Increasing tone while mixing sugar in water

[billr]

TL;DR

Tone increases while mixing sugar into water -

I make Hummingbird feed by mixing 1/4 cup granulated sugar into 1 cup of water. I use a small glass pyrex measuring cup with one cup of water measured. It's heated for 1 minute in a microwave and 1/4 cup of sugar is added (no red dye!). I then vigorously stir the sugar and water using a metal spoon, clanging the spoon on the inside of the glass cup, making a tone. Over about a minute or two, as I stir, the tone of the clang noticably increases in frequency. Why? Is it the sugar granules growing smaller or a change in the speed of sound in the sugar solution as the sugar dissolves or something else? I wonder if salt might have a similar effect.

Thanks

 

[Baluncore]

Welcome to PF.

My guess is that the speed of sound is higher in the sugar solution.

The dissolving sugar will cool the water. Monitor the tone and the temperature during the process. Once all sugar is dissolved, reheat the solution in the microwave, and see if temperature or sugar concentration changes the tone, stirring unnecessarily as it cools again.

The solid sugar in the cup, also effectively changes the size of the 'bell' that you ring with the spoon.

 

[Dale]

That would be my first thought as well. A good syrup is much more viscous than water. Before the sugar dissolves it will scatter sound, but the water will be less viscous. So scattering will reduce and viscosity will increase as the process goes on.

One other thing it could be is the temperature change. You could try the same process, but without the sugar. If the sound changes just with water then it is probably the temperature

 

[Baluncore]

When I fill a bucket with water from a tap in summer, it sounds somehow different to when I fill it in winter. The note changes as the water level rises, but it is more than that, it has a different character when the water is cold. I suspect that is due to the change in viscosity with temperature of fresh water.

 

[sophiecentaur]

My guess is that the speed of sound is higher in the sugar solution.

That was my initial reaction, too. However, the speed of sound in water would be too high to dominate the frequency of the 'ding'. As far as I can see, the relevant resonance is due to the lateral vibrations in the thin glass cylinder and a bulk effect of the mass of the glass and its stiffness.

The musical wineglass effect is easier to discuss but in that case the stem stiffness would contribute to the resonance perhaps. Here I would expect increasing the mass of water to reduce the frequency and the link seems to agree.

For the cup i think that the situation is more complicated. The relevant quantities are the mass of the glass wall and the height of the column (equivalent of) as it flexes about a circular cross section (there is no stem to produce an audible ping. The water level acts as a 'stop' as in a guitar fret. The Q of that resonator will be much lower so I'd expect a short ping.

IMO there are three stages in the op's experinent. Firstly, the plain water is added, which will produce an initial frequency. Then, as the sugar is added, the volume will increase, the stop moves up and the mass increases and the frequency would drop . But, once the sugar is dissolved, the total volume will return to near the original volume so the frequency would rise again. I cant think how to model this easily but a three stage experiment would be useful.

Then, of course, there is @Dale 's introduction of the damping effect of the syrup. Sounds very relevant but wouldnt that lower the frequency, rather than raise it? I came across a link which discussed the difference in pitch for red and white wine but i forgot to record the link.

A possible improvement could be if there were two measuring cups and one could be used as a reference with just water in it. No need for memory of the pitch.

 

[hutchphd]

My guess (we are all guessing here I think) is entrained gas bubbles. Their size and number will be quite dynamic and will certainly change the speed of sound. I have often noted and wondered about this but done nothing to suss it out. (Not much of a scientist!)

 

[hutchphd]

I have noted this effect in the following context: I will sometimes create a buffered aspirin (home-made Alka-Selzer) by dissolving aspirin in water, adding a little sodium bicarbonate and finishing with some orange juice (citric acid). The final stir of OJ makes some slight foam and the "clinck" frequency decisively goes down as I stir.
I need to make some hummingbird juice (same technique as OP) and will check if the frequency response goes upward.

 

[Baluncore]

I have often noted and wondered about this but done nothing to suss it out. (Not much of a scientist!)

I need to make some hummingbird juice (same technique as OP) and will check if the frequency response goes upward.

You are more of a scientist than you are prepared to admit. Could it be that your interest has moved you from feeling confident, to believing you are an expert imposter.

 

[hutchphd]

It is less a matter of confidence than age. I used to be really interested in everything. Now it is often difficult to summon the mental energy. Alas, I sometimes get halfway through something and just run out of interest.....this is new to me (at 72 yrs). Partially this is because stuff that used to be easy now takes more effort.
But I greatly appreciate the words. Maybe it is confidence!

 

[billr]

My guess (we are all guessing here I think) is entrained gas bubbles. Their size and number will be quite dynamic and will certainly change the speed of sound. I have often noted and wondered about this but done nothing to suss it out. (Not much of a scientist!)

I wonder if the sugar crystals could be acting like bubbles. But a post further down with bicarb and others the frequency goes the other way. If it were just the particle size I'd think it would go the same way. Perhaps some interaction with particle size and the viscosity change as they disolve.

 

[JT Smith]

I tried your experiment with just water to see if temperature alone could be the cause. There was no noticeable change in the pitch.

So then I tried it with sugar as you described. Again, there was no change. I can't replicate your result.