Quoting from the Media:
"At 15-20 ft altitude and at .93-Mach (well over 600 MPH) you can clearly see the shock wave being pushed in front of the jet in the grass, much like the wave a ship pushes ahead of it. Look closely and you can see the grass rolling over just like a wave"

"...go much faster and the jet passes that wave and you would hear a sonic boom and the jet would be supersonic ...and yes, that's fast!"