[Stephen Brooke]

Stephen Brooke  asked the Naked Scientists:
   Dear Dr Smith.

I hope that this is the correct way of posing an intriguing question, ie email.

Here goes: When I tap the bottom of my coffee cup(just filled with warm coffee) with a teaspoon quite fast, repeatedly, the tone/frequency/pitch changes from low to high.

This puzzles me every morning....Can you explain this?...Is there a link between thermodynamics and sound/kinetic energy? 

Thanks in anticipation,

Stephen Brooke  (702 listener, The Netherlands)
What do you think?

[RD]

The note depends on where you hit the cup/mug if it has a handle ...
http://www.thenakedscientists.com/HTML/content/kitchenscience/exp/sounds-from-a-mug/

If you are consistently hitting it in the same place, differential heating of the mug by the hot liquid may be responsible: A temperature gradient between the inner and outer surfaces of the cup would cause stress (thermal loading) which would cause the cup to stiffen causing a change in pitch and/or timbre.

[Soul Surfer]

I take the point described by RD's reference however I think that in this case the reason is different.  It is similar to the process described in a similar page where fizzy drinks fizz up violently.  In this case the high speed vibrations cause cavitation an bubble forming on the bittom of the cup but the bubbles do not grow but just stick there and decouple the bottom of the cup from the water allowing it to vibrate faster and causing a rising in pitch of the note

[RD]

If the phenomenon still occurs using a tin can* rather than a regular ceramic cup them my hypothesis can be discounted. Metal is a much better conductor of heat than ceramic, and the initial large temperature gradient in a thin-walled metal container like a tin can will only exist for a very few seconds. 

[* the can is going to get very hot very quick, you'll need something like a clamp or locking pliers to hold it]

[Stephen Brooke]

Gentlemen, thanks for the responses.
I've tried the 'experiment'again with plain hot WATER in the following containers: Porcelain cup, single walled glass cup, double walled stainless steel cup but with NO distinguishable change in tone.
I always tap the bottom of the vessel on exactly the same spot, frequency and force.

But, when I again tried hot coffee in a porcelain cup whilst holding it by the handle and not resting it on any surface the tone changed from low to high. Here's my theory:
Due to the soundwaves and vibrations in the suspension during the tapping, the particles start separating (heavy and light) and as a result, the density at the bottom of the cup increases whilst decreasing in the upper regions of the suspension. As a result, the frequency of the vibrating cup changes along with the change in density.
Do you think that I'm on the right track? I value your objective opinions, so don't hesitate to tell me that I'm way off the mark here.
Thanks, in anticipation.
Stephen.

[RD]

Here's my theory:
Due to the soundwaves and vibrations in the suspension during the tapping, the particles start separating ...

Sounds like you could be describing a 3D version of a 2D Chladni pattern ... https://www.youtube.com/watch?v=Qf0t4qIVWF4 [it's loud: turn volume to low ]

However if the rising pitch phenomenon is due to the liquid containing particles in suspension which are changing their distribution, (either just sinking or forming patterns via sound vibration), then it would also happen if the liquid was cold. If it only happens when the liquid is boiling hot, then Soul Surfer's hypothesis seems the best explanation so far. 

[Stephen Brooke]

I think I have finally come up with the reason why the tone changes whilst tapping:

I noticed that this only happened with frothy coffee from a expresso machine.  The liquid was well aerated firstly and as it settled, ie. the tiny bubbles escaped the liquid, the tone/pitch changed whilst tapping.  When agitated again by means of stiring with a spoon, thus aerating the liquid again, the same occured untill the air had escaped and a no tone change was noticed.

The tapping of the spoon has possibly got nothing to do with the separation of the bubbles from the liquid. It simply causes noise with which one can detect the change in density of the liquid.

Try it out for yourself..and compare it with a cup of water or tea (no change in tone).

Thanks for your time and comments, gents.  Your links are also interesting and have taught me a thing or two.
Regards, Stephen Brooke.

[Bill S]

Fascinating discussion, folks.  Glad you sorted it out, Stephen; but I would love to know why you were tapping your coffee cup in the first place.   

[Geezer]

I'm pretty sure RD is correct.

It's caused by stress in the mug during the rapid heating process. Once the system attains greater equilibrium, there is less apparent change in the tone.