A method and apparatus for detecting ultrasound vibrations of a subject (5) is disclosed. The method comprises passing a modulated beam (6) from a vibrating subject through a resonant medium (7). The beam (4) has a carrier frequency component and two major sideband frequency components. The resonant medium (7) imparts a phase shift on the carrier frequency component and passes the two major sideband frequency components to output an amplitude modulated signal. The ultrasound vibrations of the subject (5) can be detected from the amplitude modulated signal.