If you travel at mach 1 (the speed of sound) away from the source, you'll be going the same speed as the soundwaves and will hear nothing. It stands to reason that travelling at half that speed would halve the frequency and travelling mach 1 towards would double it.
Therefore:
doppler_corrected_frequency = source_frequency * (1 + approach_speed / speed_of_sound)
EDIT: Whoops. This is incorrect. Something travelling towards you at mach 1 wouldn't be heard at double the frequency - it wouldn't be heard at all: SONIC BOOM!
Therefore:
doppler_corrected_frequency = source_frequency * (1 + approach_speed / speed_of_sound)
EDIT: Whoops. This is incorrect. Something travelling towards you at mach 1 wouldn't be heard at double the frequency - it wouldn't be heard at all: SONIC BOOM!