Class 9 Sound Notes

Notes Important Questions

Sound Notes


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


A kind of rapid to and fro motion of an object is called vibration.

  • Sound is produced by vibrating objects.


A wave is a disturbance that moves through a medium when the particles of medium set neighboring particles into motion.

Compression:- It is a region of high pressure.

Rarefaction:- It is a region of low pressure.

  • Sound needs a material medium to propagate.

Mechanical wave:-

A wave which need a material medium to propagate is called mechanical wave.

Types of mechanical wave:-

  • Transverse wave
  • Longitudinal wave:-

Transverse wave:-

The type of wave in which the direction of movement of particles is perpendicular to the direction of propagation of wave is known as transverse wave.
Ex:- Water wave, wave produced in a stretched string etc.

Longitudinal wave:-

The type of wave in which the direction of movement of particles is parallel to the direction of propagation of wave is known as Longitudinal wave.
Ex:- Sound wave, wave produced in a slinky etc.

  • Light is a transverse wave but not a mechanical wave.

Trough:- A valley in the graph of a wave is called trough.

Crest:- A peak in the graph of a wave is called crest.

Wave length (λ):-

The distance between two consecutive compressions or two consecutive rarefactions is called wavelength.

  • S.I. unit:- metre (m)

Frequency (ν):-

The number of oscillations made by a vibrating body per unit time is known as frequency.

  • Frequency = Total number of oscillations/Total time taken
  • S.I. unit:- Hertz (Hz)

Time period (T):-

The time taken by a vibrating body to complete one oscillation is known as time period.

  • Time period = Total time taken/Total number of oscillations
  • S.I. unit:- Second (s)

Frequency = 1/Time Period
ν = 1/T Or, T = 1/ν

Amplitude (A):-

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


How the brain interprets the frequency of an emitted sound is called its pitch.

  • Pitch of a sound is determined by its frequency.


The measure of the response of the ear to the sound is called loudness.

  • Loudness of a sound is determined by its amplitude.


The characteristics of a sound which enables us to distinguish one sound from another having same pitch and loudness is known as quality or timber.

  • The sound which is more pleasant to the ear is said to be of a rich quality like music.
  • The sound which is unpleasant to the ear is said to be of a poor quality like noise.

Tone:- A sound of single frequency is called a tone.

Note:- The sound which is produced due to a mixture of several frequencies is called a note.

Speed of sound:-

The distance traveled by a point on a wave per unit time is called speed of sound.

  • Speed of sound = Wavelength/Time Period
    v = λ/T
  • Speed of sound = Wavelength x frequency
    v = λν

Speed of sound depends on:-

  • State of medium:- Speed of sound is more in solid than in liquid and than in gas.
  • Temperature:- Speed of sound increases on increasing the temperature of the medium.


The amount of sound energy passing each second through unit area is called the intensity of sound.

Supersonic speed:-

When the speed of an object is more than the speed of sound than its speed is said to be supersonic speed.

Shock wave:-

The wave produced by an object in air moving with supersonic speed is called shock wave.

Sonic boom:-

When an object moves with supersonic speed then the shock wave produced by it produces a very loud and sharp sound called sonic boom.

  • Shock waves can shatter glass & even damage buildings.


The repetition of sound due to the reflection of original sound after striking on a large & hard obstacle is known as echo.

Persistence of hearing:-

The sensation of sound persists in our brain for about 0.1 s. This time is called persistence of hearing.

Minimum distance between source & obstacle to hear an echo:-

Speed of sound in air = 344 m/s
Persistence of hearing = 0.1 s
So, distance traveled by sound in 0.1 s = Speed x time
= 344 x 0.1
= 34.4 m
Hence, minimum distance between source & obstacle to hear an echo = 34.4/2 = 17.2 m


When echoes follow so closely behind the original sound so that the original sound appears to be prolonged then this effect of sound is known as reverberation.

Reverberation can be controlled by:-

  • covering the roof and walls of the auditorium with sound absorbent materials like rough plaster, compressed fiberboard etc.
  • using selective seat material on the basis of sound absorbing properties.
  • using heavy curtains.

Uses of multiple reflection of sound:-

  • It is used in megaphones, horns, musical instruments, trumpets, shehanais to send sound in a particular direction.
  • It is used in stethoscope.
  • Generally the ceilings of concert halls, conference halls and cinema halls are curved so that sound after reflection reaches all corners of the halls.

Audible range of hearing for humans:- 20 Hz to 20000 Hz.

Infrasonic sound:-

Sound of frequency less than 20 Hz is called infrasonic sound.

  • Some animals which produce infrasonic sound are rhinoceros, whale, elephant.

Ultrasonic sound:-

Sound of frequency more than 20 kHz is called ultrasonic sound.

  • Some animals which produce ultrasound are dolphin, bat, moth and porpoise
  • Children under the age of 5 can hear sounds up to 25 kHz.

Uses of ultrasound:-

  • It is used to clean parts located in hard to reach places.
  • It is used to detect cracks and flaws in metal blocks.
  • It is used in echocardiography to form the image of heart.
  • It is used to form the images of internal organs such as liver, gall bladder, kidneys, uterus, etc. The technique used in this is called ultrasonography.
  • Ultrasonography is also used to examine the growth of foetus during pregnancy.
  • It is used to break small stones formed in the kidneys into fine grains.

SONAR (Sound Navigation And Ranging):-

It is a device used to determine the depth of sea and to locate underwater hills, valleys, submarine, ice bergs, sunken ship etc.

Echo-ranging:- The method used in SONAR to determine the depth of sea and to locate underwater objects is called echo-ranging.

Human Ear:-

  • The outer ear is called pinna. It collects the sound from the surroundings.
  • Auditory canal drives the sound to the ear drum (tympanic membrane).
  • The middle ear contains three bones hammer, stirrup and anvil which amplify the sound.
  • Cochlea turns the pressure variations into electrical signals.
  • Auditory nerve sends the electrical signals to the brain.


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