Why does the pitch of a spoon tapping the inside of a coffee mug get lower as you stir and then go back up as the liquid settles back down?

You know how putting more or less water in a cup can control the pitch it makes when you tap it, so you can even make music just with cups of water?

Well the pitch isn’t controlled by how much water there is, it’s how far the water reaches up the side of the cup that matters. You can see this yourself by filling a cup halfway with water and tapping it to hear the pitch, then if you put something in the water large enough to raise the water level and tap again, the pitch will be lower (though the geometry of some glasses works out that this pitch change is actually backwards and adding more water raises the pitch, such as in this video.)

And what happens when you stir your coffee? It makes a little whirlpool, the center of the coffee goes down, and the edge of the coffee goes up! You can even stir really fast to make the whirlpool bigger and change the pitch even more… just don’t get so enthusiastic you wind up splashing coffee all over the counter 🙂

Hehe, there is a video by Steve Mould on that subject. I’m sure somebody will find the link…

Edit: https://youtu.be/7Hr_zFdNW5Q

I did a project on this for my engineering degree
When you stir coffee you add microbubbles in it, changing its density
By doing so you also change the speed of the sound inside the water/bubble mixture, which influences the frequency of the sound you hear
You can do the same thing just by gently pouring hot water from the tap in a glass
Inside the pipes, the water is under pressure and will absorb some gas
When you pour it into a glass the pressure is lower than in the pipes and the gas are released, forming microbubbles which have the same effect as stirring coffee

I think it has to do with changing density from dissolving sugar, and micro bubbles in the liquid rising to the surface. Also, I think that heat changes the pitch of both the water and the mug. And lastly, there are solids from the coffee beans all through the liquid. As it sits, it starts to settle and stratify. As you stir, it changes the mix quickly until it settles into a homogenous equilibrium.

These are just my guesses. I have often wondered the same thing. I also noticed that I can tell a tiny difference in the sound of rushing water when it is hot. Like a running sink sounds different when the water started to get hot. And, I grew up in Yellowstone and could tell if a stream was hot or not by the sound while approaching.

I have no expertise, but I would guess that the shape of the empty space inside the cup changes the sound.