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Old 02-20-2006, 05:44 PM   #19
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Originally Posted by 4kV
The train would not be out travelling the sound, or you'd hear a sonic boom when it went by. The speed of sound is approximately 740 miles per hour, so I doubt a train would be going that fast, varying on different things like atmospheric density and composition, but negligible in this context.
Let's see... carry the one. Move the decimal point, and HA! 743 miles per hour. It was breaking the sound barrier. No, really, I claimed not to be a space/time/sound continuum knowledgable guy. I did a poor job of stating my idea. While speed does have something to do with difficulty hearing sound ahead of a moving object, sitting at a crossing 90 degrees to the tracks, the horn will be heard better. And not peeping through a viewfinder focusing all of one's attention to composition helps as well.

EDIT:I found this at Wikipedia

[caption:Sound waves emanating from an ambulance moving to the right. The perceived frequency is higher on the right, and lower on the left.]

Everyday Applications

[Caption:A stationary microphone records moving police sirens at different pitches depending on their relative direction.]

The siren on a passing emergency vehicle will start out higher than its stationary pitch, slide down as it passes, and continue lower than its stationary pitch as it recedes from the observer. Astronomer John Dobson explained the effect thus:

"The reason the siren slides is because it doesn't hit you."
In other words, if the siren approached you directly, the pitch would remain constant (as vs, r is only the radial component) until the vehicle hit you, and then immediately jump to a new lower pitch. The difference between the higher pitch and rest pitch would be the same as the lower pitch and rest pitch. Because the vehicle passes by you, the radial velocity does not remain constant, but instead varies as a function of the angle between your line of sight and the siren's velocity:

where vs is the velocity of the object (source of waves) with respect to the medium, and θ is the angle between the object's forward velocity and the line of sight from the object to the observer.
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Last edited by busyEMT; 02-20-2006 at 05:55 PM.
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