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Sound - Class 9 Science Notes


  • Sound is a form of energy which produces sensation of hearing in our ears

  • It is produced due to vibration of different objects.

  • Law of conservation of energy is also applicable to sound.

  • Sound travels in the form of wave.

Production of Sound

  • Sound is produced when object vibrates. (Vibration means a kind of rapid to and fro motion of an object)

  • The energy required to make an object vibrate and produce sound is obtained from sources like hand, wind, etc.


i. Striking the tuning fork produces sound.

ii. Sound of our voice is produced by vibration of two vocal cords in our throat.

Other examples:

Propagation of Sound

  • The matter or substance through which sound travels is called medium. It may be solid, liquid or gas. Air is the most common medium through which sound travels.

  • A particle of the medium in contact with vibrating object is first displaced from its equilibrium position and then exerts a force on the adjacent particle. As a result, of which the adjacent particle gets displaced from its position of rest.

  • After displacing the adjacent particle, the first particle comes back to its original position. This continues till the sound reaches the receiver.

  • Sound waves are characterised by the motion of particles in the medium and are called mechanical waves.

  • When a vibrating object moves forward, it pushes and compresses the air in front of it creating a region of high pressure. This region is called a Compression (C).

  • When a vibrating object moves backwards, it creates a region of low pressure called Rarefaction (R).

  • Compression is the region of high pressure whereas Rarefaction is the region of low pressure.

  • Thus, the propagation of Sound is the propagation of density variation or pressure variation in the medium

Sound needs a medium to travel

  • Sound is a mechanical wave.

  • It needs material medium like air, water, steel, etc. for its propagation. It cannot travel through vacuum.

The Bell Jar experiment:

  • An electric bell is suspended inside the airtight glass bell jar and it is connected to a vacuum pump.

  • When glass jar is filled with air, we can hear the sound but when the air pumped out of the glass jar, then no sound is heard.

  • Hence, medium is necessary for propagation of Sound, sound cannot travel in vacuum.

Sound waves are longitudinal waves

  • The region where the coil becomes closer are called compression and the regions where the coil is further apart is called rarefaction.

Are Sound Wave Longitudinal Waves?

The wave in which the individual particles of the medium move in the direction parallel to the direction of propagation of the disturbance is known as longitudinal waves.

The transverse wave is the one in which the individual particles of medium move in the direction perpendicular to that of wave propagation.

The particles oscillate back and forth about the position of rest.

In transverse wave particles do not oscillate along the direction of wave propagation but oscillate up and down

Example: Sound is a longitudinal wave

Examples: Light isa transverse wave

Characteristics of Sound Wave

  • Sound wave can be characterized by its:

i. Frequency

ii. Amplitude

iii. Speed

  • When a wave travel in air the density and pressure of air changes from their mean position.



Particles are crowded together

Particles are further apart

Crest represent compression

Trough represent rarefaction

Region of maximum density or pressure

Region of minimum density or pressure

  • A peak is called the crest and a valley is called the trough of a wave.

  • The distance between two consecutive compressions (C) or two consecutive rarefaction (R) is called a wavelength.

The wavelength is represented as l (lambda). The S.I. unit is metre (m).

  • The number of oscillations per unit time is called the frequency of sound wave. Or

Number of one complete wave produced in one second or number of vibrations per second is also called frequency. Or

Number of compression or rarefaction passed in one second is also frequency.

The S.I unit of frequency is Hertz (Hz). The symbol of frequency is u (nu).

  • Time taken to complete on vibration is called time period. Or

Time required to pass two consecutive compressions or rarefaction through a point is called time period.

The S.I unit is second.

Frequency of wave is reciprocal of time period

  • The maximum displacement of the particle of the medium from their original undisturbed position is called amplitude of the wave.

It is represented by A and the S.I. unit is metre. The other unit is density or pressure.

  • Sound has characteristics like pitch, loudness or softness and timber (quality).


The pitch of sound depends on the frequency of sound and the pitch is directly proportional to frequency.


- It depends on the amplitude of sound wave.

- It is measure of the sound energy reaching the ear per sec. Greater is the amplitude, greater the energy and louder the sound and vice-versa.

- It is measured in decibel, ‘dB’.

Quality or Timber:

- The timbre of sound depends on the shape of sound wave produced by it.

- It helps to distinguish between two sounds of same pitch and loudness.

- Sound of single (same) frequency is called tone while a mixture of different frequencies is called note.

- Noise is unpleasant while music is pleasant.


- The distance travelled by a wave in one second is called the velocity of wave.

- Its S.I. unit is metre per second (ms-1)

Velocity = distance travelled by wave / time taken

Speed of Sound in different media

- Speed of sound depends on nature of material through which it travels. (Solid > Liquid > Gas)

- Speed of sound depends on temperature. So, speed increases with rise in temperature.

- Speed of sound depends on humidity. Thus, the speed increases with increase in humidity in air.

Speed of light is faster than the speed of sound.

Sonic Boom:

  • When the speed of any object exceeds the speed of sound it is said supersonic speed. Bullets, jet aircrafts, etc travel at speed of supersonic speed.

  • When a sound, producing source moves with speed higher than that of sound, it produces shock waves in air. These waves carry large amount of energy and air pressure variation in shock waves produces a very sharp and loud sound called the “Sonic Boom”.

Reflection of Sound

Like light, sound also bounce back when it falls on a hard surface. It is called reflection of sound.

The laws of reflection of light are obeyed during the reflection of sound.

- The incident sound wave, reflected wave and the normal at the point of incidence lie in the same plane.

- Angle of incidence of sound is equal to angle of reflection of sound.


- The repetition of sound caused by the reflection of sound wave is called echo.

- There is a time gap of 0.1 second between original sound and echo (reflected sound).

- Echo is produced when sound strikes at hard surfaces (brick wall, mountains, etc) as soft surfaces tend to absorb sound.

Minimum distance to hear Echo:

- Speed of sound = 344 m/s (at 22oC)

- Time = 0.1 second

- Distance = Speed × Time

= 344 × 0.1 = 34.4 m

- So, distance between reflecting surface and audience = 34.4/2 = 17.2 m.


o The persistence of sound in a big hall due to repeated reflection of sound from the walls, ceiling and floor of the hall is called reverberation.

o If reverberation is too long, sound becomes blurred, distorted and confusing due to overlapping of different sound.

o Methods to reduce reverberation in big halls or auditorium:

i. The roof and walls should be covered with sound-absorbing materials like compressed fiberboard, rough plaster or draperies.

ii. Heavy curtains on door and windows.

iii. Carpets on the floor.

iv. Seats made of materials that absorb sound.

Uses of multiple reflection of sound

i. Megaphones (loudhailers), horns, musical instruments such as trumpets and shehanais, are all designed to send sound in a particular direction without spreading it in all direction.

All these instruments have funnel tubes which reflects sound waves repeatedly towards audience.

ii. Stethoscope is a medical instrument used for listening to sounds produced within body, mainly in the hearts or lungs.

In these, the sound of the patient’s heartbeat reaches the doctor’s ears by multiple reflection of sound.

iii. The ceilings of concert halls, conference halls and cinema halls are curved so that sound after reflection reaches all corners of the hall.

iv. Curved soundboard placed behind the stage so that sound reflected from soundboard spread evenly across the width of the hall.

Range of Hearing



The range of hearing in humans is 20 Hz to 20000 Hz.

Children younger than 5 years and dogs can hear upto 25 kHz.

The sound of frequencies lower than 20 Hz are known as ‘Infrasonic sound’.

A vibrating simple pendulum produces infrasonic sounds.

Rhinoceros communicate using frequencies as low as 5 Hz.

Elephants and Whale produces infrasonic waves.

Earthquake produces infrasonic wave which some animal can detect.

The sounds of frequencies higher than 20 kHz are known as ‘Ultrasonic sound’.

Dogs, parpoises, dolphins, bats and rats can produce ultrasonic sound.

Hearing Aid:

- It is battery operated electronic device used by persons who are hard of hearing,

- Microphone convert sound into electrical signals, then those are amplified and sent to the speaker of hearing aid. The speaker converts the amplified signals to sound and sends to the ear.

Application of Ultrasound

(i) It is used to detect cracks in the metal blocks in industries without damaging them.

(ii) It is used in industries to clean ‘hard to reach parts’ of objects such as spiral tube, odd shaped machine, electronic components, etc.

(iii) It is used to investigate internal organs of human body such as liver, gall bladder, uterus, kidney, etc.

Echocardiography: These waves are used to reflect the action of heart and its images are formed. This technique is called echocardiography.

Ultrasonography: The technique of obtaining pictures of internal organs of the body by using echoes of ultrasound waves is called ultrasonography.

(iv) Ultrasound is used to split tiny stones in kidney into fine grains.


  • The acronym SONAR stands for SOund Navigation And Ranging.

  • Sonar is a device that uses ultrasonic waves to measure the distance, direction, and speed of underwater objects.

  • It consists of transmitter and a detector and is installed in a boat or a ship.


  • The transmitter produces and transmits ultrasonic waves that travel through water and after striking the object on the seabed, get reflected back and are sensed by the detector.

  • The detector converts the ultrasonic waves into electrical signals.

  • The distance of the object that reflected the sound wave can be calculated by knowing the speed of sound in water and the time intervals between transmission and reception of the ultrasound.

Let, time intervals between transmission and reception of the ultrasound signal be ‘t’.

The speed of sound through seabed is ‘v’

Total distance travelled by ultrasound is ‘2d’

Then, 2d = v × t

Structure of Human Ear

  • The Ears are the sense organs that helps us in hearing sound.

  • The ear consists of three parts: the outer ear, the middle ear and the inner ear.

  • The outer ear is called Pinna. It collects the sound from the surroundings and passes through the auditory canal.

At the end of the auditory canal there is a thin membrane called the eardrum or tympanic membrane.

  • The middle ear contains three tiny bones – hammer (malleus), anvil (Incus) and stirrup (Stapes) linked with one another.

Free end of hammer touches the eardrum and that of stirrup is linked to membrane of oval window of inner ear.

  • The lower end of the middle ear has a narrow ‘Eustachian tube’.

  • The inner ear has a coiled tube called cochlea connected to oval window. Cochlea is filled with a liquid containing nerve cells and other side of cochlea is connected to auditory nerve that goes to brain.

Working of Ear:

Pinna – Ear canal – Eardrum – Hammer (malleus) – Anvil (incus) – Stirrup (stapes)– Oval window – Cochlea – Auditory nerve – brain.

  • When a compression of the sound waves strikes the eardrum, the pressure on the outside membrane increases and pushes the eardrum inwards.

  • While during rarefaction eardrum moves outwards. Thus, eardrum starts vibrating back and forth.

  • These vibrations are increased by the three bones and the middle ear transmits the amplified pressure variations to inner ear.

  • In the inner ear the pressure variations are turned into electric signals by cochlea. These signals are sent to brain via auditory nerve and finally brain interprets these signals as sound.


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