NCERT Class 9 Science Sound Revision Notes

Sound is a form of mechanical energy which produces the sensation of hearing. It is produced due to vibrations of different objects. It travels in the form of waves.

Propagation of Sound 

A material medium is necessary for the propagation of sound. It can be solid, liquid or gas.

The disturbance which moves through a medium when the particles of the medium set the neighbouring particles into motion is known as a wave

A sound wave can be considered the propagation of pressure or density variations in the medium, i.e. it propagates in a medium as a series of compressions and rarefactions.

A region of compressed air (increased density or pressure) is called a compression (C) and that of rarefied air (decreased density or pressure) is called a rarefaction (R).

A vibrating object produces a series of compressions and rarefactions in the medium.

Example: When the prongs of a tuning fork move forward, compression is formed, and when the prongs move backward, rarefaction is formed.

As sound propagates, it is the sound energy which travels in the medium and not the particles of the medium.

Sound waves are longitudinal waves as the particles of the medium through which the wave propagates vibrate in a direction parallel to the direction of propagation of waves.

Variations in Pressure and Density of a Medium due to Sound Waves

The variations of pressure and density when a sound wave moves in a medium are as shown below:

The portion of the medium where density (or pressure) has a value larger than its average value is called a crest.

The portion of the medium where density (or pressure) has a value smaller than its average value is called a trough.

The magnitude of maximum disturbance in the medium on either side of the mean position is called the amplitude (A).

When a sound propagates through a medium, the density of the medium oscillates between a maximum value and a minimum value.

The change in density (or pressure) from the maximum value to the minimum value and again to the maximum value is called an oscillation.

The number of complete oscillations per second is called the frequency (ν) of the sound wave. Its unit is hertz (Hz).

The time taken for one complete oscillation in the density (or pressure) of the medium is called the time period (T) of the wave.

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

wavelength (λ) of the wave. Its SI unit is metre (m).

Frequency (ν) and time period (T) are related as

Speed of Sound in Different Media

  • Speed of sound is finite and is much less than the speed of light.
  • Speed of sound in solids > speed of sound in liquids > speed of sound in gases
  • The speed of sound increases with increase in temperature.

Characteristics of Sound

Sounds can be distinguished from each other by three characteristics—loudness (intensity), pitch (frequency) and quality (timbre).

  • The intensity of sound at any point is the amount of sound energy passing per unit time per unit area in a direction perpendicular to the area. Its unit is watt/metre2 (W/m2).
  • The physiological response of the ear to the intensity of sound is called loudness.It is determined by the amplitude of the wave.
  • Pitch is the physiological sensation which helps in distinguishing a shrill sound from a flat sound. It is determined by the frequency of the wave.
  • Quality (timbre) distinguishes one sound from another sound of the same pitch and loudness. It is determined by the wave form of the sound.
  • A sound of single frequency is called a tone.
  • The sound produced by a mixture of several frequencies is called a note.

Reflection of Sound

  • The laws of reflection for sound are the same as those for light.
  • The repetition of sound caused by reflection of sound waves from an obstacle is known as an echo.
  • The time interval between the original sound and the reflected one must be at least 0.1 s for an echo to be heard distinctly.
  • Multiple echoes are heard when sound is repeatedly reflected from several obstacles at suitable distances.
  • The phenomenon of persistence or prolongation of audible sound after the source has stopped emitting it is called reverberation.

Uses of Multiple Reflection of Sound

In megaphones, horns, musical instruments and stethoscopes, the mechanism of multiple reflection of sound is used.

Range of Frequencies

Applications of Ultrasound

  • Ultrasound finds applications in industry, medical science and communication (SONAR).
  • SONAR stands for SOund Navigation And Ranging. It is used to measure the distance, direction and speed of objects under the sea. It is also used in ship-to-ship communication.

Human Ear

The human ear can be divided into three parts:
  • The outer ear which collects the sound waves.
  • The middle ear which amplifies the sound waves about 60 times.
  • The inner ear which converts the amplified sound energy into electrical energy and conveys it to the brain as nerve impulses for interpretation.