The cochlea of the inner ear converts sounds into electrical signals. This process is triggered by sound-evoked nanoscale vibrations in the sensory epithelium inside the organ. The epithelium contains outer hair cells that have mechanosensory hair bundles at the apical surface. The deflection of the bundles enters cation through ion channels. The epithelial vibrations are modulated by cation-induced elastic motions in the cell bodies. How the vibrations are regulated in vivo has not yet fully elucidated. Here we develop an advanced laser interferometry that precisely detects the vibrations. When a live guinea pig was exposed to acoustic stimuli, the interferometer quantitatively recorded the vibration amplitude of the sensory epithelium as described elsewhere. Additionally, an upward baseline shift of several nanometers was also detected. This motion was observed with loud sounds of >70 dB, and it was negligible when the animal was dead or under pharmacological perturbation of hair-bundles or cell body motions. A theoretical approach further suggested that the shift protects the epithelium from injury induced by strong stimuli.