What is Mach Speed? Sound Barrier Explained Simply

June 25, 2025 • 4 min read

When a fighter jet goes "Mach 2," it sounds impressive — but what does it actually mean? Mach numbers measure speed relative to the speed of sound, and they are more nuanced than you might think.

What is Mach 1?

Mach 1 is not a fixed speed — it changes with temperature and altitude. The speed of sound decreases in colder air. At cruising altitude (35,000 feet / 10,700 m), Mach 1 is only about 295 m/s (1,062 km/h) because the air is much colder up there.

Speed Ranges by Mach Number

• ▶ Subsonic: Below Mach 0.8 (below 980 km/h at sea level)
• ▶ Transonic: Mach 0.8-1.2 (mixed subsonic and supersonic airflow)
• ▶ Supersonic: Mach 1.2-5.0
• ▶ Hypersonic: Mach 5.0-10.0
• ▶ High-hypersonic: Above Mach 10

Examples of Mach Speeds

• ▶ Commercial aircraft (Boeing 787): Mach 0.85 (about 900 km/h)
• ▶ Concorde: Mach 2.04 (2,180 km/h) — retired in 2003
• ▶ F-22 Raptor: Mach 2.25 (2,410 km/h)
• ▶ SR-71 Blackbird: Mach 3.3 (3,529 km/h) — fastest air-breathing crewed aircraft
• ▶ Space Shuttle re-entry: Mach 25 (about 27,000 km/h)
• ▶ Bullet: Mach 2-3 depending on calibre

What is the Sound Barrier?

The "sound barrier" is not a physical wall. As an aircraft approaches Mach 1, air cannot move out of the way fast enough, creating a shock wave. This manifests as:

• ▶ Increased drag: Early aircraft needed enormous power to push through
• ▶ Sonic boom: A loud thunder-like sound heard on the ground
• ▶ Control difficulties: Aerodynamics behave differently in transonic flow

Chuck Yeager first broke the sound barrier on October 14, 1947, in the Bell X-1 rocket plane.

Why Mach Numbers Matter

Engineers use Mach numbers instead of km/h because aerodynamic forces depend on the ratio of speed to sound speed, not absolute speed. The same aircraft behaves very differently at Mach 0.8 versus Mach 1.2 regardless of what altitude (and therefore what absolute speed) those Mach numbers correspond to.