I've looked across the internet for an answer to why changing the water level in a wineglass affects the frequency measured when the rim of the glass is rubbed. Here's what I've found so far:
- Adding water to the glass causes the inertia of the glass to increase which causes the frequency of oscillation of the wine glass to decrease, therefore decreasing the frequency at which the air is displaced. This results in a lower-pitched sound.
However, I've seen some people say the standing wave of the air column inside the glass decreases in frequency since the length of it decreases as water is added. As a result, since the oscillations of the air column standing wave are transferred to the wine glass (and to the air), this is what causes the frequency of the sound to decrease. (https://physics.stackexchange.com/a/312048/254292).
I'm not sure which explanation is the primary factor that affects the frequency heard, or if they are in fact related to each other in some way. I'd like to have a single explanation that incorporates both of these concepts.
Best Answer
To enlarge upon Alephzero's comments: The wavelength of a sound of frequency ~1 KHz is about ~1 foot. A wavelength of order ~1 inch is about 12kHz which for most most of us old-timers is inaudible. So if one ramps the water level up and down by ~1 inch in a water-filled glass the effect on the glass resonance frequency be small and hard to hear.
The dominant effect is instead the inertial coupling between the glass walls and the mass of water contained within the glass. As the level of water rises in the glass, so does the degree of coupling, and hence the effective mass of the glass increases relative to its stiffness. This decreases the resonant frequency of the glass as a resonator.