This is the time of year when people in the Northern Hemisphere like to enjoy hot beverages like hot chocolate. [The Action Lab] uses hot chocolate to demonstrate an odd acoustic effect. Tapping a container of hot chocolate — or even just hot water — will make a sound at a certain frequency. But if you keep tapping, the frequency of the sound will gradually increase. Don’t know why? Don’t worry, neither did scientists until around 1980.
The secret is bubbles and the speed of sound through air vs a liquid. The speed of sound in the liquid and the height of the liquid in the cup set the frequency. However, the speed of sound changes based on the bubbles, which alters the frequency.
This is one of those subtle things you probably never noticed and, if you did, you probably didn’t think much about. Still, there’s a physics lesson around every corner. Besides, this is one experiment that you can end with a tasty beverage. You only need some mix and your ears or, if you prefer, a cell phone app to read audio frequencies. You can also just use hot tap water, but what fun is that?
Usually, we are talking about bubbles in the context of futuristic display devices. Then again, not always.
The Hot Chocolate Effect Explained
Source: Manila Flash Report
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