Question

The frequency of a source is 20 kHz. The frequencies of sound wave produced by it in water and air will be:
A). Same as that of source $ \simeq $ 20 kHz
B). > 20kHz
C). <20kHz
D). Depends upon velocity

Answer 1

Hint: The frequency of that sound depends beyond source. The frequency of the sound does not depend beyond their medium in which it propagates.

Complete answer:
When the sound travels from one medium to the different medium, its frequency remains the same.
When the sound travels from the less dense medium to the higher denser medium, then there is an increase in the speed of the sound and the wavelength of the sound is decreasing. But, their frequency is the same as the source.
When the sound travels from the higher denser medium to the less dense medium, then there is a decrease in the speed of the sound and the wavelength of the sound is increasing. But, their frequency is the same as the source.
When the sound wave of the sound propagates in a different medium, then the frequency of the sound does not change. The frequency does not depend on a medium and the frequency of the sound in a medium is dependent on the frequency of the source.
 So, the frequency of the sound in air and water will be the same as the frequency of the source.
So, the frequency of a source of 20 kHz remains the same when produced in water or in air.
So, the option (A) is correct.

Note: 20 kHz is the highest frequency of the most humans can hear. The sound of frequency is greater than 20 kHz then it is known as Ultrasonic Waves. The frequency is the number of the vibrations counted as per second. The frequency of the wave is the total number of the produced waves in one second.