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).

cold

hot

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.
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Cool stuff at the Audio Engineering Society Convention in Berlin

aesberlin17_IDS_headerThe next Audio Engineering Society convention is just around the corner, May 20-23 in Berlin. This is an event where we always have a big presence. After all, this blog is brought to you by the Audio Engineering research team within the Centre for Digital Music, so its a natural fit for a lot of what we do.

These conventions are quite big, with thousands of attendees, but not so big that you get lost or overwhelmed. The attendees fit loosely into five categories: the companies, the professionals and practitioners, students, enthusiasts, and the researchers. That last category is where we fit.

I thought I’d give you an idea of some of the highlights of the Convention. These are some of the events that we will be involved in or just attending, but of course, there’s plenty else going on.

On Saturday May 20th, 9:30-12:30, Dave Ronan from the team here will be presenting a poster on ‘Analysis of the Subgrouping Practices of Professional Mix Engineers.’ Subgrouping is a greatly understudied, but important part of the mixing process. Dave surveyed 10 award winning mix engineers to find out how and why they do subgrouping. He then subjected the results to detailed thematic analysis to uncover best practices and insights into the topic.

2:45-4:15 pm there is a workshop on ‘Perception of Temporal Response and Resolution in Time Domain.’ Last year we published an article in the Journal of the Audio Engineering Society  on ‘A meta-analysis of high resolution audio perceptual evaluation.’ There’s a blog entry about it too. The research showed very strong evidence that people can hear a difference between high resolution audio and standard, CD quality audio. But this brings up the question of why? Many people have suggested that the fine temporal resolution of oversampled audio might be perceived. I expect that this Workshop will shed some light on this as yet unresolved question.

Overlapping that workshop, there are some interesting posters from 3 to 6 pm. ‘Mathematical Model of the Acoustic Signal Generated by the Combustion Engine‘ is about synthesis of engine sounds, specifically for electric motorbikes. We are doing a lot of sound synthesis research here, and so are always on the lookout for new approaches and new models. ‘A Study on Audio Signal Processed by “Instant Mastering” Services‘ investigates the effects applied to ten songs by various online, automatic mastering platforms. One of those platforms, LandR, was a high tech spin-out from our research a few years ago, so we’ll be very interested in what they found.

For those willing to get up bright and early Sunday morning, there’s a 9 am panel on ‘Audio Education—What Does the Future Hold,’ where I will be one of the panellists. It should have some pretty lively discussion.

Then there’s some interesting posters from 9:30 to 12:30. We’ve done a lot of work on new interfaces for audio mixing, so will be quite interested in ‘The Mixing Glove and Leap Motion Controller: Exploratory Research and Development of Gesture Controllers for Audio Mixing.’ And returning to the subject of high resolution audio, there is ‘Discussion on Subjective Characteristics of High Resolution Audio,’ by Mitsunori Mizumachi. Mitsunori was kind enough to give me details about his data and experiments in hi-res audio, which I then used in the meta-analysis paper. He’ll also be looking at what factors affect high resolution audio perception.

From 10:45 to 12:15, our own Brecht De Man will be chairing and speaking in a Workshop on ‘New Developments in Listening Test Design.’ He’s quite a leader in this field, and has developed some great software that makes the set up, running and analysis of listening tests much simpler and still rigorous.

From 1 to 2 pm, there is the meeting of the Technical Committee on High Resolution Audio, of which I am co-chair along with Vicki Melchior. The Technical Committee aims for comprehensive understanding of high resolution audio technology in all its aspects. The meeting is open to all, so for those at the Convention, feel free to stop by.

Sunday evening at 6:30 is the Heyser lecture. This is quite prestigious, a big talk by one of the eminent people in the field. This one is given by Jorg Sennheiser of, well, Sennheiser Electronic.

Monday morning 10:45-12:15, there’s a tutorial on ‘Developing Novel Audio Algorithms and Plugins – Moving Quickly from Ideas to Real-time Prototypes,’ given by Mathworks, the company behind Matlab. They have a great new toolbox for audio plugin development, which should make life a bit simpler for all those students and researchers who know Matlab well and want to demo their work in an audio workstation.

Again in the mixing interface department, we look forward to hearing about ‘Formal Usability Evaluation of Audio Track Widget Graphical Representation for Two-Dimensional Stage Audio Mixing Interface‘ on Tuesday, 11-11:30. The authors gave us a taste of this work at the Workshop on Intelligent Music Production which our group hosted last September.

In the same session – which is all about ‘Recording and Live Sound‘ so very close to home – a new approach to acoustic feedback suppression is discussed in ‘Using a Speech Codec to Suppress Howling in Public Address Systems‘, 12-12:30. With several past projects on gain optimization for live sound, we are curious to hear (or not hear) the results!

The full program can be explored on the AES Convention planner or the Convention website. Come say hi to us if you’re there!