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Sound Class 9 Notes, NCERT Physics Chapter 12

Important topics to study:

  • Intro to sound
  • Production of sound
  • Propagation of sound
  • Need of Medium
  • Longitudinal wave.
  • Characteristics of sound wave
  • Speed of sound
  • Reflection of sound
  • Echo
  • Reverberation
  • Uses of multiple reflection of sound
  • Range of hearing
  • Application of ultrasound
  • Sonar
  • Structure of human ear

What is sound?

  • The form of energy which causes sensation of hearing in our ear is said to be sound.
  • Sound is mechanical energy.
  • The law of conservation of energy is also observed in sound.
  • Sound travels in the form of waves.
  • Sound class 9 notes contain what is specific for class 9, more you will read in higher sections.

Waht is sound?

Production of sound:

  • Due to vibration of objects sound is produced.
  • To produce vibration external source of energy is required.
  • Vibration: The periodic back and forth motion of a particle about a fixed position is said to be vibration. It is also known as
  • Some external source which provides energy to vibrate are hammer, our hand, wind etc.
  • Do you know our voice is also produced by vibration of our vocal-chord of our throat.
  • When we beat drum, produces vibration which then cause sound to produce.

Methods of production of sound:

  • The vibrating body causes material medium such as air, water etc. around it to vibrate thus produces sound.
  • The vibrating string of sitar.
  • Vibration of air in flute.
  • Drum’s vibrating membrane.
  • Metallic plates of bell when vibrates produce sound
  • Sound produces when stretched rubber band left.

methods of production of sound

What is medium?

  • Medium is the substance or material through which sound travels.
  • The material medium may be solid, liquid or gas.
  • For sound to travel, medium is necessary.
  • Sound doesn’t travel in vaccum.

medium of travel

Mechanical wave:

  • Wave is the disturbance which travels through a medium and carries energy.
  • The wave which needs material medium to travel or propagate is said to be mechanical wave.

mechanical wave of sound as water

Propagation of sound:

  • We are already familiar that we can even hear the sound produced at distant place.
  • The traveling of sound from one place to another is called “propagation of sound”.
  • When anything vibrates the medium around it also vibrates in the same manner.
  • The to and fro motion of particles in the medium exerts force on the nearby particle causes sound to propagate.
  • Only disturbance caused by particle of the medium travels, the particle comes back to its original position.
  • This disturbance in the medium continues till the sound reaches our ears.
  • When the body vibrates, a series of compression and rarefaction occurs in air which results in formation of sound wave.
  • The propagation of sound is nothing but the propagation of density change.

propagation of sound

Compression and rarefaction:

1. Compression:

  • As the object or membrane vibrates it sets high density to the nearby area of air and is called “compression”.
  • It is the region of wave with particles are much closest to each other and thus possess high pressure.
  • These wave outwards from the body.
  • The compression is represented by upper portion of the curve called crest.


2. Rarefaction:

  • As the object vibrates forming an area of high pressure and density leaves behind the area or region of low pressure. This low-pressure region is called “rarefaction”.
  • The particles of this region are far apart from each other.
  • It is represented by lower portion of the curve known to be trough.

rarefaction in slinky

Types of waves:

There are two types of waves. They are:

Longitudinal wave:

  • The waves in which the particles of the medium vibrate back and forth along the direction of propagation of the wave are longitudinal wave.
  • Sound waves are longitudinal waves.
  • The direction of vibration is same or parallel to the wave direction as we see in case of slinky.
  • When we pull or push slinky compression and rarefaction are formed.
  • The regions where the coils become closer are called compressions (C) and rarefactions (R) are the region with coils are apart.

longitudinal wave

Transverse wave:

  • The wave in which the particles of the medium vibrate in a direction perpendicular to the direction of propagation of the wave are called transverse wave.
  • The waves produced in a stretched string is an example of transverse wave.
  • Transverse wave can be transmitted in solid or liquid surfaces.

transverse wave

Difference between longitudinal and transverse wave:

Longitudinal wave

Transverse wave

The particles of the medium
vibrates in the
direction same as
the direction of wave

The particles of the medium has
perpendicular to the
direction of propagation 
of wave.

These waves propagates in the
form of alternate
and rarefactions.

These waves travel in the form
of alternate crest 
and troughs.

Longitudinal waves can be
transmitted through 
all media
i.e. solids, liquids and gases.

Transverse waves can be
transmitted through
or over liquid surfaces.

They cause changes in the
pressure and density 
of the
different regions of the medium
which they pass.

There is no change in the
pressure of medium
transverse wave.

(i) Waves formed over surface
of water,

(ii) Waves in stretched strings.

(i) Sound waves propagation
in air,

(ii) Waves formed along a
compressed slinky.

Characteristics of a Sound wave:

The exceptional characteristics possessed by sound wave are:

  • Amplitude
  • wavelength
  • Frequency
  • Time period
  • Velocity

Let’s discuss about each orderly:


  • It is the maximum displacement of each particle from its mean position.
  • In other words, it is the height of crest.
  • It is denoted by
  • Its SI unit is metre(m).
  • Amplitude tells about the loudness and softness of sound.


  • It is the distance between centers of two successive compressions or two consecutive rarefaction.
  • Wavelength is denoted by λ (lamda).
  • The SI unit of wavelength is metre.


  • It is the number of vibrations per second.
  • We can also define frequency as the reciprocal of time period.
  • It is denoted by ν (nu).
  • The SI unit of frequency is Hz (Hertz).

Time period:

  • It is the absolute time taken to complete one complete vibration.
  • It is denoted by T.
  • Its SI unit is
  • Now it is clear that ν = \(\frac{1}{T}\)


  • It is the distance travelled by a wave in one second.
  • It is denoted by v.
  • It’s SI unit is m/s.
  • v = \(\frac{ λ}{T}\) (here λ is wavelength and T is time period)
  • So, v = λν (where ν = \(\frac{1}{T}\) )
    Velocity = Wavelength x Time

Pitch and loudness of sound:


  • Pitch represents shrillness or flatness of sound.
  • It depends upon the frequency of vibration.
  • Higher the frequency of sound higher will be its pitch and vice versa.


  • Loudness is the measure of the sound energy reaching the ear per second.
  • It depends on the amplitude of the sound wave.
  • Greater the amplitude of the sound wave greater will be its loudness and vice versa.
  • It is measured in decibel (‘dB’).

Quality (timbre):

  • The quality or timbre of sound depends upon the shape of sound wave produced by it.
  • It is the feature or characteristic of musical sound.
  • By timbre we can distinguish between two sounds of same loudness and pitch.

Tone node Note of sound:

  • Pure sounds having same frequency is called
  • An impure sound made from mixture of different frequencies are called
  • Note is pleasant to hear.

Music and Noise:

  • Music is the sound that is pleasant to hear. Sound from musical instruments are music.
  • Noise is the sound which is much unpleasant to hear. Sound from bikes are noisy.

music and noise

Speed of sound in different media:

The speed of sound depends upon the following conditions:

  • Nature of the material medium: Sound travels with least speed in gases, faster in liquid and fastest in solids.
  • Temperature: Speed of sound increases with rise in temperature.
  • Humidity: As the humidity of air increases speed of sound also increases.

Speed of light is faster than that of sound. The speed of sound is 344m/s at 22⁰C in air.

Why we hear the sound of a thunder a little later than the flash of light is seen?

Sound travels with a speed which is much less than the speed of light. This is the reason we first observe lightning than hear the sound.

Sonic Boom:

  • When the speed of any object is greater than that of sound then it is said to have travel with “supersonic speed”.
  • Aircrafts, rockets have supersonic speed.
  • When the object travels with supersonic speed it produces shock waves in air.
  • Shock waves produces explosive noise which in-turn called sonic boom.
  • A tremendous amount of energy is released by sonic boom which breaks glass panes, windows etc.

sonic boom

Audible frequency of sound:

  • Human’s ear has audible range of frequency from 20Hz and 20,000Hz.
  • Sound class 9 notes are full of theory and illustration.

Reflection of sound:

  • When sound waves after striking a surface returns back to the same medium then it is said to be refraction of sound.
  • Like light we can also observe the reflection of sound.

Laws of reflection of sound:

  • The incident sound wave, Reflected sound wave and normal at the point of incidence all lies in the same plane.
  • The angle of incidence of sound is always equal to the angle of reflection of sound.

reflection of sound

Applications of multiple reflections of sound:

There are several applications of law of reflection of sound. They are:

  • In loudspeakers:
    Loudspeakers are designed to send sound by multiple reflection in a particular direction. It prevents sound from spreading in all directions.


  • In stethoscope:
    Doctors use this device to hear the heartbeat from human body. Due to multiple reflections sound reaches to doctor’s ear.


  • Soundboard:
    The reflection of sound is used by cinema halls or auditorium as its roof is made curved. The curved roof reflects and send sound to different parts of the hall.


  • The repetition of sound waves caused by reflection of sound is known to be Echo.
  • The condition for listening an echo is the gap between original sound and the echo must be at least 0.1 sec.
  • When sound is reflected from a hard surface (i.e. mountain, large walls etc.) then echo is heard.
  • Soft surfaces have tendency to absorb sound.
  • If we shout near tall wall or mountains, we hear the same sound again this is called echo.

echo of sound

Minimum distance to hear an echo:

  • To hear an echo the distance between listener and the reflecting surface must be 17.2 m.
  • As sound travels 344m in 1 sec in air. Thus in 0.1 sec, sound would cover a distance of 344 m x 0.1 = 4 m. Thus before it reaches our ears it must travel 34.4 m.
  • When we are at 17.2 m away from the wall the sound will travel 17.2 m to reach the reflecting surface and again 17.2 m to reach our ears.


  • We may hear echoes more than once due to repeated or multiple reflections of sound from several reflecting surfaces.
  • The persistence of sound for a long time due to repeated (multiple) reflection of sound are said to be
  • In large concert halls we can’t hear the sound appropriately as the sound gets distorted due to multiple reflection (reverberation).


Prevention of reverberation:

We can prevent the reverberation of sound by several methods. They are:

  • By using soft sound absorbing materials in the auditorium such as fireboard, carpets, curtains, plant fibre etc.
  • These material absorbs noise as well as reflections.
  • Chapter 12 sound class 9 notes are here.

Some categories of sound which are not heard by human ear or non-audible are:

Infrasonic sound or Infrasound:

  • Sound which lies below the frequency of 20 Hz is called infrasonic sound.
  • Very low vibration produces infrasound.
  • Simple pendulum produces infrasonic sound below 20 Hz of frequency.
  • Humans are not adapted to hear infrasound.
  • Some animals like elephant, whale, rhinoceros, dog etc. produces as well as hear infrasound.
  • Earthquakes produce infrasonic waves. So, elephant feels it earlier than humans.

Ultrasound or ultrasonic sound:

  • The sound which has a frequency greater than 20KHz is said to be ultrasound or ultrasonic sound.
  • Humans cannot listen ultrasonic sounds.
  • Human children can hear these sounds.
  • Dogs, dolphins, bats can hear ultrasounds.
  • Bat and rats produce ultrasounds.

One of the difference between ultrasound and audible sound is that audible sound bends at edges but ultrasound doesn’t.

Uses of Ultrasond/Ultrasonic sounds:

There are several applications of ultrasound. They are:

  • It is used to detect the cracks in metal sheets, blocks without any damage in it.
  • In big machinery industry it is used to clean tough to reach parts of objects such as odd shaped machines, spiral tubes.
  • It is used to investigate internal organs of our body such as liver, kidney, uterus and heart.
  • In echocardiography: Ultrasonic waves are used in investigating heartbeats and convert its behavior into images. This technique is called echocardiography.
  • Ultrasonography: The process of obtaining pictures of internal organs is said to be ultrasonography.
  • Splitting stones in Kidneys: Doctors make use of ultrasound to split tiny stones in kidney to fine grains.
  • Echolocation: Bats and porpoises use ultrasonic waves for navigation as well as to locate food in darkness.

ultras sound applications


  • SONAR stands for “Sound Navigation and Ranging”.
  • It is used to determine the distance, direction and speed of the underwater objects.
  • SONAR consists of a Transmitter, Receptor and receiver installed at the bottom of the ship in downward direction.
  • The transmitter produces and transmits ultrasonic waves and these waves after striking the objects lying in the bottom of the sea comes back and received by the receptor.
  • The detector converts reflected waves into electrical signals.
  • The SONAR measures the time taken by ultrasound to travel down the bottom of sea and come back. Half of this time is the time taken by ultrasound to reach bottom.
  • Now let’s assume that the time taken by ultrasound to reach back to receiver is T.
    Speed of sound in Sea water is v
    Distance travelled by waves = 2d
    Then, 2d = v x t and it is called Echo ranging.
  • SONAR is used to find the depth of sea, underwater hills, valleys, submarines, icebergs and sunken ships.
  • In the dark night bats fly by emitting high pitched ultrasound waves which are reflected from the obstacle or prey and returned to bats ear. The nature of reflection of waves tells bat the kind of target.

Structure of human ear:

  • Human ear is the most sensitive part of human body as it enables us to hear.
  • It consists of three parts: (i) outer ear (ii) middle ear and (iii)inner ear.
  • The outer ear which is called pinna which collects sound from the surrounding.
  • The collected sound then passes through the auditory canal.
  • Ear drum which is thin elastic membrane (also called tympanic membrane) present at the end of the auditory canal.
  • The middle ear consists of three membrane which are hammer anvil and stirrup attached to each other.
  • The ear drum is linked with hammer and stirrup is linked with oval window of inner ear.
  • ‘Eustachian tube’ is attached in the lower part of middle ear.
  • All the bones present in the middle ear acts as lever which amplifies vibrations many times.
  • The pressure difference in air is converted into audible frequencies and then it is converted into electrical signals and it is transferred to the brain through auditory nerves.
  • The inner ear has cochlea which is a coiled tube and connects with oval window.
  • Other side of cochlea is connected to auditory nerve which goes to brain.


Working of Human ear:

working of human ear

  • When compression reaches the ear drum high pressure is created outside of the ear drum which pushes it inwards.
  • While during rarefaction the ear drum tend to move outwards .
  • Thus scheduled compression and rarefaction sets the eardrum in back and forth motion i.e vibration.
  • Cochlea turns the inner ear pressure into electrical signals.
  • These signals sent to brain via auditory nerves.
  • The brain interprets these signals as sound.
  • Physics 9th class sound notes from CBSE takes a dig on all important topics.
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