We report the use of the photo-electromotive-force effect in a speckled pattern of light on a photorefractive Bi12TiO20 crystal to measure the amplitude of transverse vibrations in a self-calibrating mode. Our theoretical model shows that, for vibrations much faster than the photorefractive response time, the photo-emf signal exhibits a maximum at a characteristic vibration amplitude value that may be used to calibrate the photo-emf sensor. Small differences between experiment and theory are still under research.