I'm a physics major first year. We were discussing doppler shift, so I started putting it in terms of limits (limit as vsource -> 0 is unshifted, obviously). But then i tried when vsource -> v or when vsource -> c and things got weird. When it approaches the speed of sound it approaches an infinite pitch shift, ie, when the source of sound moves towards you at the speed of sound it sounds really high pitch until it cannot be heard. But as vsource approaches the speed of light, the frequency is multipled by a shift that goes to 0.

So, the frequency goes very high towards the speed of sound, then past the speed of sound the frenquency is negative (traveling in opposite direction?), gradually slowing as it approaches the speed of light? Am I missing something here? Graph and reasoning attached