Deriving the doppler shift equation for sound

[21joanna12]

I am looking at derivations of the Doppler effect for sound and light, and I am very confused about what stays the same when it comes to sound.

In this video, at 5:10, it is said that the wavelength will be the same.

I'm not sure I agree with this because I'm pretty sure both the observed frequency and observed wavelength change, but on the other hand perhaps the wavelength would stay the same and it would be the observed frequency and velocity that would change, because I know that the Doppler shift equations for light and sound are different, and perhaps that is because all observers measure the speed of light to be the same but they measure different speeds for sound?

Then in this video, the equations on the board suggest that the speed of sound will be absorbed to be the same.


Could this discrepancy also have something to do with the fact that the first deals with a moving observer and the second deals with a moving source?

Thank you in advance :)

 

[mfb]

Wavelength stays the same if the observer is moving, but the source does not (then the speed relative to the observer changes). If the source moves but the observer does not, the wavelength changes but the speed relative to the observer stays the same. If both move, you get both effects together.