Doppler Effect or Not?

[Andrew Troup]

Originally Posted by Dennis.G

If one is in a boat and moves towards oncoming waves, the frequency at which one encounters the waves increases. Would not the same be the case if one remains stationary and the surrounding water instead moved towards you?

I think that's covered further up the thread, by the observation that such a result would require that extra waves be somehow conjured up from nowhere. The (fixed) source of the waves can only supply so many waves every hour: if a fixed observer in another location sees a different number per hour, what happens to the waves not seen? or where do the extra waves come from?

[Dennis.G]

OK, you guys changed my thinking on this.

Assuming constant conditions, the number of waves coming into an inlet have to equal the number of waves hitting the beach. Frequency therefor unchanged. If current is different upstream than at mouth of the inlet, the wave height and wave length will change, but not frequency.

From this I guess I would say:

• Frequency is dependent on relative motion between source and observer.
• Wavelength is dependent on frequency as well as relative motion of wave-medium (water) to observer.

[Paul Elliott]

This *has* been an interesting discussion. Thanks to the O.P. for posing the original question, and thanks to everyone who participated.

The original question was about whether there was a Doppler-type frequency change when a fog horn was sounding in the wind (moving air). We've recently shifted the analysis to waves in the water, probably because we can see and feel them. Along the way we mentioned other wave effects, but most of the discussion has centered on "ideal" waves, where shallowing, breaking, etc, are conveniently ignored (at least that's what I've been doing). I think this is appropriate, as we can look at these different effects in isolation, which makes it much easier to answer the original (and follow-on) questions.

We should of course remember that waves in the water do *not* behave as classical sinusoidal waves, especially when they start interacting with the wind, the bottom, or other non-water features. Still, I'm comfortable with the conclusion that the frequency does not change when we have fixed (relative to each other) wave source and detector. This holds true even when we have other "non-ideal" factors at play.

[jongleur]

Originally Posted by Fortytwo

"For waves that propagate in a medium, such as sound waves, the velocity of the observer and of the source are relative to the medium in which the waves are transmitted. The total Doppler effect may therefore result from motion of the source, motion of the observer, or motion of the medium."

Doppler effect - Wikipedia, the free encyclopedia

I've been thinking about this again.

The ticker tape explanation: If the tape all ran
at the same speed, whether forward or backward,
no change, of course.

If however, as the tape is
moving, you tore sections out of it and those
sections moved faster toward the listener, the
holes in that torn section would go by the listener
faster, hence higher pitch. This is what occurs in
the case of gusty wind.

So, steady wind, no pitch change. Gusty wind
pitch changes.

Thanks Fortytwo and whoever put up the ticker
tape analogy.

[David M]

If the boat were connected to the same medium that the sound travels, then there would be no Doppler shift. But the boat is connected to a different medium, the water. Therefore there is a Doppler shift.

A hot air balloon floating in the air drifting over the boat would not hear a Doppler shift. The boat floating in the water below the balloon would hear a shift.

If the balloon threw a sea anchor in the water and started drifting with the water, it would then hear a shift.

Less time to get to you, same frequency, same velocity through medium would equal a higher frequency if you are going into the wind towards the source.

With sound as with light, if the wind is blowing from the source, and you are connected to the water, you would get a blue shift (higher frequency). If the wind is traveling towards the source, you would get a red shift.

The only thing sound senses is the air that it propagates in and not the water or land below it.

This is starting to sound somewhat like Relativity.

[sailingpzas]

Originally Posted by Matt sachs

Someone should do a simple experiment to settle this.

No need to do this. (Enjoy your popcorn anyway.)

Originally Posted by Richard_W

Interesting, and simple explanation even without the formulas ... the frequency shifts of the source and the receiver created by moving medium cancel each other.

Right ...

Richard actually hit the nail on its head! This is exactly the answer.
(Assuming source and emitter at rest relative to each other, and the wind being constant.)

As has been pointed out already:

Originally Posted by Albro359

wrong !!!
It has NOTHING TO DO WITH THE MEDIUM

It has EVERYTHING to do with the movement of the source and/or receiver.

IT IS ONLY the relative velocities of the source and receiver that cause the Dopper effect.

This is BASIC physics.

Just my 2 ct.

[Sailor g]

This has been really interesting. I appreciate the discussion. Something to keep the brain active.