# In terms of the Doppler effect, what happens when the source is moving faster than the wave?

I'm just trying to understand this problem from a qualitative perspective. The Doppler effect is commonly explained in terms of how a siren sounds higher in pitch as it is approaching a particular observer. I understand this is because the velocity of the wave is constant and so the frequency of the waves increase as they are bunched together. What would happen if a siren was mounted on say a plane traveling at a supersonic speed? To clarify what would the observer observe/hear? Apologies if my question is not phrase very well my knowledge of physics is very rudimentary.

• – Nicolau Saker Neto Apr 21 '13 at 1:20
• The thread linked to by Nicolau is about a moving observer. This question is about a moving source. The equation given in Eudoxos's comment on the other question is the one for the moving source case, so it would actually be more relevant here. – user4552 Apr 21 '13 at 2:29

The first image shows an object traveling at Mach 1 ($v=c$). The second one shows the object traveling at some supersonic velocity ($v>c$). For both the cases, the longitudinal pressure waves pile up. Say the observer is standing in the ground and the object is traveling at $c$. The observer can't hear the pitch of sound because, the waves reach him all at once and hence, he'd hear a loud "bash". The most necessary thing is that he had to wait until the source arrives. When the source is directly overhead, he hears the shock waves.