Can you hear the difference between hot and cold water?
Next time you make tea listen carefully as you add hot water to the cup. Try pouring the same volume of cold water into the same type of cup. Can you hear a difference? We carried out this experiment during our Blue Planet II episode. Recent research suggests that 93% of adults can tell whether water is hot or cold just by listening to the pouring sounds and this ability develops over the first six years of life. So why can we hear the difference? Let’s take a look at the science.
What sound does water make? You may be surprised to hear that water by itself produces very little sound. When a droplet of water hits the flat surface of a body of water there is a soft impact sound which is caused by supersonic shock waves right after the impact. What we hear when listening to water is the sound of bubbles of trapped air, which pulsate and vibrate emitting a sound. If any of these bubbles form close to the air-water boundary and rise towards the surface, the pitch of the bubble sound also rises. This causes the ‘blooink’ when a stone is thrown into the water.
Why do we hear a difference between hot and cold water? It’s all down to the frequency of the sound. There are three sources of sound during pouring: the resonance of the air, the vibration of the container and water as a whole, and the water sounds (including bubbles). Hot and cold water both produce these same frequencies but with different strengths. When cold water is poured the dominant sound is the vibration of the container and water, whereas when hot water is poured the resonance of the air is the dominant sound.
OK, but why is there a difference in the strength of these frequencies? Most people think the difference is due to the viscosity (thickness or gloopiness) of the water. Believe it or not cold water is 5 times more viscous than hot water. This affects the size of the water bubbles produced, the more viscous the solution the larger the bubbles and the lower the resonant frequency (the natural vibrating frequency of an object). It is also possible that the steam produced by the hot water has an impact on the frequency of the air in the container. Either way, there is definitely room for future research!
