 Subsonic flow

A subsonic flow has Mach number less than one, so its speed is below the local sound speed.

For uniform gas moving at $U{\mathbf{e}}_x$ past a resting body, an upstream-directed acoustic wave has body-frame velocity $(U-c){\mathbf{e}}_x$. When $0\le M<1$, this velocity points upstream. This is why the body can influence the upstream fluid flow through small pressure disturbances.