# Speed of Sound

by on September 20, 2009

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The speed of sound is dependent on two things: the type of medium in which the sound waves propagate and the temperature of the said medium.

For example, the speed of sound in air at a temperature of 20ºC is about 344 m/s. This value changes to 332 m/s if the air temperature is 0ºC. On the other hand, if we compare the speed of sound in Hydrogen and in Carbon dioxide when both are at 0ºC, Carbon dioxide allows a slower speed at 258 m/s, compared to Hydrogen’s 1,270 m/s.
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Since the atmosphere’s temperature varies among its different layers (e.g. troposphere, stratosphere, etc.), so does the speed of sound. Generally speaking, the speed of sound is constant for a given altitude. That is, you can have one speed in one altitude and another in a higher altitude.

To express the speeds of hyper sonic aircraft that exceed the speed of sound, the speeds are expressed in terms of a ratio called the Mach number. The Mach number is the ratio between the aircraft’s air speed and the speed of sound at the particular altitude the aircraft is flying. Thus, Mach 1 is equal to the speed of sound, Mach 2 is twice, Mach 10.5 is 10.5 times, and so on.

Remember though that, strictly speaking, that value is dependent on altitude. That is the reason why in most cases, when you see speed comparisons of aircraft traveling above the speed of sound, you may have noticed that their altitudes are specified.

When aeronautical engineers group aircrafts in terms of speed, they classify them as subsonic (below Mach 1), transonic (approximately Mach 1), supersonic (greater than Mach 1 but less than Mach 3), high supersonic (greater than or equal to Mach 3 but less than Mach 5), hypersonic (greater than or equal to Mach 5 but less than Mach 10), and high-hypersonic (greater than or equal to Mach 10 but less than or equal to Mach 25).

The fastest jet in the world, the X-43A is nearly high-hypersonic since it has set a high-speed record of Mach 9.6. The Space Shuttle, which hits Mach 25 during re-entry, reaches high-hypersonic speeds. Most jets are only supersonic.

The making and breaking of aircraft high-speed records is monopolized by NASA, particularly through their X-planes (like the X-15 and the X-43A). Currently they are on a quest to develop propulsion systems and subsequently, aircrafts that can cruise in the hypersonic levels.

We have some articles in Universe Today that are related to the speed of sound. Here are two of them:

Speed of sound references, brought to you by NASA. Here are the links:

Tired eyes? Let your ears help you learn for a change. Here are some episodes from Astronomy Cast that just might suit your taste:

Source: NASA

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