Cocoa stir

[beckyman]

If you stir a mug of cocoa fast into a small whirlpool and then tap the edge of the mug repeatedly with the spoon which you used for stirring why does the pitch level of the sound produced change upwards?

[gsr600]

dunno - I will give it a try at 9.00 tonight -cocoa time

[wildwood]

As the speed of the revolving cocoa slows down, there is more of the cup inside exposed to change the reverberation of the tapping sound.

[roger]

Happens when making instant coffe too - I assume it's because as the powder dissolves it causes the density of the liquid to change.

[mibn2cweus]

I suspect the damping (not dampening) properties of the liquid are increased by virtue of being in motion, perhaps by disturbing the ability for resonances to be established within the liquid?

[mibn2cweus]

One of my links got dropped in the process of submission . . . here's the other ~

[beckyman]

All very interesting but because of the variety of answers nothing seems totally convincing. Perhaps there is scope for a research project. Wonderful benefits for mankind might eventuate!

[jomifl]

Have just done a little experiment, the more liquid there is in a glass the lower the 'ring' frequency probably due to the density difference between the glass and the water. When the water is stirred it is higher at the wall of the glass, it slows down the level drops so the frequency goes up.

[beckyman]

There's still a bit to go on this. "probably" implies non-proven; also there are so many variables - e.g. the type of liquid; the material of the container; temperature and air pressure. I think I'll sleep better if I just drink the cocoa and shut up!

[jomifl]

Beckyman, the 'probably' referred to the reason that the height of liquid determined the ring frequency. It is not unreasonable to assume that the ring frequency is related to the density of the fluid in contact with the vessel. Nevertheless the ring frequency is related to the depth of liquid, whatever it's density as my experiment demonstrated. Try this, take a wine glass (because they ring well), half fill it with water then get a small poly bag and trap some air in one corner and seal it by twisting the body of the bag. Now tap the wine glass as you insert the air filled bag into the centre of the water so that the water level goes up. You will notice the frequency going down although the amount of water in the glass has remained the same. This is the same phenomenom as the stirred liquidi ie. there is a conical depression in the water surface, though in the case of the stirred liguid it is caused by centripetal force.

[wildwood]

Very interesting and probably true mibns, but the OP asked about tapping the edge of the mug. I am with jomifl on this one.

[mibn2cweus]

//Very interesting and probably true mibns, but the OP asked about tapping the edge of the mug. I am with jomifl on this one.//

Me too, Wildwood. His technique of using an object to displace the water, causing the water level to rise produced an obvious lowering of pitch for me as well.

While I concur with Jomifl's explanation for the relationship between water level and pitch, I have not been able to reproduce or account for the bizarre effect demonstrated in the videos I submitted which might well be demonstrating an entirely different phenomenon which I have not been able to duplicate and the cause of which for me remains a mystery.

[jomifl]

I have to admit that the video seems to show an exaggerated effect. One of them shows the pitch increasing although the liquid is virtually still. I have my suspicuons that there may be a bit of jiggery pokery going on.

[jomifl]

Roger, if the density of the liquid goes up as the sugar etc. dissolves then the frequency should go down not up.