MP Board · Class 9 · Science · Sound(a) Describe an experimental activity using a Bell Jar setup to demonstrate that sound requires a material medium to propagate and cannot travel through a vacuum. (b) How does the speed of sound vary in different states of matter (solids, liquids, and gases)? State the effect of temperature on the speed of sound in air. (c) Numerical: A person standing near a vertical mountain wall claps his hands and hears the echo after $0.6\text{ seconds}$. Calculate the distance of the mountain wall from the person if the speed of sound in air is $344\text{ m/s}$.

Part (a): Bell Jar Experiment (Sound Requires a Medium)

• Objective: To prove that sound waves require a material medium for propagation and cannot travel through vacuum.

• Apparatus Required: An airtight glass bell jar, an electric bell, a battery, a switch, and a vacuum pump connected through a tube at the bottom.

• Procedure:

  1. Hang an electric bell inside an airtight glass bell jar and connect it to a battery through a switch.
  2. Connect the base of the bell jar to a vacuum pump.
  3. Press the switch to complete the circuit. The hammer strikes the gongs, and a clear ringing sound is heard outside.
  4. Now, start operating the vacuum pump to gradually pump out the air from inside the bell jar while the bell is ringing.

• Observations:

  • As air is gradually removed, the sound of the bell becomes fainter and fainter.
  • When almost all the air is pumped out (creating a vacuum inside), no sound is heard at all, even though the hammer is still seen striking the gong continuously.
  • When air is allowed to re-enter the bell jar, the ringing sound becomes clearly audible again.

• Conclusion: Sound cannot travel through a vacuum; it requires a material medium (like air) for propagation.

Part (b): Speed of Sound in Different Media and Temperature Effect

• Variation Across States of Matter:

  • Sound travels through elastic media as mechanical waves. The speed of sound depends on the elasticity and density of the medium.
  • Speed of sound is maximum in solids, intermediate in liquids, and minimum in gases: $$\text{Speed in Solids} > \text{Speed in Liquids} > \text{Speed in Gases}$$
  • Example: Speed of sound in Aluminum $\approx 6420\text{ m/s}$, in Water $\approx 1500\text{ m/s}$, in Air $\approx 344\text{ m/s}$.

• Effect of Temperature:

  • As the temperature of the medium increases, the speed of sound increases.
  • Reason: Higher temperature increases the kinetic energy of gas molecules, allowing sound vibrations to transmit faster.
  • In air, the speed of sound increases by approximately $0.61\text{ m/s}$ for every $1^\circ\text{C}$ rise in temperature.

Part (c): Step-by-Step Numerical Solution

Given:

• Speed of sound in air ($v$) = $344\text{ m/s}$
• Time taken to hear echo ($t$) = $0.6\text{ s}$

Formula: $$\text{Distance } (d) = \frac{\text{Speed } (v) \times \text{Time } (t)}{2}$$

Calculation: $$d = \frac{344 \times 0.6}{2}$$ $$d = 172 \times 0.6$$ $$d = 103.2\text{ meters}$$

Answer:\nThe distance of the mountain wall from the person is $103.2\text{ meters}$.