The sound that rushing water makes

[Richard DiMaria]

What actually caused the sound made by rushing water, for example in rapids. Is it molecules bumping against each other? Or something else?

 

[hutchphd]

Welcome. I think that is a wonderful question and I will leave it to the experts here to answer adequately. There is much interesting Physics to be examined.
What I do know that turbulent water contains a large mix of velocities and directions of flow producing a wide range of sound tones (frequencies). These pretty well span the range of our hearing producing the "white" noise one hears. (White meaning an equal admixture of all frequencies of sound )

 

[BWV]

No way am I anything close to an expert, but got curious as well, particularly how the sounds were synthesized and found this:

A physically based liquid sound synthesis methodology is developed. The fundamental mechanism for the production of liquid sounds is identified as the acoustic emission of bubbles. After reviewing the physics of vibrating bubbles as it is relevant to audio synthesis, a sound model for isolated single bubbles is developed and validated with a small user study. A stochastic model for the real-time interactive synthesis of complex liquid sounds such as produced by streams, pouring water, rivers, rain, and breaking waves is based on the synthesis of single bubble sounds. It is shown how realistic complex high dimensional sound spaces can be synthesized in this manner.

https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.90.5684&rep=rep1&type=pdf

 

[Arjan82]

Well, there are bookshelfs full on this topic. But at least there are two important mechanisms, both already mentioned. One are air (gas) bubbles. These are exited by the (turbulent) flow and resonate, deform etc. This produces sound waves in the water which transfer to the air, or if you're on a ship, they excite the hull which exites the air on the other side again.

The second is indeed turbulence. Turbulence is really just small fluctuations in pressure (and velocity, and possibly density, which can look amazing). These can excite the surrounding fluid and thus radiate away, which is indeed what noise is. This is usually much lower in volume.

The typical 'splashing' noise of water is actually more the interaction of the free surface (of the water) with air. The water constantly encloses air and releases it, this generates noise as well.

 

[boneh3ad]

Sound waves do not transmit efficiently between water and surrounding air, so any sound created by the bubbles mentioned here is likely to be inaudible. The acoustic impedance ($\rho c$) of air is so much smaller than that of water that the interface is an almost perfect reflector for acoustic energy. Some energy escapes but it's very, very small.

Turbulence is not really just fluctuations in pressure (et al.). It's a cascade of viscous eddies that starts at relatively large length scales and passes energy down to increasingly small scales until it is dissipated. This is often measured as fluctuations in pressure, velocity, density, etc. Ultimately, though, you can have fluctuations in any of those things and still not be turbulent. Rushing water is often (usually?) turbulent, true, but the important thing here is the many disparate scales rather than fluctuations.

The noise you hear is just how your brain processes small pressure waves, aka sound waves. These are generated when the water interacts with other objects or itself (i.e. splashes against rocks and water splashing back into more water), thereby displacing air and generating small pressure waves you can hear. It generally sounds like white(ish) noise because of all the disparate scales inherent in turbulent flows, meaning pressure waves of many different strengths and wavelengths/frequencies. If you hear the noise from a laminar flow of water, it is typically substantially quieter and less distinctively "white noisy."

 

[hilbert2]

Here's a video with quite realistic water flow simulations including sound.



I think the sound of flowing water is just a fluid version of a metal object vibrating and producing sound waves after being hit by something.

And here's an actual publication about something related to this: https://www.nature.com/articles/s41598-018-27913-0

 

[tech99]

I believe the small bubbles burst with great violence, creating enormous pressure, and there is a huge number of them in. say, a breaking wave.

 

[Richard DiMaria]

I believe the small bubbles burst with great violence, creating enormous pressure, and there is a huge number of them in. say, a breaking wave.

Uneducated much on physics I thought it might be the molecules smashing into each other. I see I was very wrong.