Sonic boom facts for kids
A sonic boom is a loud sound. It happens when something moves through the air faster than the speed of sound. Imagine a huge explosion or a loud thunderclap – that's what a sonic boom can sound like!
You might have heard small sonic booms already. The crack of a bullwhip or a fast-moving bullet are tiny examples.
Big sonic booms from supersonic aircraft can be very loud. They can even wake people up or cause small damage to buildings. Because of this, planes are usually not allowed to fly supersonically over land. Scientists are working on ways to make sonic booms quieter. This could allow faster flights over land in the future.
A sonic boom isn't just a single sound when an object first breaks the sound barrier. Instead, it's a continuous sound created as long as the object flies faster than sound. You only hear it if you are in a specific area, shaped like a cone, behind the fast-moving object. As the object flies, this "sound cone" moves with it. When the cone passes over you, you hear the "boom."
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How Sonic Booms Happen
When a plane flies, it pushes air out of the way. This creates invisible pressure waves in front and behind it, like the waves a boat makes in water. These waves normally travel at the speed of sound.
But if the plane goes faster, the waves get squished together. They can't move out of the way fast enough! Eventually, they combine into one big shock wave. This special speed is called Mach 1. It's about 1,192 kilometers per hour (740 mph) at sea level when it's 20 degrees Celsius (68°F).
When a plane flies smoothly at supersonic speeds, this shock wave forms a Mach cone. Think of it like a cone of sound with the plane at its tip. The faster the plane flies, the narrower and pointier this cone becomes.
The shock wave creates a quick change in air pressure. First, there's a sudden increase in pressure, then it drops to a lower pressure, and finally, it returns to normal. This pattern is called an "N-wave" because of its shape. You hear the "boom" when these pressure changes hit your ears. An N-wave actually causes two booms: one from the pressure rising and another when it returns to normal. This is why you sometimes hear a "double boom."
As long as the plane flies faster than sound, it keeps making this boom. The sound spreads out on the ground in a narrow path, like an unrolling red carpet. This is called the boom carpet. How wide it is depends on how high the plane is flying.
Most supersonic planes today create a pressure change of about 50 to 500 Pascals (1 to 10 pounds per square foot) for an N-wave. Very strong sonic booms have been recorded, but they usually don't cause serious harm. For example, a powerful boom of 7,000 Pascals (144 psf) was recorded from an F-4 jet flying very low. It didn't injure the researchers nearby.
Buildings in good condition are usually safe from sonic booms under 530 Pascals (11 psf). Most sonic booms heard by communities are much weaker, below 100 Pascals (2 psf).
The loudness of a sonic boom depends on the plane's size and shape. The faster and higher a plane flies, the weaker the boom can become. Longer planes tend to spread out their booms more, which leads to a less powerful boom.
Planes can also create smaller shock waves at other points, like the front edge of the wings or engine openings. These smaller waves can combine with the main shock wave, making the overall boom seem louder.
What Are Supersonic Aircraft?
Supersonic aircraft are planes that can fly faster than Mach 1, which is the speed of sound. These planes can reach speeds up to five times the speed of sound, or Mach 5!
Most supersonic planes fly between 700 to 1,500 miles per hour (1,126 to 2,414 km/h). Some are designed with very sleek shapes to cut through the air easily. Others rely on powerful engines to push them to incredible speeds.
Measuring Sonic Booms
The pressure from a sonic boom is usually measured in Pascals (Pa) or pounds per square foot (psf). Planes flying higher up create less pressure on the ground. This is because the shock wave spreads out and becomes weaker as it travels further.
Here are some examples of sonic boom pressures from different aircraft:
| Aircraft | Speed | Altitude | Pressure | |
|---|---|---|---|---|
| SR-71 Blackbird | Mach 3+ | 80,000 feet (24,000 m) | 0.9 lbf/ft2 | 43 Pa |
| Concorde (SST) | Mach 2 | 52,000 feet (16,000 m) | 1.94 lbf/ft2 | 93 Pa |
| F-104 Starfighter | Mach 1.93 | 48,000 feet (15,000 m) | 0.8 lbf/ft2 | 38 Pa |
| Space Shuttle | Mach 1.5 | 60,000 feet (18,000 m) | 1.25 lbf/ft2 | 60 Pa |
| Ref: | ||||
Making Sonic Booms Quieter
In the past, people thought that flying planes higher would solve the problem of loud sonic booms. But tests with planes like the North American XB-70 Valkyrie in the 1950s showed that booms were still a problem, even at very high altitudes.
Scientists like Richard Seebass and Albert George developed ways to measure how loud a sonic boom would be. They created a "figure of merit" (FM) based on a plane's weight and length. A lower FM meant a quieter boom. For example, the Concorde had an FM of about 1.4, which was considered too loud for regular flights over land. This led to rules that stopped most supersonic flights over populated areas.
Today, researchers are still working to make supersonic planes quieter. They are trying new designs to spread out the shock waves. One idea is to shape the plane's nose and wings in special ways. This could make the boom sound more like a gentle thud instead of a loud crack.
NASA and Lockheed Martin are working together on a project called the Low Boom Flight Demonstrator. Their goal is to reduce the sonic boom to sound like a car door closing. The first flight of this special plane was planned for 2024.
How People Hear Sonic Booms
The sound of a sonic boom can change depending on how far away you are and the shape of the aircraft. It's often heard as a deep "double boom," similar to fireworks going off. It's important to remember that the boom isn't just a single event; it's continuous along the plane's flight path. Pilots inside the plane don't hear the boom themselves; they only see instruments showing the pressure waves.
In 1964, NASA and the Federal Aviation Administration conducted the Oklahoma City sonic boom tests. For six months, eight sonic booms occurred daily over the city. While scientists gathered important information, people complained a lot. This even led to a lawsuit against the government.
Sonic booms were also a problem in parts of the UK, like North Cornwall and North Devon, which were under the flight path of the Concorde. Windows would rattle, and sometimes even parts of roofs would shake loose.
Scientists are now looking at how loud and annoying a sonic boom feels to people. They measure something called perceived loudness. This takes into account how quickly the pressure changes and the different sound frequencies. A quick, sharp pressure change feels louder than a slower, smoother one, even if the overall pressure is the same.
Sonic booms usually last less than a second. For fighter jets, it's about 100 milliseconds (0.1 second), and for larger planes like the Space Shuttle or Concorde, it's about 500 milliseconds.
The strength and width of a sonic boom path depend on the plane and how it's flown. Generally, the higher a plane flies, the weaker the boom on the ground. However, flying higher also spreads the boom over a wider area. The boom is strongest directly under the plane's path and gets weaker as you move further away.
Weather conditions can also affect sonic booms. Temperature changes, humidity, and wind can all change how a boom sounds on the ground. Even the ground itself plays a role. Hard surfaces like concrete can make the boom sound louder due to reflections. Soft surfaces like grassy fields can help make it quieter.
Currently, there are no agreed-upon rules for how quiet a sonic boom needs to be. More research is needed to understand how people react to these sounds. Until then, it's unlikely that rules preventing supersonic flights over land will change in many countries, including the United States.
The Bullwhip's Crack
The sharp cracking sound a bullwhip makes is actually a small sonic boom! The very tip of the whip, called the "cracker," moves faster than the speed of sound. This creates a tiny shock wave, just like a supersonic jet.
A bullwhip gets thinner from the handle to the cracker. When you swing it sharply, the energy moves down the whip. As the whip gets thinner and lighter, its speed increases dramatically, allowing the tip to break the sound barrier.
Images for kids
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NASA F-5E modified for DARPA sonic boom tests
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New research is being performed at NASA's Glenn Research Center that could help alleviate the sonic boom produced by supersonic aircraft. Testing was completed in 2010 of a Large-Scale Low-Boom supersonic inlet model with micro-array flow control. A NASA aerospace engineer is pictured here in a wind tunnel with the Large-Scale Low-Boom supersonic inlet model.
_DARPA_sonic_tests_04.07R.jpg)
See also
In Spanish: Explosión sónica para niños
- Cherenkov radiation
- Hypersonic
- Supershear earthquake
- Ground vibration boom
- Christine Darden
