Physics Notes For Competitive Exams

Speed of Sound

Speed of sound depends on what factors ?

Factors which influence speed of sound are
Effect of density
Effect of humidity


   The speed of sound is directly proportional to temperature of air

    V α √T

    For every 1⁰C          0.61 ms⁻¹ ,

                    25⁰C        330 ms⁻1 ,

                    26⁰C        330.61 ms⁻¹

The speed of the sound is more in summer where atmosphere temperature is more.


   v  =  √γ p/d

   v√d = √γ p

Speed of sound is independent of pressure

Effect of density

The Speed of the sound is inversely proportional to square root of density of gas.

The density of gas varies one gas to another, so the speed of the sound also changes from one gas to another gas.

v α 1/√d

Speed of sound in hydrogen is 1230 ms⁻¹

Speed of Sound in Air is 330 ms⁻¹

Effect of humidity

At same temperature the speed of the sound is more in the wet day(a day in the rainy season) then dry day ( a day in the summer season)

Lets density of air in summer d1 = 1 units
Density of air in rainy d2 = 0.6 units


In which one of the following days the speed of the sound is more

a) a dry day at 0⁰ C

b) a wet day at 0⁰ C

c) a dry day at 30⁰ C

d) a dry wet at 0⁰ C

Because, in this day both the percentage of humidity and temperature are more 

Speed of the Sound in Solids

   Vs = √y/d

   y = Young’s Modulus
   d = density of solid

Speed of sound is directly proportional to young modulus

Speed of different materials

   In Diamond   v = 12000 ms⁻¹
   In glass          v = 5500 ms⁻¹
   In steel           v = 5000 ms⁻¹

Speed of Sound in Liquids

          V =√k/d

    k = Bulk Modulus

    d = density of liquids 

    where   k = bulk Modulus.
                d  = density of liquid.

Speed of sound more in water

     water v = 1435 ms⁻¹
     salt water v=1485 ms⁻¹

Question: In which one of the material the speed of the sound is more

a) Hydrogen 

b) Water

c) Plastic material

d) Air

Types of Speed of Objects

Depending on the speed of the sound, we have 5 types

1.   Sub-sonic speed

2.   Sonic Speed

3.   Supersonic Speed

4.   Hyper sonic speed

5.   High Hyper sonic Speed.

Subsonic Speed

If the speed of the object is lesser than the speed of the sound then its called subsonic speed

v = 330 ms⁻¹

Mach = object/sound

Say object speed v = 10 ms⁻¹

 mach  = 10 ms⁻¹/300 ms⁻¹

mach < 1

Sonic Speed

If the speed of the object is equal to the speed of sound then it is called as sonic speed.

mach = 1

Supersonic speed

If the speed of the object lies between 1 to 5 time of sound then it is called as supersonic speed

    330 × (1-5)times


Hyper sonic speed

If the speed of the object lies between 5 to 10 times of the speed of the sound then it is called as hyper sonic speed
330 × (5-10) times

India want to develop Brahmos II of 6 – 7 mach speed

High hyper sonic speed

If the speed of the object is more than 10 times of the speed of sound then it is called as high hyper sonic speed.
330 × 10 times

Characteristics of sound

1.   Intensity
2.   Pitch
3.   Quality


I α a²

a = amplitude 

Units of intensity is Bell

1/10th of Bell is Decibel(db)

Intensity of sound is independent of frequency and it is measured by Sound-meter or Noise-meter.


It is the frequency of sound wave

n = hz

Either Increasing or decreasing of amplitude, the frequency will not changed

Frequency is measured by Tanometer

Pitch of sound is independent of amplitude of sound.


The Ability of Eardrum to identify the source of sound when multiple sounds are received simultaneously is called as quality or timber

It depends on the sensitivity of hearing of sound of the person.


The tone of female, children, mosquito, amplitude is less and frequency is more,they contain shrillness.

Doppler’s Effect

The comparison of velocity of a body with another body is called as the relative velocity.

The relative velocity of the two bodies in the same directions

v = v₁ ∽ v₂

and the relative velocity of approach  v = v₁ + v₂

The apparent frequency of sound received by the observer due to relative motion between the source of the sound and observer is called as Doppler Effect.

It can be applied in all most all the types of waves including the sound and light.

Applications of speed of sound

1. The sonar works on the principle of Doppler effect and it is use to locate the position of submarine in the sea.
3. To find the speed of vehicle the instrument namely speed gun is used in which low energy laser rays are used.
4. To hear the heart beat of fetus, the Doppler approach is used.
5. Doppler color photo instrument used to find the defect in the blood circulation of human beings.
6. Doppler effect is used to detect the direction of self rotation of sun.
7. To study the properties of the binary stars or double stars.
8. To study the colored rings present around the planet.
9. Doppler cyclone detecting radars are used to produce predicts the formulation of cyclone in sea.
10. To determine the radius of the orbit and range of an artificial satellite the Doppler effect is used.

Next Topic: Light  

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