Which one of the following statements about sound is INCORRECT?

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:

• As sound waves are longitudinal waves, the air particles vibrate to and fro in the direction of propagation of sound. Therefore option 1 is correct.

• The relation between the temperature and the velocity of sound in a gas is given by

\(\Rightarrow \frac{v_t}{v_o}=\sqrt{\frac{T}{T_o}}\)

Where vt = velocity of sound at temperature T, v0 = velocity of sound at 0°, and T, and To = temperature at 0° and T° 

• From the above relation, it is clear that the velocity of sound is directly proportional to the square root of its absolute temperature. Therefore option 2 is correct.
• Sound is a mechanical wave and needs a material medium like air, water, steel, etc. for its propagation. It cannot travel through a vacuum. Therefore option 3 is incorrect.
• 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 4 is correct.