The Magic Sound of Hot Chocolate
Published on 7 July 2023
Today (7 July) is the International Chocolate Day. Let's celebrate with a cup of hot chocolate! Grab a bag of chocolate premix and pour it into a mug. Next add some hot water and stir well with a spoon. Then, here comes the magic. Keep tapping the bottom of your mug, and listen carefully.
Can you hear the pitch of your tapping gets progressively higher? Well, this has nothing to do with magic, it's physics!
The "hot chocolate effect", also known as the "allassonic effect", is a fascinating phenomenon of wave motion. Sound is a mechanical wave that requires a medium (solid, liquid or gas) to travel. The closer the particles in a medium are, the lesser time it takes for vibrations to propagate through the medium. This explains why sound travels faster in water (about 1,500 m/s) than in air (about 340 m/s).
↑ The distribution of particles in different states of matter
Now, let's get back to our chocolate. As we stir it, not only did we add sweet, delicious flavour, but also introduce gas bubbles in the liquid. If we consider the mug as a tube with one closed end, its fundamental frequency (f) depends on the speed of sound (v) and the length of tube (L), which in this case is the height of the mug. While the height of the mug is fixed, sound travels through the bubbly liquid at a slower speed, thus giving a lower pitch sound.
↑ The relationship of fundamental frequency and speed of sound
As we tap the mug filled with hot chocolate, gas bubbles rise to the surface and escape, leaving mostly liquid hot chocolate behind. The speed of the tapping sound gradually increases, thereby raising its pitch.
Now stir your hot chocolate and try the magic sound trick again. You may also try with other instant premixes like coffee and matcha powder, or even with soft drinks, and see which one works best for your sound magic.