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I've noticed that unlike other liquids, when pouring olive oil for example, I don't hear any sound at all from it.

Usually you can hear an audible sound as a cup gets filled with water, as the sound increases in pitch.

What makes the oil behave this way?

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    $\begingroup$ To add to the answers - watch the cup and look for "splashing" vs. just a gentle fill $\endgroup$
    – Carl Witthoft
    Sep 17, 2021 at 13:25
  • $\begingroup$ Pour it from a greater height, and less flow (giving a thinner column of oil), so it breaks into droplets by the time it hits the pan; you'll hear it.. Now turn your tap on fairly slow so there is a smooth column of water and fill your pan by placing it at such a height that the water hasn't broken into droplets before it hits the pan.. silence.. $\endgroup$
    – Caius Jard
    Sep 20, 2021 at 11:31

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The noise is generated by turbulent flow. Turbulence in the flow generates turbulence in the air at the interface between the air and the liquid surface, and that turbulence in the air is what we hear as the splashing sound.

So how much noise you hear depends on how turbulent the liquid flow is, and this is inversely related to the viscosity of the liquid. Turbulent flow is exceedingly hard to describe mathematically (indeed you can win a million dollars if you can do this) but as a general rule for a given flow rate the amount of turbulence decreases as the fluid viscosity increases.

And oil has a higher viscosity than water so when pouring oil into a cup we get less turbulence than when pouring water at the same rate, and hence we hear less noise.

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    $\begingroup$ So honey is even more silent... $\endgroup$
    – George Menoutis
    Sep 20, 2021 at 14:29
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Actually, two questions:

  1. The existence of the sound: The most common source of the flowing liquid sound is air bubbles popping. Surface waves (both over the open surface and inside the bubbles forming in the jet in less than spherical form) also create some sound.

Both of these phenomena are connected to the surface tension of the liquid/air interface and the viscosity of the liquid.

If you use hot oil (dangerous!) which is less viscous and has less surface tension, you can hear generally the same sound. You can as well hear that hot water (less viscosity and surface tension) makes different sound and a soapy water (less surface tension only, a lot of bubbles simply refuse to pop) is different, too.

  1. The sound pitch.

It is generally not a single pitch, but an open tube resonance. The noise of the popping bubbles just gets multiplied by the "tube" response. And, human ears are good at assigning a single pitch to a rather complex spectrum having equally separated maximums.

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    $\begingroup$ May I suggest this YouTube video as an example of the auditory difference between hot and cold water? $\endgroup$
    – fyrepenguin
    Sep 18, 2021 at 1:14
  • $\begingroup$ @fyrepenguin I don't have a device that can play video with sound right now, but I will consider adding it as soon as I have chance to listen to it. Of course, there is an youtube for everything, even for things I didn't even try searching. $\endgroup$
    – fraxinus
    Sep 18, 2021 at 6:35
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    $\begingroup$ @fraxinus most definitely: what's interesting about that video is that it shows that most people even know the difference in sound between hot and cold water pouring, even if they don't know that they know it. $\endgroup$
    – fyrepenguin
    Sep 18, 2021 at 20:56
  • $\begingroup$ turbulence in the air at the interface between the air and the liquid surface=air bubbles popping? $\endgroup$
    – cipricus
    Sep 19, 2021 at 21:12
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The combined effects of viscosity, density, and surface tension tend to suppress the formation of vortices in air near the surface and that can reach your ears. The weaker the vortices, the softer the sound.

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