What Is the Speed of Sound?

Chuck Yeager and the X-1 research plane that broke the sound barrier in 1947. It can be seen today at the Smithsonian Air & Space Museum in Washington, D.C. (Image credit: Air Force Test Center History Office)

The speed of sound varies depending on the temperature of the air through which the sound moves.

On Earth, the speed of sound at sea level — assuming an air temperature of 59 degrees Fahrenheit (15 degrees Celsius) — is 761.2 mph (1,225 km/h).

Because gas molecules move more slowly at colder temperatures, that slows the speed of sound; sound moves faster through warmer air. Therefore, the speed required to break the sound barrier decreases higher in the atmosphere, where temperatures are colder.

Scientists are interested in the speed of sound, according to NASA, because it indicates the speed of transmitting a "small disturbance" (another way of describing a sound wave) through a gas medium.

The transmission of the disturbance takes place as molecules in the gas hit each other. The speed of sound also varies depending on the type of gas (air, pure oxygen, carbon dioxide, etc.) through which the sound moves.

The first controlled flight to break the speed of sound — also known as Mach 1 — took place Oct. 14, 1947, when test pilot Chuck Yeager breached the barrier using Glamorous Glennis, an X-1 aircraft.

NASA's X-43A aircraft flew more than nine times as fast on Nov. 16, 2004, flying Mach 9.6 or almost 7,000 mph. That stands as the fastest speed achieved to date by a jet-powered aircraft.

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Elizabeth Howell
Live Science Contributor
Elizabeth Howell is a regular contributor to Live Science and Space.com, along with several other science publications. She is one of a handful of Canadian reporters who specializes in space reporting. Elizabeth has a Bachelor of Journalism, Science Concentration at Carleton University (Canada) and an M.Sc. Space Studies (distance) at the University of North Dakota. Elizabeth became a full-time freelancer after earning her M.Sc. in 2012. She reported on three space shuttle launches in person and once spent two weeks in an isolated Utah facility pretending to be a Martian.