Class 9 Science Note


Sound is the form of energy which is produced due to the vibration of molecule.

Sound wave is the mechanical wave which needs material medium to propagate.

Propagation of sound takes place through compression and rarefaction.

It shows the properties of reflection, refraction diffraction and interference.


 Factors influencing the velocity of sound

Density: The velocity of sound decreases with increase in the density of the medium.

Temperature: The velocity of the sound increases with increase in temperature.

Humidity: The velocity of sound in air increases with increase in humidity.

Pressure: The velocity of sound is independent o pressure.

Direction of air flow: If the sound propagation take place in the direction of air flow then its velocity increases.


Pitch of the sound

 It is the property of sound that distinguishes between the sharp of shrill sound from dull or flat. Pitch the sound depends on the frequency .Higher the frequency higher will be the pitch. Pitch is not the measurable quantity.


Intensity of sound

It is the measure of the loudness of the sound .It may be defined as the rate of flow of sound energy through the unit surface area normal to the direction of propagation. It is measured in w/m2

Factors determining the loudness of the sound:

Amplitude of the sounding body

 Distance between the source and observer

Density of the medium

Surface area of the sounding body

 Presence of the other sounding body




Reflection of sound

 When the sound wave strike on the rigid surface a part of it is absorbed and a rest is reflected back is called reflection of sound. It follows the laws of reflection. Sound can be reflected from walls of big building, hills or row of trees.


Refraction of sound waves

Sound waves also follow the property of refraction. Sound travels in a constant speed in a uniform medium. When the medium is not uniform, it changes its speed. Because of the temperature there is no uniformity in the air medium at different layers. So, the bending of sound wave in a layer of different temperature is called refraction.


Noise   pollution

 The sound which produces an unpleasant effect is called noise. It is a unwanted sound.


Effects of sound pollution

Hearing loss

 Mental illness

Causes the different disease like, increase in sugar level, blood pressure e.t.c



Ways to reduce the sound pollution

 By using the sound absorbing materials

Shifting the noisy source away from the sources

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