Which one of the following statements about sound is NOT correct?

CONCEPT:

• Sound wave: The longitudinal wave in an elastic medium that produces an audible sensation is called a sound wave.
  • As sound waves are longitudinal waves, the air particles vibrate to and fro in the direction of propagation of sound.
  • It is a wave of compression and rarefaction.
  • Compressions and rarefactions are part of a sound wave.

• Compression: A region in a longitudinal wave where the particles are closest together is called compression. Compression has high density and high pressure

  Rarefaction: A region in a longitudinal wave where the particles are furthest apart is called rarefaction. The rarefaction has a low density and low pressure

EXPLANATION:

• The speed of sound is in dry air, which is like 343 m/s whereas the speed of light is 3 × 10⁸ m/s. Therefore, sound travels at a speed slower than the speed of light. Hence option 1 is correct.
• As sound waves are longitudinal waves, the air particles vibrate to and fro in the direction of propagation of sound. Therefore option 2 is incorrect and option 3 is correct.

Effect of density: 

• The velocity of sound decreases, as density increases.

\(v = \sqrt {\frac{{\gamma P}}{\rho }} \Rightarrow v \propto \frac{1}{{\sqrt \rho }}\)

• From the above equation, it is clear that the velocity of sound in a gas is inversely proportional to the square root of its density, and as we know the density of moist air is less than that of dry air, therefore, the velocity of sound in moist air is greater than that in dry air. Hence option 4 is correct.