# Important Questions and Answers: Mechanical Waves

1. Do sound waves undergo reflection, rarefaction and polarization phenomena?

Ans: Sound wave undergoes reflection and refraction but it does not follow polarization. Since sound wave is longitudinal wave, the vibration of particle occurs to and fro in the direction of the propagation of the wave. The vibrations of such type cannot be limited or controlled by any barriers and so polarization is not possible in them.

2. Is velocity of sound more in damp air or in dry air? Explain.

Ans: The velocity of air is more in damp air than in dry air. In damp air, the density of air molecule is less than that of dry air due of the presence of humidity. Also, the velocity of sound is inversely proportional to the square root of density as per the relation given below.

$$\text{v} \propto \frac{1}{\sqrt{\rho}}$$

Hence, the velocity of sound is more in damp air than in dry air.

3. Although the density of solid is high, the velocity of sound is greater in air. Why ?

Ans: We know that the velocity of sound in solid is given by $\text{v} \propto \frac{\text{Y}}{\sqrt{\rho}}$ where Y is the Young’s modulus and ρ is the density of the solid. Though density of solid is high, the velocity of sound is greater in solid because the value of Young’s modulus is very high in case s of solid.

4. Sound is heard better on wet days than in dry day. Why?

Ans: On a wet day, the air contains large number of water vapour. So, the density of air molecule is less than that in dry days. As per the relation, $\text{v} \propto \frac{1}{\sqrt{\rho}}$, the velocity of sound is inversely proportional to density. So, sound is heard better on wet days than in dry days.

5. A person walking on a railway line hears two sounds for the same explosion occurring far away, why?

Ans: The sound of explosion travels through metal and air. The velocity of propagation of sound in metal is high due to high value of Young’s modulus while the velocity of sound in air is less in compared to that of solid. So, listener hears two sounds.

6. Do sound wave need medium to travel from one medium to another medium in space? What properties of the medium are relevant ?

Ans: Yes, since sound wave is a longitudinal wave, it needs medium for its propagation in space. Some of relevant properties of the medium are as follows.

• The medium should have property of elasticity.
• The medium should possess the property of inertia.
• The medium should have minimum friction.

7. Why explosions on other planet are not heard in earth?

Ans: We know that sound waves require material medium for its propagation. Since there is a vacant space between earth and other planets, sound produced in other planets cannot be heard in earth.

8. Why sound made at a distance can be heard distinctly at night than at day time?

Ans: It is based on the variation of velocity of sound with temperature. The speed of sound and temperature have direct relation given by $\text{v} \propto \text {T}$. So, if temperature is high, the velocity of sound is high and if less, the velocity is also less.

At night, the ground is cooler than the air above  as air high above the air is hotter than those at near ground. So sound waves travelling in slanted manner will travel faster whereas those travelling near the ground will travel slowly. This makes the sound waves become more near each other near the sound but farther apart high above the ground. In addition the waves which travelled up will head back to earth after bending caused due to the difference in velocities.

So, an observer at a distance will get sound that has travelled along the ground and also that has ascended up into air, making the sound to be heard distinctly. Moreover, the sound seems to come from the sky rather than the ground itself. So sound is heard clearly at night than in day time.

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