The above illustration shows how a time varying stress due to a time varying (sinusoidal) acceleration changes the frequency of a resonator, i.e., how the vibration frequency modulates the oscillator’s frequency.
	R. L. Filler, "The Acceleration Sensitivity of Quartz Crystal Oscillators:  A Review," IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 35, No. 3, pp. 297-305, May 1988.