Described are a sensor and a method for measuring a vibration of a surface obscured from view. The sensor includes a narrowband source of a terahertz beam, a beamsplitter, a beam combiner and a terahertz detector. The beamsplitter splits the terahertz beam into a sample beam for irradiating the surface and a reference beam. The beam combiner combines the sample beam scattered from the surface and the reference beam. The terahertz detector generates an electrical signal based on a modulation of the power of the combined beams due to the vibrating surface. The electrical signal indicates a characteristic of the surface vibration. Homodyne or heterodyne detection can be utilized. Advantageously, the sensor can see surfaces that are covered, concealed or otherwise obscured behind optically opaque materials, including plastic, cloth, foam, paper and other materials. Thus the sensor has a wide variety of applications where conventional vibrometers are not practical.