# Independence of frequency in sound waves?

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?

• In regards to your second question, sound waves are dispersionless (in most cases). This means that a frequency vs. wavenumber plot would show a straight line for the range of frequencies/wavenumbers of interest. In dispersionless waves, the frequency can only change if it depends on the medium, which it does for sound waves. – honeste_vivere Oct 23 '14 at 21:11
• In regards to your last question, the sound wave will not care about your motion, only your measured frequency/wavenumber will care (due to the typical Doppler effects). That is, unless you move faster than the speed of sound. In which case, you have created a shock wave which can be dispersive and it locally alters the medium, thus altering the incident sound wave. – honeste_vivere Oct 23 '14 at 21:12

Frequency is an intrinsic property of a vibrating source. Waves (of a certain kind) can be differentiated only with their frequency and not their wavelength. It cannot be changed unless there's a relative velocity between you and the source. That's the Doppler effect, which is the apparent change in the frequency when observed with a certain velocity relative to the source. It applies to both sound and light with the only difference that we'll take SR's corrections for relativistic speeds near $c$.
The only reason that the velocity of sound waves differ with medium is only because they're just mechanical pressure waves. Hence, they depend on the properties of medium like pressure, density, elasticity, etc. This can be noticed in the Laplace-Newton equation $$v=\sqrt{\frac{\gamma p}{\rho}}$$
(It should be noted that as you increase density, you don't just decrease $v$, but you're also increasing the pressure. Hence, the velocity changes accordingly)