1. Can people tell hot from cold water just by listening to it being poured?
Do you notice a difference when filling a bath with hot water versus cold water? In 2013, Velasco and colleagues reported three experiments where participants listened to water being poured into glass, ceramic, paper, or plastic containers and had to say if it was hot or cold. People were able to judge the temperature by sound alone [1]. In a separate experiment, when the sound was artificially changed, the perceived temperature also changed [1].
However, because the full text of that paper is not easily verified here, specific numbers on temperature and accuracy are not included. A more interesting question is whether the people who guessed correctly actually knew they were doing it. It is important to note that this experiment worked even when the container material was changed, suggesting that the material of the cup itself is not the only clue [1].
Safety Note: If you try listening to pouring sounds, be careful with hot water. Always have an adult help you and make sure the water is not hot enough to burn.
2. Why people who said they couldn't tell still guessed correctly
In a 2024 study by Wenger and others, participants were first asked if they could distinguish between the sounds of pouring hot and cold water. About 80% of them answered that they could not [2]. But when they actually listened to the sounds and had to say if it was "hot" or "cold," their answers were surprisingly accurate.
For water at 85°C (185°F), 71% correctly identified it as hot. At 90°C (194°F), 72% were correct. For cold water at 5°C (41°F), only 26% said "hot," and at 10°C (50°F), 31% did. Since guessing randomly would result in about 50% accuracy, these results show that the ability to hear the difference is significantly better than chance [2].
Researchers think this skill might be "perceptual learning," a type of knowledge we pick up from daily life without realizing it. However, they also note that previous studies have not fully explained why the sounds are different [2]. So, while the difference in sound is real, the reason for it is still being investigated.
3. The three sources of the sound made by pouring water
When you pour water, you hear a "glug-glug" or splashing sound. Research suggests this noise comes from three main sources: the resonance of air inside the container, vibrations of the container and water, and the sounds made by bubbles trapped in the water [3].
Bubbles are created when the stream of water hits the surface and pulls in air. Another study suggests that if the water flow breaks into droplets before hitting the surface, it creates more bubbles and a louder sound [7].
When hot water is poured, the air inside the container warms up. This might make the speed of sound faster and raise the pitch of the air resonance. However, the air does not actually reach the same temperature as the water, so this difference is thought to be small [3]. This makes bubbles a strong candidate for explaining the sound change.
4. How bubble size changes the pitch of pouring water
A large flute makes a low note, and a small flute makes a high note. Bubbles work similarly. The air inside a bubble vibrates to create sound, and the pitch is inversely proportional to the radius of the bubble. This is called Minnaert resonance [4]. Note that a bubble is not like a flute; it is just a vibrating mass of air.
In water bubbles, the product of the frequency and the radius is about 3.26 m/s. This means a bubble with a 1 mm radius makes a sound of about 3.3 kHz, while a 5 mm radius bubble makes a sound of about 650 Hz (calculated as 3.26 divided by 0.001 and 0.005 respectively) [4]. If the radius is 5 times larger, the pitch becomes roughly one-fifth as high.
This formula is attributed to Dutch physicist Minnaert, who published it in 1933 [3]. If pouring hot water creates more large bubbles, this could explain why the sounds are different.
5. Does hot water make bigger bubbles? The viscosity theory
A 2024 study used high-speed cameras to watch hot and cold water being poured. It reported that hot water created more large bubbles. However, this paper was not yet peer-reviewed [3].
The reason given is the viscosity (thickness) of the water. Viscosity decreases as temperature rises. At 20°C (68°F), it is about 1.00 mPa·s, but at 80°C (176°F), it drops to about 0.35 mPa·s. This means hot water is about one-third as viscous as cold water [5].
Other researchers (Karahasan et al., 2014) found that in water jets between 5°C and 30°C, higher temperatures created larger bubbles. They explained this by saying lower viscosity makes the water surface rougher [3]. The idea that "lower viscosity leads to a rougher surface, which leads to bigger bubbles" is currently the leading theory. However, final confirmation awaits analysis by fluid dynamics experts [3].
But there is a problem: sources disagree on whether hot water sounds higher or lower.
6. Does hot water sound higher or lower? Scientists disagree
If the theory above is correct, hot water should have more large bubbles, which produce lower-pitched sounds. Indeed, the 2024 preprint reported that hot water had more noticeable low-frequency components [3].
However, some science websites explain that hot water makes a "higher" sound. They argue that because it is less viscous, the water splashes into finer droplets, creating higher-pitched sounds [6]. In a 2017 video that popularized this idea, the experimenters themselves admitted that fluid movement is complex and an exact explanation is difficult [3].
So, there is no settled conclusion on whether hot water sounds higher or lower. The authors of the 2024 preprint call their work "citizen science," admitting they lack professional equipment and that their reasoning involves subjectivity. They also describe the explanation about bubbles making low sounds as a guess [3]. This shows that even common everyday sounds can have unanswered scientific questions.
7. An experiment you can try yourself with cold and lukewarm water
If you want to explore this, try pouring cold water and lukewarm bath-temperature water into the same cup one after another. Close your eyes and listen. Record whether you can tell them apart and if the sound seems higher or lower. This might help you understand why sources disagree.
You can also observe bubbles and sound when filling a kitchen sink or bathtub. Changing the force of the tap water might change how the bubbles form [7].
For more information, look for books on physics and sound in libraries or check university public lectures on bubble sounds and viscosity.
Safety Note: When doing this experiment at home, use cold water and hot water that is safe to touch (not boiling). Always do this with an adult. Record what you hear exactly as it sounds to help compare the different explanations.