Abstract: An input device comprises a plurality of optical vibration sensors mounted in a common housing. Each optical vibration sensor comprises a diffractive optical element; a light source arranged to illuminate the diffractive optical element such that a first portion of light passes through the diffractive optical element and a second portion of light is reflected from the diffractive optical element; and a photo detector arranged to detect an interference pattern generated by said first and second portions of light. The optical vibration sensor is configured so that in use, after the first portion of light passes through the diffractive optical element, the first portion of light is reflected from a reflective surface onto the photo detector. The input device is placed in contact with a surface of a solid body, and an object is brought into physical contact with the surface of the solid body, thereby causing vibrations in the solid body.