An optical microphone assembly (2; 64) comprising: a substrate (4; 50; 66); an interferometic arrangement, the interferometric arrangement comprising a membrane (6; 68) and at least one diffractive optical element (20, 22; 78, 80) spaced from the membrane (6; 68), wherein the at least one diffractive optical element (20, 22; 78, 80) comprises a plurality of lines formed in or disposed on a surface of the substrate (4; 50; 66), the plurality of lines being arranged in a first pattern; the optical microphone assembly (2; 64) further comprising: a light source (8; 70) arranged to provide 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 different optical path via said interferometric arrangement, thereby giving rise to an optical path difference between the first and second optical paths which depends on a distance between the membrane (6; 68) and the diffractive optical element (20, 22; 78, 80); and at least one photo detector (10; 72) arranged to detect at least part of an interference pattern generated by said first and second portions of light (28, 30) dependent on said optical path difference; characterized in that the substrate (4; 50; 66) comprises one or more holes (32; 54; 82) extending fully through the substrate (4; 50; 66), the one or more holes (32; 54; 82) being arranged in a second pattern that is different from the first pattern, wherein the one or more holes (32; 54; 82) are positioned such that at least one of said first and second optical paths at least partly overlaps with the one or more holes (32; 54; 82). a substrate (4; 50; 66); an interferometic arrangement, the interferometric arrangement comprising a membrane (6; 68) and at least one diffractive optical element (20, 22; 78, 80) spaced from the membrane (6; 68), wherein the at least one diffractive optical element (20, 22; 78, 80) comprises a plurality of lines formed in or disposed on a surface of the substrate (4; 50; 66), the plurality of lines being arranged in a first pattern; the optical microphone assembly (2; 64) further comprising: a light source (8; 70) arranged to provide 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 different optical path via said interferometric arrangement, thereby giving rise to an optical path difference between the first and second optical paths which depends on a distance between the membrane (6; 68) and the diffractive optical element (20, 22; 78, 80); and at least one photo detector (10; 72) arranged to detect at least part of an interference pattern generated by said first and second portions of light (28, 30) dependent on said optical path difference; characterized in that the substrate (4; 50; 66) comprises one or more holes (32; 54; 82) extending fully through the substrate (4; 50; 66), the one or more holes (32; 54; 82) being arranged in a second pattern that is different from the first pattern, wherein the one or more holes (32; 54; 82) are positioned such that at least one of said first and second optical paths at least partly overlaps with the one or more holes (32; 54; 82). a light source (8; 70) arranged to provide 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 different optical path via said interferometric arrangement, thereby giving rise to an optical path difference between the first and second optical paths which depends on a distance between the membrane (6; 68) and the diffractive optical element (20, 22; 78, 80); and at least one photo detector (10; 72) arranged to detect at least part of an interference pattern generated by said first and second portions of light (28, 30) dependent on said optical path difference; characterized in that the substrate (4; 50; 66) comprises one or more holes (32; 54; 82) extending fully through the substrate (4; 50; 66), the one or more holes (32; 54; 82) being arranged in a second pattern that is different from the first pattern, wherein the one or more holes (32; 54; 82) are positioned such that at least one of said first and second optical paths at least partly overlaps with the one or more holes (32; 54; 82).