B Question about the allassonic effect

[bunburryist]

In the allassonic effect, is it the column of water that is actually creating the sound (the surface of the water passing vibrations into the air), or is it that the water acts as a damper on the cup, and that it is the vibration of the cup, like a bell, that actually creates the sound waves? Another way of putting the question is - is the water's roll that of primary vibrator with the cup merely holding the water, or is it that the "system" (cup and water) that vibrates, and thus creates the sound. Would there be a way to create the allassonic effect with a container that itself was not allowed to vibrate, or at least was damped as much as possible, so that it was effectively the water only that is vibrating?

Am I thinking about his wrong? Is it really that the cup and the water vibrate relatively independently from one another, each creating their own sound waves, and that I am actually hearing a combination of their vibrations?

I found an interesting version of this effect with cold water from our deep sink faucet. (It doesn't work with water from kitchen sink.) If I put cold water in the cup and tap, the pitch initially slowly drops, and then after a while, starts going up again. My guess is that there are air bubbles and that the initial dropping is from the consolidation of air bubbles into larger bubbles, and that the raising of the pitch is a result of the bubbles leaving the water. Does this make sense?

 

[sophiecentaur]

My guess is that there are air bubbles and that the initial dropping is from the consolidation of air bubbles into larger bubbles, and that the raising of the pitch is a result of the bubbles leaving the water. Does this make sense?

That seems to be the sort of thing that's happening. There could also be the effect of the dissolving coffee displacing dissolved air and forming small bubbles due to the shock of the spoon hitting the bottom. When I was a lad, we used to tap the sides of our fizzy drinks bottles (glass) and stimulate the formation of bubbles of dissolved CO₂. (Idiot kids - losing the fizz so pointlessly.
This link (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4549848/) suggests that the speed of sound would be inversely proportional to 1/R² but also that it should reduce with the density of bubbles (that's fairly obvious). So both of your ideas could be right qualitatively. The pitch change in a couple of the YouTube links is small (perhaps 1/4 tone?) and that is around 0.01%. That wouldn't need a big change in bubble concentration or size.
You'd have to read that article more fully than I did but I reckon that what you need to know is probably in there. (I now step aside for a younger man to complete the job.)

 

[bunburryist]

Unfortunately, I'm not a mathematician, so I don't understand all the equations. Thanks for your response.

 

[sophiecentaur]

Unfortunately, I'm not a mathematician, so I don't understand all the equations. Thanks for your response.

I could sum up that part of the article by saying that the sort of change in the frequency of the resonance is tiny and that the formulae seem to suggest that you only need a low change in concentration of small bubbles to achieve that change (much lower density than you could actually see).
As with many things in Physics that need 'explaining', Maths comes into the explanation. It's like love and marriage.