The Sound Barrier

The sound barrier is the speed of sound in air. In still air at sea level and 20 degrees Celsius, sound travels at about 343 metres per second, or 1,235 km/h. Anything moving faster than this is travelling at supersonic speed. Anything slower is subsonic. Going faster than sound is one of the more dramatic things a vehicle can do: it produces a famous sonic boom, a thundering crack that can rattle windows on the ground. The first plane to fly faster than sound did so in 1947, and the achievement remains one of the most famous in aviation history.

  • Speed of sound343 m/s1,235 km/h, sea level, 20 C
  • Mach 1Equal to speed of soundMach 2 is twice as fast
  • First broken byChuck Yeager, 1947Bell X-1 rocket plane
  • Concorde top speedMach 2.042,180 km/h, retired 2003
  • Fastest manned aircraftSR-71 at Mach 3.33,540 km/h
  • Sonic boomHeard, not produced onceHeard whenever a plane passes overhead

What is the sound barrier?

The "barrier" is not a physical wall, but it does behave in some ways like one. As a plane approaches the speed of sound, the air in front of it cannot get out of the way quickly enough. The air piles up into a shock wave that the plane has to push through. The pile-up makes drag and pressure rise sharply, and it makes the plane shake.

Early jets in the late 1940s struggled with this effect. Some planes broke up trying to dive past Mach 0.95. Pilots called the strange behaviour a "wall" they could not push through. Engineers responded by redesigning wings (sweeping them back into an arrow shape) and reinforcing planes against the new stresses. The redesigns finally allowed planes to safely go faster than sound.

Mach number

Aircraft speeds at high speeds are usually measured in Mach numbers, named after the Austrian physicist Ernst Mach. The Mach number is simply how many times the speed of sound you are travelling at.

  • Mach 0.5 = half the speed of sound (about 600 km/h, typical passenger jet take-off speed)
  • Mach 0.85 = 85 per cent of sound speed (typical airliner cruise)
  • Mach 1 = equal to sound speed (the sound barrier itself)
  • Mach 2 = twice the speed of sound (Concorde cruise)
  • Mach 3 = three times (SR-71 spy plane)
  • Mach 5 and above = "hypersonic", a regime where the air actually heats up enough to ionise around the vehicle
  • Mach 25 = roughly orbital speed (the Space Shuttle reaching low Earth orbit)
Fact The very first person to break the sound barrier was American pilot Charles "Chuck" Yeager on 14 October 1947. He flew the experimental rocket-powered Bell X-1 to Mach 1.06 over California. Just two days before the flight, he had broken two ribs falling off a horse, but he flew anyway, using a sawed-off broomstick to help him close the cockpit door. He died in 2020, aged 97.

Sonic booms

When a plane is travelling faster than sound, it is constantly outrunning its own sound waves. The waves cannot get ahead, so they pile up into a cone-shaped shock wave trailing behind the plane. Anyone on the ground who is suddenly swept by this shock wave hears a sharp, loud "BOOM" sound: the sonic boom.

Important misunderstanding: the sonic boom is not a one-off event at the moment the plane breaks the sound barrier. The boom is heard continuously by anyone the plane flies over while it remains supersonic. A supersonic jet leaves a continuous "carpet" of booms behind it. Anyone standing under the carpet hears the boom as the plane passes overhead.

Supersonic flight over land is banned in most countries because the booms can frighten people, scare animals and even crack windows or plaster on the ground. Concorde flew supersonic only over oceans.

How fast does sound really go?

The speed of sound varies with conditions:

  • Temperature: sound moves faster in hotter air. At -20 C, sound is about 318 m/s. At +30 C, sound is about 349 m/s.
  • Air density: sound moves at the same speed in thin or thick air, but the air mass changes how long pressure pulses last.
  • Altitude: at the cruise altitude of an airliner (around 11 km up), air is cold (around -56 C) so sound is around 296 m/s. A plane cruising at Mach 0.85 there is doing about 900 km/h.
  • Other gases: sound moves faster through lighter gases. In helium, sound is nearly 3 times faster than in air, which is why a helium-filled voice sounds so squeaky.
Did you know? The tip of a stockmans whip moves so fast (over the speed of sound) that the crack it makes is actually a tiny sonic boom. The shape of the whip lets you transfer energy through it efficiently, with the tip building up to supersonic speeds as the wave reaches the end. Whips are probably the first human-made supersonic objects, used for centuries before aircraft existed.

The era of supersonic flight

After Chuck Yeagers historic flight in 1947, military aircraft quickly began to break the sound barrier routinely. By the 1960s, several fighter jets could exceed Mach 2.

Civil aviation followed for a while. The Anglo-French Concorde began passenger service in 1976, carrying passengers across the Atlantic at Mach 2 (twice the speed of sound), cutting the New York to London flight to about 3.5 hours. The Russian Tupolev Tu-144 also flew supersonic with passengers briefly. Both were eventually retired (Concorde in 2003) due to high costs, low capacity and noise complaints.

In 2025, several companies are working on a new generation of supersonic airliners (Boom, Aerion and others), aiming for quieter sonic booms and better fuel efficiency. Whether they succeed commercially remains to be seen.

Try this Watch a high-speed video of a bullet being fired into a still pond or piece of fabric. The shock cone trailing the bullet is the same shape as the shock wave behind a supersonic aircraft, just much smaller. Many slow-motion videos online show this beautifully. You can also see the wake from a fast boat as the same cone-shape, only on water; gentle ripples cannot keep up with the boat, so they pile up into a V-shape behind it. Same physics, just slower waves.
Deeper dive: the rise and fall of Concorde

For 27 years, between 1976 and 2003, the most glamorous way to cross the Atlantic was on the Concorde: a needle-shaped white airliner that flew at Mach 2 (twice the speed of sound) at 18 km altitude, where the sky overhead is nearly black. It was an engineering marvel, a political project and a luxury experience all rolled into one.

Concorde was built jointly by British and French companies starting in the 1960s, at a cost of around 1.3 billion pounds in 1970s money (perhaps 15 billion in todays prices). Only 20 were ever built; 14 entered commercial service with British Airways and Air France.

At Mach 2, Concorde could fly New York to London in about 3.5 hours, compared with 7 to 8 hours for a normal airliner. The cabin was small (100 passengers), narrow and noisy, but the food was excellent and the experience legendary. A return ticket cost about 4,000 pounds.

The problems were many. The plane was extremely thirsty (4 huge afterburning engines drinking fuel like rockets). The sonic boom forced it to slow to subsonic speed over land, so its main use was transatlantic flights. The aircraft body got so hot at Mach 2 that it actually grew 25 cm in length during cruise. Maintenance was expensive.

The end came after a tragic crash near Paris in July 2000 that killed all 109 people on board. Although the cause was a stray metal strip on the runway that punctured a tyre (not a design fault), the disaster shook confidence. Combined with rising fuel costs after the September 2001 terror attacks, both airlines decided to retire Concorde in 2003.

For 22 years now, no civilian airliner has flown faster than sound. Several startups are trying to bring back supersonic passenger flight with quieter, more fuel-efficient designs. Whether they will succeed where Concorde struggled remains an open question.

For more, see what is sound and echoes.