# eli5: Why does the pitch of the “mixing noise” decrease over time as a stir a hot drink?

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I’ve noticed this with lots of hot liquids. Once I start stirring, the clanking noise of whatever I’m stirring with just gets lower and lower in pitch, possibly as the beverage cools off. What is that? Thanks

In: Physics

As the powder dissolves, it changes the density of the liquid, which affects the pitch as the spoon hits the glass or cup.

For the same reason that the sink’s cold tap sounds different than the hot tap. The temperature of water affects its viscosity so that effects the sound as it pours as well as when it’s running through a sink valve.

This is known as the “Hot Chocolate Effect.”

Your spoon hitting the mug generates the sound, but most of it is formed by the cylinder of liquid inside (in the same way that plucking a guitar string generates a sound, but the hollow body of the guitar is what amplifies and shapes it).

When you start stirring, air bubbles get dissolved in the liquid. The speed of sound depends on the medium it’s traveling through; liquid with air bubbles in it has a slower speed of sound than liquid without. As you stir, the bubbles clear, and the speed of sound gradually increases.

Since the liquid vibrating is what’s actually causing the sound, and the speed of that vibration depends on the speed that sound can travel through it, as the speed of sound increases the pitch increases.

You question, though, mentions the pitch *decreasing* which is the opposite of what people usually observe! But the sauce is the same – something is happening to change the speed of sound in that liquid. Maybe you’re mixing in air rather than releasing it, or mixing in sugar or another substance that will lower the speed of sound?