Can you tell if you're hiking upstream via Doppler effect?

Factual Questions
1

Hiking in the Rockies last year, I managed to somehow get lost while in a tight valley. I was following the stream and it was out of sight but not out of hearing range. Yet somehow I could “hear” that I was hiking upstream rather than down.

Is it possible to hear the difference between walking upstream at 3 mph (say) versus downstream at 3 mph? I guess this is a two parter:

– Part 1: Assuming that the water rushing over the rocks creates a noise from the frame of reference of the water, then when I’m walking upstream, and assuming the water is moving at 5 mph, the upstream water will be generating noise going toward me at 8 mph, whereas the downstream water’s noise will be generated in a frame of reference away from me at 8 mph. Whereas walking downstream the two sides will be moving at 3 mph away and 2 mph toward, respectively.

A quick calculation leads me to believe that it may be physically possible to differentiate between these two situations because of the Doppler effect. But am I mistaken in the limits of human physiology, if the sounds of the babbling brook were generated from the frame of reference of the rushing water (as opposed, to, say, the stationary rocks it is hittng?)

– Part 2: What frame of reference is the sound of water rushing down a rocky stream generated from? The rocks, or the water, or both?

2

The Doppler shift isn’t going to be very large for a mere few miles per hour from the motion of the water or you hiking. You might be able to hear the frequency shift if the water-noise were a pure tone, and you were hearing one right after the other to compare. But with the noise being a, well, noisy mix of different frequencies, and without having a reference to compare to, I can’t see how you could tell.

3

The sound is produced at a stationary point: where the water falls. It’s not moving (relative to the medium, which is air, or to a listener stationary on the ground) so there’s no Doppler effect to a stationary listener.

So there’s no way to tell, from a Doppler effect, whether you’re going upstream or downstream.

Now, the shape of the streambank will be different looking up or downstream, and water noises are generally going to be made at the bottom (downstream) of large rocks,etc., so it’s barely conceiveable that the mix of frequencies and whatnot could be different from upstream and downstream, as the rocks tend to block the sound from travelling upstream and reflect the sound back downstream. Maybe even to the point where it’s discernable by the human ear. But there wouldn’t be any Doppler effect.

4

You had a 50% chance of being right. As it happens you were.

5

Might there not have been other subtle cues that you were picking up on, such as a slight gradient, or the valley narrowing in the upstream direction? (Or, indeed, something neither of us have yet thought of.) It is actually quite common to pick up environmental cues without being consciously aware of what they are.

6

My rather dubious memory/impression and limited experience paddling rushing water tells me that downstream of a shoal/rapid sounds different than upstream of it.

7

In that case, doppler effect would change your perception, since you’re paddling in a boat that’s moving considerably faster than someone walking.

8

True if the boat is going pretty fast. But even paddling speeds seem a bit on the slow side for actual Doppler effect to be noticeable

But, what I am talking about is just the way the sound sounds, even standing still upstream of it vs downstream. Where’s an observant whitewater person when you need one?

9

If you can’t see the water, the sounds could be echoing off of rocks. So even if you pick up the doppler effect (which I doubt), you could just end up going the wrong direction anyway.

If you can hear the water, just walk in that direction until you can see it.

10

Can you tell the difference between sound coming from in front of you versus in back of you?

And, how fast is a mountain stream really running?

Because if it’s more like 15 mph rather than the 5 mph I posited in the OP, and the frame of reference of the sound production is the running water (which other posters have doubted), then I would think that you could hear the difference between water moving away from versus water moving toward you – a difference of 30 mph.

So if I could have subconsciously distinguished between water coming toward me versus water moving away from me, I could have determined the direction of the stream based on my orientation rather than the direction I was moving.

11

Consider Figure 1 (:)) below. Point X is upstream of a small waterfall, and point Y is downstream. Point Z is where the water generates sound from falling. A person at Y will hear the water falling clearly, while a person at X will hear it muffled. The higher frequencies will be more attenuated at X than the lower frequencies, so it will sound higher pitched at Y than at X. If you are hiking upstream, as in the OP, this is the same relative frequencies as if it were caused by the Doppler effect. Hiking the other direction, though, it would sound opposite.



X
____________
             \
              \
               |
               |Z_________Y____

          Figure 1.
12

Brilliant! Now, I vaguely recall people not being able to tell the difference between sounds coming from behind versus in front of them in the absence of visual cues. But combining the muffling effect with orientation could explain why streams sound different when you’re facing one direction versus the other (note that I am not claiming to be able to tell which is which, only that if I turn around they sound quite different.)

13

Stand next to a rock in stream that has water gurgling around it. Wait a minute. Is that rock still there with water gurgling around it? Yes. The water is moving, yes, but the sound creation point is not.

If you are going to experience the Doppler Effect off of this source, either you or the source needs to be moving. Since the source isn’t moving, you can only experience the Doppler Effect as you move past the source. And since you would only be at walking speed, the effect will not be very pronounced. And it still won’t tell you which direction the water is flowing in, only whether you are approaching or leaving the source of the sound.

Rip and/or shred the above, as needed.

14

I’ve also predicted the flow direction by sound before and I think you’re right, it’s due to the difference of the sound being shielded/unshielded by the rocks. Scale it up to a larger waterfall and think of the sound at the top and at the bottom. Completely different.