# Speed of Sound Varies?

I was reading the commuter newspaper on Monday and it has little factoids scattered through it one of them said

“Its true: Sound travels 15 times faster through steel than air”

Is this correct and if so how is the speed of sound actually measured?

I thought that the speed of sound was a used to measure things but how can this be if it varies?

It’s true that it varies, but I don’t know if 15 is the right factor offhand. It could be. The speed of sound in diamond is about 35 times that in air.

When you use the speed of sound to measure some other speed, as you do when dealing with Mach numbers, you measure it with respect to the medium it’s in. So for instance, when a jet travels at Mach 2, that’s 2 times the speed of sound in the air surrounding the jet.

The speed of sound varies according to the medium in which it travels; sound is just vibrating molecules; in air, these molecules are more loosely packed.

Like this (each numbered row represents the same row of molecules, the red ones are moving and are carrying a sound impulse.

``````
less dense material:
1   O   O   O   O
2    O  O   O   O
3     O O   O   O
4      OO   O   O
5   O    O  O   O
6   O     O O   O
7   O      OO   O
8   O   O    O  O
9   O   O     O O
10  O   O      OO

more dense material:
1   O O O O O O O
2    OO O O O O O
3   O  OO O O O O
4   O O  OO O O O
5   O O O  OO O O
6   O O O O  OO O
7   O O O O O  OO

``````

Oops; I posted when I meant to preview; just to add that each numbered row represents a sequential timeframe; the impulse is passed on more quickly in the more dense medium because each molecule has less distance to travel before it passes on the energy to the next.
(Actually, I have no idea if this is correct, but it seems to make sense, perhaps some kindly passing physicist could either confirm this or shoot me in the head)

[sub]and yes, I know that the imparting particles shouldn’t just
‘snap’ back into place, but I tried it the other way and it was even harder to look at.[/sub]

Thanks Achernar and Mangetout it all makes a bit more sense now.

Also, remember that the ‘speed of sound’ is just the speed at which a vibration will travel through a particular medium, which will vary with changes in density, temperature or other factors. The speed of sound in Miami on a humid summer day, for example will be different from the speed in Denver on a dry winter day.

Generally, sound (or any vibration, as Sublight pointed out) travels faster in dense and/or rigid material. Inside a neutron star, sound speed is close to the speed of light.

Even though speed of sound varies, it’s still a useful reference with which to measure speed. For example, if you are measuring speed of an aircraft, knowing how close you are to the speed of sound is more important than knowing the exact speed in mph. So you want to express your speed as a fraction of the local speed of sound, i.e. as Mach number.

And if you’re flying your aircraft through steel, the numbers change accordingly.