The sound that rushing water makes

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In summary, the sound produced by rushing water, such as in rapids, is caused by a combination of air bubbles and turbulence. The bubbles are excited by the turbulent flow and resonate, producing sound waves that transfer to the air. Turbulence, on the other hand, is a cascade of viscous eddies that generates small pressure waves. The noise we hear is a result of our brain processing these pressure waves, which are of various strengths and frequencies due to the many disparate scales in turbulent flows. This is why the sound of flowing water is often described as "white noise." Additionally, the bursting of small bubbles in a breaking wave also contributes to the sound. Contrary to popular belief, the sound is not caused by molecules
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Richard DiMaria
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What actually caused the sound made by rushing water, for example in rapids. Is it molecules bumping against each other? Or something else?
 
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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 )
 
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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
 
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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.
 
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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."
 
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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
 
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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.
 
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tech99 said:
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.
 

FAQ: The sound that rushing water makes

What causes the sound of rushing water?

The sound of rushing water is caused by the movement of water molecules as they flow over and around objects in a river or stream. This movement creates vibrations in the water, which then travel through the air and are perceived as sound by our ears.

Why does the sound of rushing water vary in intensity?

The intensity of the sound of rushing water can vary based on a few factors. The speed and volume of the water, as well as the size and shape of the objects it flows over, can all affect the intensity of the sound. Additionally, the surrounding environment, such as the presence of trees or rocks, can also impact the sound.

Can the sound of rushing water be used to measure the speed of the water?

Yes, the sound of rushing water can be used to estimate the speed of the water. This is because the frequency of the sound is directly related to the speed of the water. As the water moves faster, the frequency of the sound increases.

Why does the sound of rushing water change in pitch?

The sound of rushing water can change in pitch due to the Doppler effect. As the water is in motion, the sound waves it produces are compressed in front of it and stretched behind it. This results in a higher pitch in front of the water and a lower pitch behind it.

Can the sound of rushing water be used for communication?

The sound of rushing water is not typically used for communication as it is a natural sound and can be difficult to control. However, some animals, such as certain species of fish, use the sound of rushing water to communicate with each other. Humans can also use man-made devices, such as water wheels or turbines, to harness the sound of rushing water for communication purposes.

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