NASA Armstrong Fact Sheet: Sonic Booms

Summary

A sonic boom is a thunder-like noise a person on the ground hears when an aircraft or other type of aerospace vehicle flies overhead faster than the speed of sound, or “supersonic.”

Air reacts like fluid to supersonic objects. As those objects travel through the air, molecules are pushed aside with great force and this forms a shock wave, much like a boat creates a wake in water. The bigger and heavier the aircraft, the more air it displaces.

The Cause

The shock wave forms a “cone” of pressurized or built-up air molecules, which move outward and rearward in all directions and extend all the way to the ground. As this cone spreads across the landscape along the flight path, it creates a continu­ous sonic boom along the full width of the cone's base. The sharp release of pressure, after the buildup by the shock wave, is heard as the sonic boom.

The change in air pressure associated with a sonic boom is only a few pounds per square foot -- about the same pressure change experienced riding an elevator down two or three floors. It is the rate of change, the sudden changing of the pressure, which makes the sonic boom audible.

“Double” Booms

All aircraft generate two cones, at the nose and at the tail. They are usually of similar strength and the time interval between the two as they reach the ground is primarily dependent on the size of the aircraft and its altitude.

While some people on the ground may perceive the sound as a single sonic “boom,” many sonic booms produced from NASA’s research flights are easily heard as distinct “double” booms, similar to what was created by the space shuttle. This is the result of the two separate cones generated, at the nose and the tail of the aircraft.

General Factors Associated With Sonic Booms

There are several factors that can influence sonic booms -- weight, size, and shape of the aircraft or vehicle, plus its altitude, attitude, and flight path, and weather or atmospheric conditions.

A larger and heavier aircraft must displace more air and create more lift to sustain flight, compared with small, light aircraft. Therefore, they will create sonic booms stronger and louder than those of smaller, lighter aircraft. The larger and heavier the aircraft, the stronger the shock waves will be.

Altitude Effect

Altitude determines the distance shock waves travel before reaching the ground, and this has a significant effect on intensity. As the shock cone gets wider, and it moves outward and downward, its strength is reduced. Generally, the higher the aircraft, the greater the distance the shock wave must travel, reducing the intensity of the sonic boom.

Sonic Boom Carpet

The width of the boom “carpet” beneath the aircraft is about one mile for each 1000 feet of altitude. For example, an aircraft flying supersonic at 50,000 feet can produce a sonic boom cone about 50 miles wide. However, parts of the sonic boom carpet are typically weaker than others.

Maximum intensity for traditional supersonic aircraft is directly beneath the aircraft, and decreases as the lateral distance from the flight path increases, until it ceases to exist. The lateral spreading of the sonic boom depends upon altitude, speed, and the atmosphere – and is independent of the vehicle’s shape, size, and weight.

Size, Speed, and Atmosphere

As described earlier, the size and weight of the aircraft influence sonic booms. The ratio of aircraft length to maximum cross-sec­tional area also influences the intensity of the sonic boom. The longer and more slender the aircraft, the weaker the shock waves. The fatter and more blunt the vehicle, the stronger the shock wave can be.

Meanwhile, increasing speeds above Mach 1.3 results in only small changes in shock wave strength.

The direction of travel and the strength of shock waves are influenced by wind, speed, and direction, as well as by air temperature and pressure. At speeds slightly greater than Mach 1, their effect can be significant, but their influence is small at speeds greater than Mach 1.3.

Distortions in the shape of the sonic boom signatures can also be influenced by local air turbulence near the ground. This, too, will cause variations in the overpressure levels.

Aircraft maneuvering can cause distortions in shockwave patterns. Some maneuvers -- pushovers, accel­eration, and "S" turns -- can amplify the intensity of the shock wave. Hills, valleys, and other terrain features can create multiple reflections of the shock waves, and can likewise affect intensity.

Measuring Sonic Booms

Sonic booms are measured in pounds per square foot (psf) of “overpressure.” This is the amount of the increase that occurs over the normal atmospheric pressure which surrounds us (2,116 psf/14.7 psi).

At one pound overpressure, no damage to structures would be expected.

Overpressures of 1 to 2 psf are produced by supersonic aircraft flying at normal operating alti­tudes. Some public reaction could be expected above 1 psf.

Rare minor damage may occur with 2 to 5 psf overpressure.

As overpressure increases, the likelihood of structural damage and stronger public reaction also increases. Tests, however, have shown that structures in good condition have been undamaged by overpressures of up to 11 psf.

Typical overpressure of aircraft types are:

  • SR-71: 0.9 psf, speed of Mach 3, 80,000 feet
  • Concorde SST: 1.94 psf, speed of Mach 2, 52,000 feet
  • F-104: 0.8 psf, speed of Mach 1.93, 48,000 feet
  • Space Shuttle: 1.25 psf, speed of Mach 1.5, 60,000 feet, landing approach
Last Updated: Aug. 15, 2017
Editor: Yvonne Gibbs