonic sound waves to detect an obstacle or its prey.

Uses of ultrasonic sound

Ultrasonic sound is used to clean objects like electronic components. The components to be cleaned are kept in a cleaning solution and ultrasonic waves are sent into the solution. Due to the high frequency, the dirt particles get detached from the components.

Ultrasonic sound is used to detect cracks in metal blocks. Ultrasonic waves are sent through the metal blocks and if there are cracks, the waves are reflected back and the cracks can be detected

Ultrasonic sound is used in ultra sound scanners for getting images of internal organs of the human body.

Ultrasonic sound is used to break small stones formed in the kidneys into fine grains so that they are removed through the urine.

SONAR

Sonar stands for Sound Navigation And Ranging. It is a device which uses ultrasonic waves to measure distance, direction and speed of underwater objects.

Sonar has a transmitter and a detector installed in ships. The transmitter produces ultrasonic sound waves which travel through the water and after striking the object in the sea bed is reflected back to the detector.

The distance of the object can be calculated by knowing the speed of sound in water and the time taken between the transmission and reception of ultrasound.

If the time taken for the transmission and reception of ultra sound is t and the distance travelled is 2d by the ultra sound, then 2d = v✕t
or, d = v✕t/2

Structure of the human ear

The outer ear called pinna collects the sound waves. The sound waves passes through the ear canal to a thin membrane called eardrum. The eardrum vibrates. The vibrations are amplified by the three bones of the middle ear called hammer, anvil and stirrup. The middle ear then transmits the sound waves to the inner ear. In the inner ear the sound waves are converted into electrical signals by the cochlea and sent to the brain through the auditory nerves. The brain then interprets the signals as sound.