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Old 22-03-2013, 13:24   #76
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Re: Doppler Effect or Not?

Originally Posted by Dennis.G View Post
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?

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Old 22-03-2013, 13:48   #77
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Re: Doppler Effect or Not?

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.

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Old 22-03-2013, 21:51   #78
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Re: Doppler Effect or Not?

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.

Paul Elliott, S/V VALIS - Pacific Seacraft 44 #16 - Friday Harbor, WA
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