Why does the frequency of sound wave depend only on the source? Why is the frequency and not any other "quality" independent of everything but the source?
And that said, why is velocity and wavelength of the wave dependent only on the medium?
And finally – in case of the doppler effect in case of sound, does or doesn't the independence of these quantities hold true?
Best Answer
Maybe it is better to ask the other way around. Why is the frequency the only property that is independent? Although, mentioning the Doppler effect, you already found an example, showing that this is not exactly true. Hence the frequency depends on the source-velocity relative to me, and my velocity relative to the source. So "independent" is tricky and the question somewhat vaguely defined. That the speed depends on the medium should be clear, when looking at textbooks on acoustics. The frequency is constant, because source and observer have the same absolute time. If the source sends a pressure maximum every 2 ms, this is the same for the observer. This would change, however, if you consider relativistic effects. In short, the source can only fix output energy (amplitude) and time between two wavefronts, i.e. frequency.