I'm a bit confused about this. So say there's an observer and a source of sound. If the observer moves towards the source of sound, the frequency seems to increase because he encounters more wavelengths in the same amount of time. In a second case, if the source moved towards the observer, the wavelengths get sort of 'bunched up' and it makes the effective wavelength shorter, again increasing the frequency. But even if their relative velocities are the same in both cases, the perceived frequency is different. My textbook (and the internet sources I looked at) didnt really explain this, though one place off handedly said it's because sound only propagates in a medium. I get how it is numerically different. Can someone help me understand kind of quantitatively how it's different? And why the same doesn't occur for light waves (where both cases would give the same answer?) Thanks for your help and time!