Sound in stirred hot fluids If a hot beverage in a cup gets stirred, the sound of the spoon changes. You can easily hear this if you repeatedly cling the spoon to the cup ground after stirring. The cling sound will raise in tune for quite a long time.
This even happens if the fluid motion is (almost) stopped after stirring, eg. by using the spoon to do so.
What is the reason for this effect? Why it last so long after stirring?
 A: I enjoy listening to this phenomenon as I wait for my teabag to brew.
What is happening is that parts of the the ceramic cup heat up and expand. Because its shape is constrained (assuming it doesn't break), this produces a stress in the material that increases its resonant frequency (like tightening a guitar string).
When it gets to Middle C, my tea's ready.
A: This is a well known effect, and most clearly happens with hot instant cocoa which has just been mixed. Stir the cocoa while tapping the spoon on the mug, and you'll hear the pitch of the tapped spoon go down. Now start tapping while not stirring; the pitch will gradually go up by an octave or more. But, if you stir again, the pitch will go back down again.
The cause is microscopic bubbles of air, which lower the speed of sound in the cocoa and thus lower the resonant frequency of the mug. Heated tap water is supersaturated with air; add cocoa or sugar and the air will come out of solution into those microscopic bubbles, lowering the resonant frequency. Let the cocoa sit, and the bubbles will rise, leaving the bulk of the mug with fewer bubbles and a higher sound velocity, raising the resonant frequency of the mug. Stir, and you'll bring the bubbles back down, reversing the effect and lowering the pitch.
A: I would suggest the surface of the stirred fluid has more area as it is in rotation. The expanded conal shaped surface area is resonating at a lower frequency driven by the sharp pings generated by the spoon against the hard bottom of the cup. As the rotation slows the fluid has less surface and the pitch rises as the level of the fluid drops at the edge of the cup, and rises in the center of the fluid vortex. This may also explain the super low frequency audible incidents during a tornado where "train engine rumble" caused by low pressure is "amplified" by the vortex of the air column. This was first observed and questioned by me 49 years ago and the answer eluded me until today. Excelsior !  Clem
