What is the sound barrier?

Felix Baumgartner jumps

This image provided by Red Bull Stratos shows pilot Felix Baumgartner of Austria as he jumps out of the capsule during the final manned flight for Red Bull Stratos, Oct. 14, 2012.

Stuntman, Felix Baumgartner became the first human projectile to break the sound barrier on Sunday 14th October, after he jumped from a helium balloon over 36,580 m (24 miles) above the surface of the Earth (the highest ever sky dive).

But what does ‘breaking the sound barrier’ actually mean?

Sound travels as a wave at different speeds through different mediums. For example, through dry air at 20 °C, sound travels at 343.2 metres per second (or, 1,236 km per hour). In liquids or non-porous solids sound travels more quickly.

The sound barrier, in aerodynamics, is the point at which an object moves from transonic to supersonic speed.

When an object is trvalleing faster than the speed of sound, it is said to be at supersonic speed. The speed of sound is Mach 1. An object travelling at between Mach 0.8 and Mach 1 is said to be at transonic speed. (Get over Mach 5 and things are said to be at hypersonic speed).

The Mach number (M) is a dimensionless quantity representing the ratio of speed of an object moving through a fluid and the local speed of sound.

M = v/a


M is the Mach number,
v is the velocity of the source relative to the medium and
a is the speed of sound in the medium.

The Mach number can therefore vary through changes in the medium, and by temperature and pressure conditions.

As an object approaches the speed of sound the pressure waves it generates spread out ahead of it. As the object reaches sonic speed those waves cause severe drag and instability as the object passes through them.

The phrase, ‘sound barrier’ stems from the 1940s when it seemed that an infinite amount of thrust would be needed to fly at the speed of sound and therefore a physical barrier existed that would prevent supersonic speeds.

Although travelling at supersonic speed is now commonplace in flight, the term has remained in popular use. Baumgartner broke the sound barrier reaching a speed of 1,342 km per hour – Mach 1.24 during his 4 minute 20 second long freefall, which he described as ‘like swimming without touching the water’.

About Derek Shirlaw

I'm passionate about science communication, social media, and my home country, Scotland. In particular, I have a real interest in astronomy, digital marketing, and the great outdoors.
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