Although the speed of sound is lower at high altitude, they mostly fly in the Mach .80-.86 range, depending on the aircraft....which means they are flying at 80%-86% of the speed of sound regardless of their groundspeed.
You have to exceed Mach 1 (actually about Mach 1.1 to get a sonic boom).
Thank you for explanation. I suspected that was the case.
Is the barrier similar to cavitation in the ocean propulsion system?