# Waves in the Air

## What Happens as an Airplane Nears the Speed of Sound?

As an airplane approaches the speed of sound, conditions around it begin to change.
The air ahead of it starts to compress. Shock waves form on its wings and drag increases dramatically.

Shock wave formation in front of the Bell X-1, the first airplane to fly faster than sound; the Concorde, a supersonic airliner; and the Space Shuttle during reentry into the atmosphere.
Credit: National Air and Space Museum, Smithsonian Institution

## An Airplane Creates a Wave of Pressure in the Air

A moving airplane causes a disturbance in the air—a wave of pressure—similar to a sound wave. Just like sound waves, any object in motion, such as an airplane, causes a chain reaction of colliding air molecules to spread outward in all directions at the speed of sound. Keep in mind that it is the wave that travels; the air simply moves back and forth. This wave of molecular collisions is called a pressure wave.

## What Is Sound?

Sound consists of waves transmitted through the air (or another substance) by molecules bumping into each other. When these sound waves reach your ear, they cause your ear drums to vibrate. Your brain “decodes” the vibrations into voices, music, and noises.

## What Is a Sonic Boom?

The shock waves created by an airplane flying faster than sound are nearly cone shaped and extend outward until they dissipate. However, if the airplane is flying low enough so the shock wave reaches the ground, anyone in the shock wave's path will experience a sonic boom. The sound is caused by a sudden, momentary change in air pressure that the ear registers as a
loud bang.

## What Is a “Mach Number”?

We use Mach numbers to describe an airplane’s speed in terms of the speed of sound.

A Mach number is derived by comparing the speed of an airplane with the speed of sound in the air it’s moving through. An airplane moving at Mach 1 is traveling at the speed of sound. The Mach number is named in honor of Ernst Mach, a late 19th century physicist who studied gas dynamics.

## Ranges of Speed

• Subsonic: Usually less than Mach 0.8. Air is flowing slower than sound over every part of the airplane.
• Transonic: About Mach 0.8 to Mach 1.2. Air is flowing faster than sound over some parts of the airplane.
• Supersonic: Greater than about Mach 1.2. Air is flowing faster than sound over the entire airplane.
• Hypersonic: Greater than about Mach 5. Heat becomes a critical factor.

Q:

## Why does sound move faster in warmer air?

A:

The speed of sound in the air mainly depends on temperature. At a typical sea level temperature, sound travels about 1,220 kilometers (760 miles) per hour. At high altitudes, where it is much colder, sound travels slower. The higher the temperature, the faster the air molecules are moving to begin with, and the quicker they bump into each other as a sound wave passes through.

# Watch a Video

## Supersonic Flight

Find out how a whip and a Slinky can help us understand the speed of sound.

## Did You Know?

The mechanical vibrations that can be interpreted as sound are able to travel through all forms of matter: gases, liquids, solids, and plasmas. The matter that supports the sound is called the medium. Sound cannot travel through a vacuum.

## Pop Quiz

The movement of sound through a medium (like air) is affected by:

A) Temperature
B) Density
C) Pressure
D) All of the above

The movement of sound through a medium is affected by temperature, density and pressure.  Since temperature, density and pressure all decrease with altitude, the speed of sound slows down the higher up you get.