Why does density change the resonant frequency of a wineglass? I’ve been looking at various sources, and they all say that density effects the energy transfer from oscillations of the wineglass (in other words, the heavier and denser the liquid, the more energy per vibrations it takes to set the molecules in motion, and therefore the slower the frequency.) However, this counteracts the belief that frequency never changes when a wave propagates from one medium to the next. The frequency of glass molecules should be the same as the frequency of liquid molecules. The only thing that changes is the speed of the wave, and therefore the wavelength. Can someone please explain how energy changes frequency, or if not, another explanation to this phenomenon?
 A: The mistaken belief here is that frequencies being used to inject energy into the glass are then converted into the glass's resonant frequency. This is wrong.
The frequencies being used to inject energy into the glass do not change when inside the glass. However there's almost always a mix of frequencies and while the the energy of the frequencies other than the resonant frequency are damped and dissipated, the energy at the resonant frequency accumulates inside the glass thus dwarfing energy of the other frequencies. That's the definition of resonance: the energy at some frequency is stored within the glass rather than dissipated.
If you were only injecting frequencies that were not the resonant frequency, the glass would not resonate as there would be no energy accumulating inside the glass.
A: Here is my way of thinking about this.
An empty wine glass' resonant frequency is determined by its density and stiffness. However, when wine is poured into it, the mass of the wine right next to the glass wall (in the near-field) is well-coupled to the glass and so this mass-loading effect lowers the resonant frequency of the glass.
Putting denser and denser liquid into the glass will progressively lower the resonant frequency because of the mass-loading.
