Mach Cone & DopplerA source slides along, pinging out a ring of sound every moment. Slower than sound, the rings just crowd ahead of it (higher pitch) and stretch out behind (lower pitch). Reach the speed of sound and every ring piles onto the same wall — a shock. Go faster and the walls fold into a trailing V, the sonic boom.Christian Doppler predicted the pitch shift in 1842; Ernst Mach photographed the shock cone of a supersonic bullet in 1887. The cone’s half-angle obeys one clean line: sin θ = 1 / M, where M is speed ÷ sound speed.