The Doppler Effect......

[Ranjr]

Right. So it is all about proximity and relitivity. 

But to someone who isn't the engineer, who is going to precieve the sound as being the same, if you are the observer, ahead of that train coming toward's you, the soundwaves are going to compress and seem louder as the approach, and sound lower and lower the longer it passes you.

Not louder but higher in pitch.  To put in terms you understand, as it approaches, the train whistle sounds like Agnetha.  After it goes by, it sounds like Bjorn.

However, it does get louder as it approaches because it experiences less attenuation over the distance, which is not related to the Doppler effect.

[Brian37]

No one will perceive the sound as being the same in that scenario.  A person ahead, on, and behind all hear different things.  A sound wave moves, and if the emitter moves as well, the leading edge of the wave is compressed and the trailing edge is enlongated.  It's not about things being bigger or louder.  Things are louder closer to you because they've lost less force in interaction between yourself and the emitter.

If there are barriers between yourself and the emitter - you won't hear anything at all...and the person behind will hear the same thing they would regardless of whether or not there are barriers between you and the emitter.  Just expunge the idea of bigger and louder entirely out of your mind.

^^^^^ This, this right here is what I think I understand.

[The Grand Nudger]

Yet another service we provide.

[HappySkeptic]

Go back and read what you wrote.


You said the frequency IS.

not the frequency APPEARS to the observer to change.

See the problem?

Sound frequency isn't some constant of the universe. Of course I was talking about the frequency at the listener. Yes, it does actually change depending on reference frame. It doesn't just "appear to".

If you want to specify the vibrational frequency of the emitter, before, it interacts with the air, you can. A bell in a vacuum has a specific vibrational frequency but no sound. That could be constant for a pure tone. But, once the vibration excites the air, the sound frequency depends on the frame of the listener (or measurement device) relative to the source, and how those relative speeds compare to the speed of sound.

Now, the still air itself could be considered to be a nice reference frame to talk about frequency. You are free to do that, as it has some physical reality -- the air molecules are actually moving back and forth slightly at a frequency with respect to the still air. But, if the source is moving, it still isn't the same as the source vibrational frequency. The medium reference frame is quite useful for understanding sonic booms when something breaks the sound barrier (the theoretical frequency goes through infinity in all reference frames going toward the item, because the vibrations are all piling up into a pressure wave in the air).

If the way you want to think of Doppler effect is with a constant frequency in the reference frame of the medium, with transformations at the source and the listener, that's fine. I don't tend to do that. The definition of "frequency" doesn't require it, and when you start to talk about light, it even makes no sense.

If you want to think of the source vibrational frequency as the only "real" frequency, that is your choice, but it isn't required by physics. Perhaps your previous work had you do this.

[arewethereyet]

My African Grey barks like a dog that's in the house, and like one that's outside, and one that's far away...she seems to "get" that there's a difference in how sound travels and is picked up by the ear.

[HappySkeptic]

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