Optical microphone assembly

A method of operating an optical microphone assembly, the optical microphone assembly comprising:

• a rigid substrate;
• an interferometric arrangement, the interferometric arrangement comprising a membrane and at least one optical element spaced from the membrane, wherein the at least one optical element comprises a surface of the substrate and/or is disposed on a surface of the substrate;
• a light source;
• at least one photo detector;
• and
• an enclosure arranged to form an acoustic cavity in fluid communication with one side of the membrane;
• wherein the acoustic cavity has a volume which is at least 3 mm multiplied by d2, where d is a diameter of the membrane;
• the method comprising:
• the light source providing light to said interferometric arrangement such that a first portion of said light propagates along a first optical path via said interferometric arrangement and a second portion of said light propagates along a second optical path via said interferometric arrangement, the second optical path differing from the first optical path, thereby giving rise to an optical path difference between the first and second optical paths which depends on a distance between the membrane and the at least one optical element;
• and
• the photo detector(s) detecting at least part of an interference pattern generated by said first and second portions of light dependent on said optical path difference.