Baluncore said:
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.