When you hear someone say that a plane is traveling at Mach 1 what exactly does that mean? It means that the plane is traveling at the speed of sound. Mach 1 equals the speed of sound whereas Mach .5 is half the speed of sound and Mach 2 is twice the speed of sound. That's some pretty cool general information to know, right?
Another cool thing about Mach numbers is that the speed of sound is not constant; it depends on temperature and altitude. In other words, sound travels at different speeds depending on where it is traveling. Here's a good trivia question to ask somebody: If two planes are both traveling at Mach 1 can they be going different speeds? If that somebody answers no, they would be wrong.
Here's some more information relating to our topic: The word sonic is a synonym for Mach 1 and speed of sound. So if you're traveling at sonic speeds, you're traveling alongside sound.
To watch Sonic the Hedgehog travel at the speed of sound check out the video below!
FYI: The speed of sound (Mach 1) at sea level and 15o C (59o F) is 761.2 mph (340.3 m/s).