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|   Hey Terrence, the train whistle question again. | Smokey
May 17, 2001 6:53 AM | | .
This post is the respond to the following post:
http://forums.consumerreview.com/crforum?14@@.ee9282c/21
You wrote:
>This is known as the doppler effect. It however is not frequency based, but amplitude based. If the whistle is at 1khz, it will remain at that level. What will change is the volume. As it comes closer, it is louder. As it passes, it becomes softer. The psychoacoustical effect makes it seem like it is changing pitch, but it is not. If you where riding on the train, the pitch would remain the same.
Your assessment of Doppler effect is partly right. The official description of Doppler Effect is as following: "The Doppler effect is the change in pitch or FREQUENCY of the sound detected by an observer becuase the sound source and observer have different velocities with respect to the medium of sound propagation."
So when train is blowing its whistle at 1 khz and it is moving away or coming toward you, the pitch and frequency will be different.
Here is the formula for Doppler effect:
F'=F[1/{1-(speed of train/speed of sound)}]....F' is the sound the observer will hear. F is the sound of train. Speed of sound is 343 meter per second
For example lets say that train is approching you at 100 m/h(44.7 meter per second)
F'=1khz[1/{1-(44.7/343)}]....=1khz[1/0.87]....F'=1.114 khz
So sound you will hear from the moving train is 1.114 khz
If the train is moving away from you, the formula is slightly different:
F'=F[1/{1+(speed of train/speed of sound)}]
F'=1 khz[1/{1+(44.7/343)}]...=1khz[1/1.13]...F'=0.88khz
Frequency you will hear when the train is moving away from you is 0.88khz.
So the train speed and its direction will dictate the frequency you will hear when the train is on the move.
Wow, I thought the frequency variation due to Doppler effect would be smaller. Oh well, you can't argue with Physics
Take care. |
| Where did you get your information? | Terrence
May 17, 2001 10:35 AM | | I would like to read it myself. I guess my perception is based on the fact that train whistles actually stimulates more than one frequency. The whistle is usually a combination of frequencies. Thanks for the information. So much for the film and audio education ;-)
Terrence |
| Where did you get your information? | Smokey
May 17, 2001 4:16 PM | | Thanks for response Terrence.
I dug it out of my old text books from college. These reference material really comes in handy at my work place and here too as I am very forgetfull. Well, I guess the memeory is the second thing that goes. :-)
BTW: you are right about multiple frequency of a train whistle. I guess it is the combination of a frequency and its harmonics that make that distinguish sound.
Keep up the good work. Take care. |
| the combination of a frequency | justin
May 17, 2001 7:01 PM | | I believe its a combination of 4 frequencies. I ve seen whistles that sound just like the real thing and they usually have 4 barrels. |
| Any thoughts on what frequencys they might be... | Smokey
May 18, 2001 2:45 AM | | ...and could you elaborate more on them barrels?
Thanks Justin |
| Where did you get your information? | mtrycrafts
May 17, 2001 10:35 PM | | Single frequency or multiple, it is affected the same way:-) |
| That is true too. | Smokey
May 18, 2001 3:22 AM | | I wonder what would happen to the frequency if the train speed become faster than speed of sound. The Doppler effect formula doesn't work if the source of sound is going faster than speed of sound as it have a negative frequency value....what ever that means.
F'=F[1/{1-(speed of train/speed of sound)}] |
| That's easy. | bigwally
May 18, 2001 9:02 AM | | You'd get hit before you heard the whistle!
>>sheepish grin< |
| Great answer LOLOLOLOLOLOLOL ouch!!! (nt) | Terrence
May 18, 2001 10:04 AM | | nt |
| Nasty. | RADAR O_Riley
Aug 11, 2002 7:05 PM | | Smokey, I registered on the forum just so I could answer your question (since no one else gave any kind of real answer).
I'm not aware of any trains that operate at supersonic speeds, but many aircraft exceed the speed of sound. As Mach I (the speed of sound) is approached, things get nasty. The air molicules start (quite literally) piling up on the front wing (and other forward facing) surfaces, and it takes a great deal of power to continue accelerating. The "pile of air molicules" gets larger, up to Mach I, and then it breaks apart and folds around the surfaces of the aircraft. This creates a shock wave that spreads out in a cone behind the aircraft, and as the wave travels along the ground, we hear what's called a sonic boom.
More to the point of your question though, sound doesn't radiate forward from an aircraft that is exceeding Mach I. So the answer that said you wouldn't hear the train until after it hit you was actually correct. The train is actually passing the sound waves as they are being created.
If you put this in the context of light instead of sound you end up with the theory of Relativity. Things do indeed get interesting when the equations produce negative frequencies. ;-)
Hope this helps the understanding a little.
RADAR O_Riley |
| Radar 101.... (nt) | SteveW
May 17, 2001 7:22 PM | | Radar 101.... (nt) |
| Another Doppler effect | justin
May 18, 2001 10:43 AM | | Supposedly the doppler effect also happens to light. I wonder what would happen if you were moving fast enough and a red light looked green or is it green that looked red. Anyways could you imageine going to a light and seeing it change color after you stop...
Or it looked green when really it was red |
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