Question

A source of sound frequency 600Hz is placed inside water. The speed of sound in water is 1500m/s and in air is 300m/s. What will be the frequency of sound recorded by an observer standing in air?

Answer 1

When sound travels from one medium to another, both its velocity and wavelength undergo changes, but its frequency stays the same.
So the freq of the recorded sound will be 600Hz.
This result makes sense if you think of what you can hear being underweater, in a pool for example, if the freq were to change you would hear a different pitch for voices or music coming from outside the pool !!