A couple of months ago I stumbled upon this video which claims you can hear the difference between hot and cold pouring water. You can check it out below:
This might sound obvious to the average tea-drinker, but it is an interesting example of people’s intuitive understanding of fluid mechanics. A slightly “lukewarm” explanation given in the video is that the viscosity of water decreases as it is heated and this results in a different sound, although I think we can add to that.
The “gurgling” sound we hear when we pour water actually comes from the vibrations of entrapped air bubbles. One of the simplest models for bubble vibrations is the Minnaert frequency, in his appropriately named paper “On musical air-bubbles and the sound of running water“:
Where R is the bubble radius, P is ambient pressure, and ρ is the liquid density. A 1mm radius bubble should produce a sound of around 2750 Hz. Taking this a step further, researchers from Cornell University were able to simulate pouring water with potential flow analysis, and additionally model the sounds from the entrapped bubbles. The main simulations start at 2:00:
The resulting sounds are impressively realistic, and their analysis goes as far as investigating what frequency would be expected at a particular depth. These bubble oscillation dynamics fall into a broader range of fluid mechanics known as cavitation, which is the focus of some our research in the Micro & Nano Flows for Engineering group.
One last interesting story on this topic, researchers from Tel Aviv University claim that tomato plants make high-pitched (ultrasound) noises when stressed, most likely caused by cavitation. I have more in common with a tomato plant than I first realised… Their research was published in the open-access journal bioRxiv doi: 10.1101/507590v