Sonic simply relates to sound — sonic waves, a sonic boom. Supersonic means travelling faster than the speed of sound, roughly 767 mph at sea level. The related terms run subsonic (slower than sound), transonic (around it), supersonic (faster) and hypersonic (from about five times faster).
The core difference
One is an adjective about sound; the other is a speed threshold.
- sonic — of or relating to sound: sonic frequency, sonic boom.
- subsonic — slower than sound.
- supersonic — faster than sound (Mach 1+).
- hypersonic — from roughly Mach 5.
What a sonic boom actually is
Not a one-off bang at the moment of breaking the sound barrier, as commonly believed. An aircraft travelling supersonically produces a continuous shockwave, and the boom is heard by each observer as that cone sweeps over them. The aircraft is booming the whole time it is supersonic.
Mach numbers
Speed is given as a Mach number — the ratio to the local speed of sound. Because that speed falls with temperature, Mach 1 is slower at altitude than at sea level, so the threshold is not a fixed mph figure.
Frequently asked questions
How fast is supersonic?
Faster than the local speed of sound — roughly 767 mph at sea level, though the threshold falls at altitude because sound travels slower in colder air.
Does a sonic boom happen only once?
No. A supersonic aircraft produces a continuous shockwave; each observer hears the boom as that cone passes over them.
What does Mach 2 mean?
Twice the local speed of sound. Mach numbers are ratios, not fixed speeds, because the speed of sound varies with temperature.