A sound source and an observer are stationary. If the velocity of sound is changed, then the frequency of sound source heard by observer will

Question

Accepted Answer

The correct answer is Option 3: "remain same."

1. **Explanation for Option 3 (Correct Answer):**
   - The frequency of a sound wave is determined by the source of the sound and is independent of the medium through which it travels. When both the sound source and the observer are stationary, the frequency of the sound heard by the observer is the same as the frequency emitted by the source. 
   - The velocity of sound in a medium affects the wavelength of the sound wave, but not its frequency. The relationship between velocity (v), frequency (f), and wavelength (λ) is given by the equation $ v = f \times λ $. If the velocity of sound changes, the wavelength will adjust accordingly to maintain the same frequency, as long as the source and observer remain stationary.

2. **Explanation for why other options are incorrect:**
   - **Option 1 (increase):** This option is incorrect because the frequency of the sound is determined by the source and not by the velocity of sound in the medium. A change in the velocity of sound does not affect the frequency heard by a stationary observer.
   - **Option 2 (decrease):** Similar to Option 1, this option is incorrect because the frequency remains constant regardless of changes in the velocity of sound when both the source and observer are stationary.
   - **Option 4 (increase if the velocity of sound is increased):** This option is incorrect because the frequency of the sound does not depend on the velocity of sound in the medium. The frequency remains constant, and only the wavelength changes to accommodate any change in sound velocity.

In conclusion, the frequency of the sound heard by the observer remains the same when both the source and observer are stationary, regardless of changes in the velocity of sound. This is why Option 3 is the correct answer.