Single diaphragm second order differential microphone assembly

Apparatus comprising a first-order differential microphone having front and rear faces, wherein the front face is in acoustic communication with first and second front acoustic conduits through a front cavity, the rear face is in acoustic communication with first and second rear acoustic conduits through a rear cavity, each conduit is terminated by a respective port for admission of acoustic energy, and the ports are so arranged that the response of the microphone will be approximately proportional to a second spatial derivative of a sampled acoustic pressure field, CHARACTERIZED IN THAT:

• a) the apparatus further comprises a slab-like base comprising an acoustically rigid material and having a substantially planar surface, to be referred to as the top base surface;
• b) a pattern of channels is formed in the base, wherein the pattern is defined by interior surfaces of the base, the channels are open at the top base surface, and the channels extend in at least one longitudinal direction substantially parallel to the top base surface;
• c) the pattern comprises four channels of equal length and cross sectional area, and each of said four channels is identified with a respective one of the acoustic conduits, so that the channels will be referred to, respectively, as the first and second front channels and first and second rear channels;
• d) the pattern further comprises a central cavity having a front end joined to the front channels and a rear end joined to the rear channels;
• e) the microphone is mounted in the central cavity such that the portion of the central cavity unoccupied by the microphone is subdivided into mutually acoustically isolated front and rear portions of equal volumes, wherein:
• the front portion adjoins the front channels and the microphone front face, and is identified with the front cavity;
• and the rear portion adjoins the rear channels and the microphone rear face and is identified with the rear cavity;
• f) each port is an opening in a surface of the base;
• g) the ports are arranged substantially along a straight line, the rear ports are intermediate the front ports or the front ports are intermediate the rear ports, and each front port is distant by the same amount from its adjacent rear port, such that the second spatial derivative to be represented by the microphone response is a uniaxial derivative;
• h) the apparatus further comprises a cover having a substantially planar surface, to be referred to as the bottom cover surface;
• and
• i) the bottom cover surface is affixed in contact with the top base surface such that the cover forms an acoustically sealing closure for the channel pattern.