Doppler Effect ....

If you think about it, it makes sense.

If i'm travelling at the speed of sound, and emitting sound in the direction i'm travelling, then the sound cannot propogate away from me, and so "collects" at a point just above the emitting surface. This "shock wave" cannot be heard by anyone who i'm travelling towards, but can be heard by people i travel past, (people who i'm not travelling towards at a speed higher than the speed of sound vectorially) in the form of a sonic boom.

I agree with that !
but we're talking about frenquencys ...
if I say : f(x) = Fe(1/x)

in my case, x tend to 0 ...
so lim f when x tend to 0 is +infinit
... does it mens that the frequency tnd to infinit?? but we cam hear only frequencys under 1000 Hz ... (I'm not sure of that point but i know it's not much... )
so how could you hear the supersonic boom ??

In the real life, infinity cannot be approached but a barrier will be formed and result in A Sonic Boom.

Sound propagates at 300 meters per second (depends on characteristics of the medium) with respect to the medium while light propagates at 300,000 kilometers per second with respect to the observer. In both cases movement of the source and/or the observer produces a dopler shift, but the speed of propagation is independent of both the speed of the source and the speed of the observer. Light does however slow somewhat in mediums other than vacuum. Sound does not propagate in a perfect vacuum, but it can in space under some conditions even at one millionth of sea level air pressure as evidinced by the bow wave that planet Earth produces in the solar wind. Please correct, refute and/or embellish. Neil

Does this mean that infinity is what we use to describe the incompetence of the mathematical formula which is applied?

Infinity in math formulas is not attained due to the "explosion" of ignored factors in the formulas as the parameters or variables go to a critical values.