Most people are very good at distinguishing between the sound of hot liquid being poured and the sound of cold liquid being poured, even if they don't realize it.
“Every time I give a talk and say, 'Surprisingly, adults can tell the difference between hot and cold water,' that's what people do,” she says, as the audience mimics shaking during a video call. said psychologist Tanushree Agrawal. Their head is no. However, her research, which she completed at the University of California, San Diego, demonstrated that three-quarters of her experimental participants were actually able to detect a difference.
You can also try it yourself. Put on your headphones or listen closely to your computer or cell phone's speakers and press the play button on this audio recording.
Did you know which sounds are hot and which sounds are cold?
If you said the first one was cold, congratulations. You are in the majority Dr. Agrawal.
In general, cold water sounds brighter and more splattered, while hot water sounds duller and more bubbly. But until recently, no one really had evidence to explain the difference.
But Xiaotian Bi, a Ph.D. in chemical engineering from Beijing's Tsinghua University last year, offers a new explanation in a paper published in March on the arXiv website. What matters is the size of the bubbles formed when pouring, he says, and this insight may influence the way we enjoy our everyday food and drinks.
Dr. Bi's paper has not yet been peer-reviewed and he acknowledges that more research is needed. But Joshua Rees, a professor of acoustics engineering at Queen Mary University of London who also studies hot and cold water acoustics, said he was “definitely on the right track”.
Discussions of the different sounds of hot and cold liquids usually point to differences in viscosity as the cause. However, Dr. Bi was not satisfied with that reasoning. He produces and stars in his own popular science videos and decided that the sounds water makes at different temperatures was a good subject. He looked around for published research on the subject, but he came back disappointed.
“No one gave an accurate explanation,” he said, adding that it was an “unsolved mystery.”
So Dr. Bi decided to conduct his own scientific research to inform the video. He used his expertise in fluid mechanics to investigate the role played by air bubbles, which actually produce much of the sound heard in moving water. This can be observed in waves. Waves glide silently until they break, at which point air is trapped and noise is created when the bubbles briefly resonate underwater.
Previous research has shown that large bubbles in liquids produce lower frequency sounds. Dr. Bi also found that the acoustic spectrum of hot water contains more infrasound than the spectrum of cold water. So he wondered if pouring boiling water into a container would trap larger bubbles than pouring cold water, and if that might explain the difference in sound.
His hunch turned out to be correct. Dr. Bi purchased a container with a stopper to dispense water in a controlled manner, first at 50 degrees Fahrenheit and then at 194 degrees. High-resolution videos and photos revealed that in hot water he always produces bubbles with a size of 5 to 10 millimeters, while in cold water bubbles of about 1 to 2 millimeters are produced. .
(That's why in the video above the cold water is on the left side of the screen and the hot water is on the right side)
This research not only provides an explanation of what people hear, but also provides insight into how we enjoy food and drink in general. Let's think about coffee.
Coffee tastes good when it's hot, but it becomes sticky and bitter when it cools down. That's because aroma molecules can more easily escape from the surface of a hot drink. And the relationship between flavor and temperature can cause a Pavlovian response in coffee drinkers.
This echoes the observations of psychologist Charles Spence, director of the Crossmodal Institute at the University of Oxford, who won an Ig Nobel Prize for his work on the relationship between sound and taste when consuming potato chips. “Temperature sounds likely help subtly set people's scent expectations,” he wrote in a 2021 paper, even if unconsciously.
“Very often, we taste what we predict,” he said. It's all part of what he calls the “acoustic seasonings” hidden in food and drink.