Correct Option
The statement that the velocity of sound in air decreases as the humidity increases is incorrect. In reality, the velocity of sound in air increases with an increase in humidity. Water vapour (H₂O) has a lower molar mass (approximately 18 g/mol) compared to the average molar mass of dry air (approximately 29 g/mol, primarily N₂ at 28 g/mol and O₂ at 32 g/mol). When humidity increases, the proportion of water vapour in the air increases, effectively decreasing the average density of the air. Since the speed of sound is inversely proportional to the square root of the density, a decrease in density leads to an increase in the speed of sound.
Incorrect Options
The velocity of sound in air increases with the increase of temperature:
The speed of sound in a gas is directly proportional to the square root of its absolute temperature. As temperature increases, the kinetic energy of air molecules increases, leading to more frequent and energetic collisions, which facilitates faster propagation of sound waves. Therefore, this statement is correct.
The velocity of sound in air is independent of pressure:
For an ideal gas at a constant temperature, the speed of sound is independent of pressure. According to the ideal gas law (PV=nRT), if temperature is constant, an increase in pressure leads to a proportional increase in density. Since the speed of sound depends on the ratio of pressure to density (or bulk modulus to density), and this ratio remains constant at constant temperature, the speed of sound does not change with pressure. Therefore, this statement is correct.
The velocity of sound in air is not affected by the change in amplitude and frequency:
The speed of sound in a medium is determined by the properties of the medium itself (elasticity and density), not by the characteristics of the sound wave such as its amplitude or frequency. Amplitude relates to the intensity or loudness of the sound, while frequency relates to its pitch. These wave properties do not influence the speed at which the sound propagates through a given medium. Therefore, this statement is correct.