CBSE Class 9 Science Notes Chapter 11 Sound

Sound is a type of energy that moves through a medium like air, water, or solids, and is perceived by our ears. It is generated by the vibrations of objects, which produce waves that travel through the medium. As a mechanical wave, sound requires a medium to propagate and cannot travel through a vacuum, as there are no molecules present to carry the vibrations.

1.0Sound as a Wave

Sound is the form of energy propagated as waves which our ears receive. Sound is produced by virtue of its vibrations. Sound waves cannot travel through vacuum, they need a material medium to travel.

2.0Propagation of Sound 

• A wave in which the particles of the medium move to and fro in the same direction as the wave's propagation is known as a longitudinal wave. Sound waves are categorized as longitudinal waves.
• The longitudinal wave travels in the form of compressions and rarefactions.
• Sound travels in the form of longitudinal waves and it requires a material medium for its propagation.

Example of Sound Wave-Tuning Fork-The longitudinal wave travels in the form of compressions and rarefactions.

3.0Basic Terms Connected to Waves 

1. Wavelength-It is the distance between two consecutive points on the wave which are in the same phase.
2. Amplitude-It is the maximum displacement of the particle from its mean position.
3. Frequency-It is the number of periodic oscillations completed in one second.

f = 1/T

T is the time taken to complete one oscillation

SI Unit of Frequency is Hertz(Hz)

4. Wave Velocity(v)-The velocity at which energy is transmitted through a medium

Relation Between Wavelength, Frequency and Speed

Speed =Frequency ✕ Wavelength

$V=f\times \lambda$

4.0Speed of Sound

• The flash of lightning due to collision of clouds is seen much before the thunder, although both occur simultaneously. This happens because the velocity of light is greater than the velocity of sound.
• The speed of sound is governed by the properties of the medium it travels through, which can vary in terms of elasticity, density, pressure, and temperature.
• The speed of sound decreases as it moves from solid to gaseous state. But in any medium the speed of sound increases with increase in temperature.

5.0Reflection of Sound

• When sound is incident on a solid or a liquid surface bounce off the surface like light rays.
• Sound waves also obey the laws of reflection and refraction. For sound waves to reflect, we need extended surface or obstacles of larger size. For example, the rolling of thunder is due to successive reflection from cloud and land surfaces.
• According to the law of reflection of sound, the angle at which sound waves strike the reflecting surface is equal to the angle at which they are reflected. Both angles, along with the normal to the reflecting surface, lie in the same plane.

6.0Echoes

• When we shout or clap near a reflective surface like a tall building or mountain, we will hear the sound repeated after a short delay. This repeated sound is known as an echo.
• Echoes are produced when sound is reflected by a hard surface. An echo can be heard distinctly only if the time interval between the original sound and reflected sound is greater than 0.1s.This is because the sensation of sound persists in our brain for about 0.1s.

7.0Reverberation

• In a large hall, a sound will continue to linger due to repeated reflections from the walls until it diminishes to a level where it is no longer audible. This phenomenon, where sound persists through multiple reflections, is known as reverberation.
• Persistence of sound after the source has stopped as a result of repeated reflections from walls, ceiling and other surfaces is called reverberation. Reverberation characteristics are important in the design of concert halls, theatres etc.

8.0Practical Application of Reflection of Sound

• Megaphone
• Hearing Aid
• Sound Board
• Stethoscope

9.0Range of Hearing

• Sound waves are emitted from a vibrating source and transmitted through air. The human ear is capable of hearing sound waves within the range of  20Hz to 20 KHz. This range is known as audible range.
• The sound wave having frequencies above the audible range are known as ultrasonic waves and it is usually referred as Ultrasound.
• The sound waves having frequencies less than the audible range are called infrasonic waves.

10.0Application of Ultrasound

1. It is used for medical diagnosis and therapy and also as a surgical tool.
2. It is used as a tool in the treatment of muscular pain.
3. It is employed to identify cracks and imperfections in metal blocks.
4. Bats and Porpoises use ultrasound for navigation and to locate food in darkness.