I have used the Doppler effect relations to calculate the frequency of the sound of an object that is moving faster than the speed of sound, by using this formula:
$frequency = (\frac{v_{sound}}{v_{sound} – v_{source}})* f_{source}$
but the result is smaller than zero (and we know it isn't possible).
The questions are how can we calculate the real frequency we are hearing? And is doppler relations limited to slower-than-sound objects?
Comment:
- *: Let's assume that the sound receiver is not moving
Best Answer
If the source is moving faster than the speed of sound then ahead of the source it produces a shock wave instead of a sound wave. This is the infamous sonic boom associated with faster than sound flight. Your equation is giving you a silly result because no propagating sound wave exists, so there is no frequency to be calculated.
If you're interested in reading more there are lots and lots of questions about shock waves on this site.