# The Naked Scientists Forum

### Author Topic: Sound wave in a sonic train traveling in the opposite direction  (Read 1555 times)

#### McKay

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##### Sound wave in a sonic train traveling in the opposite direction
« on: 13/09/2014 22:57:52 »
Greetings. I got confused. Perhaps someone can illuminate me on this one:
If I am standing next to a railway made for sonic trains and one train, a long train, is passing by, with sound waves, naturally, propagating all around the inside of the train - if I could see the variations in air pressure that are the sound wave and I would look at one particular wave traveling in the opposite direction of the train itself ( to the back), would I be looking at a, relative to me, unchanging pressure difference?
( of course, speed of sound changes in different mediums and pressures, but, in this case, the train would be traveling at the speed of sound of its inside (?) )

#### evan_au

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##### Re: Sound wave in a sonic train traveling in the opposite direction
« Reply #1 on: 13/09/2014 23:35:41 »
Sound travels at a speed relative to it's medium, in this case, about 330m/s relative to the speed of the air.

If a hypothetical train were traveling at the speed of sound towards the right (carrying it's internal air with it), and a sound wave inside the train were propagating towards the rear of the carriage, an outside observer would see it as a stationary pressure wave (assuming a hypothetical observer could see variations in air pressure).

This does not happen for light propagating in a vacuum, which always seems to be traveling at the same speed relative to any observer; we call this speed "c".

#### McKay

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##### Re: Sound wave in a sonic train traveling in the opposite direction
« Reply #2 on: 14/09/2014 00:30:08 »
Ok, I knew about light propagation, but as I said: I got confused when thinking about sound. It seems very weird that a sound wave would appear stationary.
Thank you.
Speaking of seeing sound waves - cant an observer actually see (detect) different pressure pockets by analyzing light refraction, preferably from a laser?

#### chiralSPO

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##### Re: Sound wave in a sonic train traveling in the opposite direction
« Reply #3 on: 14/09/2014 02:48:51 »
You can see the change of refractive index thusly:

#### The Naked Scientists Forum

##### Re: Sound wave in a sonic train traveling in the opposite direction
« Reply #3 on: 14/09/2014 02:48:51 »