Mach number a small area around the object's leading edge. (Fig.1b) Fig. 1. "Mach number in transonic airflow around an airfoil; M < 1 (a) and M > 1 (b)." When an aircraft exceeds Mach 1 (i.e. the sound barrier), a large pressure difference is created just in front of the aircraft. This abrupt pressure difference, called a shock wave, spreads backward and outward from the aircraft in a cone shape (a so-called Mach cone). It is this shock wave that causes the sonic boom heard as a fast moving aircraft travels overhead. A person inside the aircraft will not hear this.
Mach (kernel) as well. Mach 2.5 was also selected for the NeXTSTEP system and a number of commercial multiprocessor vendors. Mach 3 led to a number of efforts to port other operating systems parts for the microkernel, including IBM's Workplace OS and several efforts by Apple to build a cross-platform version of the classic Mac OS. Mach was originally intended to be a replacement for classical monolithic UNIX, and for this reason contained many UNIX-like ideas. For instance, Mach used a permissioning and security system patterned on UNIX's file system. Since the kernel was privileged (running in "kernel-space") over other OS servers