Speed of sound: what is and how it is measured

 We just watched the sequel to the movie "Top Gun" with Tom Cruise. It is a film that explores the world of military aviation, with spectacular battles and increasingly pioneering aircraft. One of the questions we ask ourselves after watching the film concerns the speed of sound, reached and surpassed for the first time on 14 October 1947 by Charles "Chuck" Yeager. He was the first to fly faster than sound with a Bell XS-1 rocket plane. But let’s now explain what the terms Mach and Sonic barrier mean.

A scene from the movie "Top Gun: Maverick". Credit: Eagle Pictures

The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through a medium. On Earth the speed of sound at sea level, assuming a temperature of 15 degrees Celsius, is 1,225 km/h. Sound moves faster through warmer air, as gas molecules move more slowly at cooler temperatures. But what is this sound barrier? This word was coined during World War II when aircraft began to experience transonic flight and exploded unexpectedly as if they had hit an invisible barrier. Mach’s number, however, which is often quoted in the film, defines the ratio between the velocity of the object in motion and that of the sound in the gas or fluid considered. As a result, the speed required to break the sound wall decreases as you rise into the atmosphere, where temperatures are colder.

The speed of sound also varies according to the kind of gas (air, pure oxygen, carbon dioxide) through which the sound moves. NASA's X-43A flew more than nine times faster than sound on 16 November 2004, at Mach 9.6, at over 11,000 km/h. Felix Baumgartner, on the other hand, was the first man to break the wall of sound in free fall, after having launched from a height of almost 40 kilometers at a record speed of 1,342 km/h.

References: Live Science