Why can you hear the difference between hot and cold water ?

I recently found out about an interesting little experiment where it was shown that people could identify when hot or cold water was being poured from the sound alone. This is a little surprising since we don’t usually think of temperature as having a sound.
Here are two sound samples;

Which one do you think was hot water and which was cold water? Scroll down for the answer..

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Yes, the first sound sample was cold water being poured, and the second was hot water.
The work was first done by a London advertising agency, Condiment Junkie, who use sound design in branding and marketing, in collaboration with researchers from University of Oxford, and they published a research paper on this. The experiment is first described in Condiment Junkie’s blog, and was picked up by NPR and lots of others. There’s even a YouTube video about this phenomenon that has over 600,000 views.

However, there wasn’t really a good explanation as to why we hear the difference. The academic paper did not really discuss this. The youtube video simply states ‘change in the splashing of the water changes the sound that it makes because of various complex fluid dynamic reasons,’ which really doesn’t explain anything. According to one of the founders of Condiment Junkie, “more bubbling in a liquid that’s hot… you tend to get higher frequency sounds from it,” but further discussion on NPR noted “Cold water is more viscous… That’s what makes that high pitched ringing.” Are they both right? There is even a fair amount of discussion of this on physics forums.
But its all speculation. Most of the arguments are half-formed and involve a fair amount of handwaving. No one actually analysed the audio.

So I put the two samples above through some analysis using Sonic Visualiser. Spectrograms are very good for this sort of thing because they show you how the frequency content is changing over time. But you have to be careful because if you don’t choose how to visualise it carefully, you’ll easily overlook the interesting stuff.

Here’s the spectrograms of the two files, cold water on top, hot water on bottom. Frequency is on a log scale (otherwise all the detail will be crammed at the bottom) and the peak frequencies are heavily emphasised (there’s an awful lot of noise).

There’s more analysis than shown, but the most striking feature is that the same frequencies are present in both signals! There is a strong, dominant frequency that linearly increases from about 650 Hz to just over 1 kilohertz. And there is a second frequency that appears a little later, starting at around 720 Hz, falling all the way to 250 Hz, then climbing back up again.

These frequencies are pretty much the same in both hot and cold cases. The difference is mainly that cold water has a much stronger second frequency (the one that dips).

So all those people who speculated on why and how hot and cold water sound different seem to have gotten it wrong. If they had actually analysed the audio, they would have seen that the same frequencies are produced, but with different strengths.

My first guess was that the second frequency is due to the size of water droplets being dependent on the rate of water flow. When more water is flowing, in the middle of the pour, the droplets are large and so produce lower frequencies. Hot water is less viscuous (more runny) and so doesn’t separate into these droplets so much.

I was less sure about the first frequency. Maybe this is due to a default droplet size, and only some water droplets have a larger size. But why would this first frequency be linearly increasing? Maybe after water hits the surface, it always separates into small droplets and so this is them splashing back down after initial impact. Perhaps, the more water on the floor, the smaller the droplets splashing back up, giving the increase in this frequency.

But Rod Selfridge, a researcher in the Audio Engineering team here, gave a better possible explanation, which I’ll repeat verbatim here.

The higher frequency line in the spectrogram which linearly increases could be related to the volume of air left in the vessel the liquid is being poured into. As the fluid is poured in the volume of air decreases and the resonant frequency of the remaining ‘chamber’ increases.

The lower line of frequencies could be related to the force of liquid being added. As the pouring speed increases, increasing the force, the falling liquid pushes further into the reservoir. This means a deeper column of air is trapped and becomes a bubble. The larger the bubble the lower the resonant frequency. This is the theory of Minneart and described in the attached paper.

My last thought was that for hot water, especially boiling, there will be steam in the vessel and surrounding the contact area of the pour. Perhaps the steam has an acoustic filtering effect and/or a physical effect on the initial pour or splashes.

 Of course, a more definitive answer would involve a few experiments, pouring differing amounts of water into differing containers. But I think this already demonstrates the need to test the theory of what sound will occur against analysis of the actual sounds produced.