What is Doppler effect?

Model Railroader General Discussion (Model Railroader)
1

I see in ads for sound equipped locomotives, along with all the various sounds,(horn, brakes etc) “doppler effect” listed. What exactly is that? Dan

2

Believe their talking about the change in sound as a locomotive pass’s you and is moving away. almost a echo type of sound.

3

Thats what it is. I’ve got a Proto Loco that has that feature, its kinda cool.

4

I thought it was more of a volume thing, louder as the train approaches and then fades as it moves away, although I have been know to be wrong in the past. (rarely [:D])

Have fun & be safe,
Karl.

5

Close. Doppler effect is the way a sound gets higher in pitch when the source of the sound is moving toward you, and lower in pitch when the source is moving away. It doesn’t really involve echo.

I’m not sure this is an accurate scientific explanation, but I think it helps to imagine the sound waves being stretched or compressed. If the source is moving toward you, the forward motion of the source is compressing the sound waves. Sound waves with shorter wavelength, mean higher frequency (pitch).

Since model trains don’t move fast enough to cause much of a Doppler effect, I guess the vendors are providing functions to simulate it.

6

Not volume- Frequency (what some call pitch).

Sounds are waves. Any more detail needed to understand, just lemme know. [:)] (don't want to be TOO pedantic.) When something travels towards you, the waves get squashed together. When it travels away from you, the waves get stretched apart. However, for the most part, all sound waves at a given location (i.e., temperature/atmospheric pressure/humidity) will travel the same speed. This means that, by the equation v=fL (v = velocity, f = frequency, L = wavelength), rearranging to f = v/L: If L decreases (squashed together), f increases ==> higher frequency or pitch If L increases (stretched apart), f decreases ==> ower frequency or pitch

Think of your favorite cop movie/firefighter show/war movie/racecar scene:
A siren just sitting still in front of you (or airplane motor or racecar…) has a certain pitch.
If it’s coming towards you, it sounds higher-pitched.
If it’s going away from you, it sounds lower-pitched.

Brian Pickering

7

No, the Doppler Effect is totally frequency. As the sound producer approaches you, the wave fronts get “compressed,” so they occur more often and therefore the sound has a higher pitch, or frequency. Likewise, as the sound producer moves away, the wave fronts are “stretched” and the frequency drops. This is most noticeable as the locomotive (or whatever) passes you and rapidly changes from approaching to receding. The amount of Doppler shift is proportional to the speed of the sound producer relative to the observer.

And yes, there is a loudness thing, too. The sound is louder the closer you are to the sound producer. It’s independent of Doppler, though.

Doppler is also present in light waves. That’s what causes the “red shift” of receding galaxies, and was one of the first clues to the Big Bang theory.

8

A good example of Doppler Effect would be the engine sounds you hear on TV when the Nascar or formula cars go whizzing around the track. Usually there’s a drop of about 3 1/2-steps in pitch, from the time the race car is in view (and coming at you) to the time it is out of view of the camera, or going away from you.

Tom

9

There’s a might of relativity involved. It is compressed relative to the stationary listener, but not to the person on the train.

10

The doppler effect represents a shift in frequency of the sound wave as been explained by others. The shift in pitch can happen two ways: one an object making sound comes toward you and the waves are closer together because the next wave starts closer each time (raises the relative pitch) and as the object passes the listener, the pitch lowers as each wave starts farther away from the listener lowering the frequency of the waves passing the listener. The scenario can also be that the listener is going towards a sound source and passes it creating the doppler effect. The shift in frequence is constant based on speed therefore can be used to accurately plot the speed of the movement of a sound source or sound waves bouncing off an object (like your car when the County Mountie is taking your picture or even applies to light waves and is used to show the speed of celestial objects using the electromagnetic spectrum) The Doppler Effect was dicovered by Austrian mathematician and phyicist Christian Doppler (1803 - 1853).

For a good explaination of the Doppler Effect visit: http://hyperphysics.phy-astr.gsu.edu/hbase/sound/dopp.html

11

Ray has it correct. Astronomers use the doppler effect to detect radial motion in heavenly bodies. As they move away from us, each successive wavefront emitted by the object starts a little further away, with the effect to the detector (eye or other instrument) of perceiving a shift towards the red end of the visible spectrum (light, not Bachmann). In trains and aircraft, as they appraoch, each wavefront starts its journey to our ears closer than if the object was standing still. You can imagine that a still jet engine would have a certain sound, each wavefront exactly the same distance as the one in front of it. Move it at speed X towards you, and the fronts get crowded. That is the change in pitch.

12

The doppler effect is also what makes the “ding-ding-ding” of a crossing bell change pitch if you’re on the train going past it–higher frequency as you approach the bell, lower frequency as you speed away from it.

13

eeeeEEEEYYYOOOWWWWWWWwwwwwww…
I Getcha! Thanks, Dan

14

yesitsoundslikethiswhennapproachingthen when it gets closer its normal and passe e s y o u i t g e t s s l o w e r r r r r r r r r r