A sound is a form of energy which produces a sensation of hearing in our ears. 

Sound is a disturbance or vibration that travel through any medium by transferring energy from one particle to another and can be heard when it reaches a person’s or animal ear.

Types of Sound: 

(i)  Audible Sound: Vibrations whose frequency lies between 20 Hz to 20,000 Hz (20 kHz)

(ii)  Inaudible Sound: The sounds having frequencies above 20,000 Hz and below 20 Hz cannot be heard by the normal human ear.

Production of Sound

  • A sound is produced by vibrating objects. 
  • Vibration means a kind of rapid and to and fro motion of an object. 
  • The sound of the human voice is produced due to vibration in the vocal cords. 
  • We can produce sound by striking the tuning fork, by plucking, stretching, rubbing, blowing or shaking different objects. 
  • They all produce sound due to vibration.

Propagation of Sound

  • When an object vibrates, it sets the particles of the medium (solid, liquid or gas) around it in vibrations. 
  • The particles do not travel from the vibrating object to the ear. 
  • A particle of the medium in contact with the vibrating object is first displaced from its equilibrium position
  • It 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. 
  • The process continues in the same medium till sound reaches our ear.
  • The source of sound creates a disturbance in the medium which travels through the medium. 
  • The particle of the medium does not move forward but the disturbance is carried forward. 
  • Sound waves require (solid, liquid, air) a medium to travel, so they are called mechanical waves. 
  • Sound is transmitted through air and liquid as longitudinal waves. 
  • But through solid, it is transmitted both longitudinal and transverse waves.

Introduction to waves

  • The sound is produced by vibrating objects.
  • They travel from one place to another in the form of waves. Hence, the name sound waves.


Wave and particle motion of waves

  • Mechanical waves are waves that travel through a material medium.
  • It is of two types: depending on the direction of motion of the particle of the medium and the wave propagation:

Transverse waves

  • Particle motion is to perpendicular the direction of wave motion.
  • This type of wave is a mechanical wave called a transverse wave. E.g.: Light, or even  Mexican wave in a stadium.

Longitudinal waves

  • When the particles of the medium travel parallel to the direction of the wave motion by means of successive compression or rarefaction.
  • It is also a mechanical wave.

Characteristics of Sound Waves


The distance between two successive crests or troughs (or) successive compressions and rarefactions is called as wavelength (λ).  The SI unit of wavelength is metre (m).

Time period

Time taken by two consecutive compressions or rarefactions to cross a fixed point is called a Time period (T).

The SI unit of time in seconds (s).


The number of compressions or rarefactions per unit time is called frequency (𝛎).
The SI unit of frequency is Hertz. The  SI unit is Hertz (s−1)


Speed (v), wavelength (λ) and frequency (𝛎) are related as v=λ𝛎


The magnitude of disturbance in a medium on either side of the mean value is called an amplitude (A).

As shown in the figure below, the unit of amplitude will be the density or pressure.

Distance between mean position and crest (maximum displacement).


The number of compressions or rarefactions per unit time. Directly proportional to frequency.

Representation of low and high pitch


Volume or loudness of a sound depends on the amplitude. The force with which an object is made to vibrate gives the loudness.

Higher force → higher amplitude → louder sound

The amount of sound energy flowing per unit time through a unit area is called the intensity of sound.

The Intensity of Sound

Note and Tone

A sound of a single frequency is called a tone. A sound produced with a mixture of several frequencies is called a note.

Quality of sound

The richness or timber of sound is called the quality. Sound with the same pitch and loudness can be distinguished based on the quality. Music is pleasant to the ears while noise is not. But they both can have the same loudness and pitch.

Speed of sound
  • Sound travels through different media with different speeds.
  • Speed of sound depends on the properties of the medium: pressure, density and temperature

Speed of sound: Solids > Liquids > Gases

Speed of sound in air = 331 m/s at 00C and 344 m/s at 22 C

Distance travelled by a sound is 346 ms-1 at a common atmospheric temperature 250 C

When a source emits sound with a speed greater than the speed of sound in air, it creates a sonic boom which produces shockwaves with lots of energy.

They produce a very loud noise which is enough to shatter glass and damage buildings.

Reflection of Sound Waves

Like light, sound also follows laws of reflection, it bounces off the surface of solid and liquid.


The phenomenon where a sound produced is heard again due to reflection is called an echo.
E.g: Clapping or shouting near a tall building or a mountain.

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

As sound persists in our brain for about 0.1s.

Minimum distance for obstruction or reflective surface to hear an echo should be 17.2 m. multiple echoes can be heard due to multiple reflections.

Note: The roof of theatre as well as the conference hall is intentionally made curved.

This is done so that the sound produced can be reflect from the walls so that the sound produced can be reached in all parts of theatre or the conference hall.

The walls of theatre are made up of soft or wooden materials to avoid echo so that it can absorb the sound.

Sonar and Radar

SONAR – Sound Navigation And Ranging.

It is a technique that uses sound or ultrasonic waves to measure distance. The human range of hearing is 20Hz- 20kHz.

Range of Hearing
  1. Infrasonic sound
  2. Audible sound
  3. Ultrasonic sound
Infrasonic sound

Sound waves whose frequency below 20 Hz

e.g: earth quake, heartbeat, etc

Audible sound

Frequency of sound waves is in between 20Hz to 20000 Hz. 

Ultrasonic sound

Ultrasonic sounds are high-frequency sound having a frequency greater than 20kHz (inaudible range).

Applications of Ultrasound

  • Scanning images of human organs
  • Detecting cracks in metal blocks
  • Cleaning parts that are hard to reach
  • Navigating, communicating or detecting objects on or under the surface of the water (SONAR).
  • Used in “echocardiography” (ultrasonic sound are made up to reflect from different parts of heart and then make the image of the heart)

Sonar consists of a transmitter and detector mounted on a boat or ship.

The transmitter sends ultrasonic sound waves to the seabed which gets reflected back and picked up by the detector.

Knowing the speed of sound in water, distance can be measured using:   2d=v×t.

This method is called echo-location or echo ranging.


Persistence of sound because of multiple reflections is called reverberation.

Ex: Auditorium and a big hall.

Excessive reverberation is undesirable and to reduce this, halls and auditoriums have sound-absorbing materials on the walls and roofs.  E.g: Fibreboard and rough plaster.

Doppler’s effect

If either the source of sound or observer is moving, then there will be a change in frequency and wavelength for the observer.

The frequency will be higher when the observer moves towards the source and it decreases when the observer moves away from the source.

Ex: If one is standing on a street corner and an ambulance approaches with its siren blaring, the sound of the siren steadily gains in pitch as it comes closer and then, as it passes, the pitch suddenly lowers.

Radar gun or Doplar gun:

Traffic police uses radar gun to identify the speed of vehicles.

Human Ear

The ear is a sensitive organ of the human body.

It is mainly involved with detecting, transmitting and transducing sound and maintaining a sense of balance is another important function of the human ear.

Human ear includes:

  • The outer ear or the visible part of the ear is called the pinna.
  • Pinna collects sound from the surroundings.
  • Sound passes through a tube called an auditory canal.
  • Eardrum (tympanic membrane) vibrates in response to incident sound waves.
  • Vibrations are amplified and transmitted further by three bones hammer, anvil and stirrup in the middle ear to the inner ear.
  • In the inner ear, cochlea converts pressure signals into electrical signals.
  • Electrical signals are transmitted by the auditory nerve to the brain for interpretation.