Unidirectional condenser microphone unit

A unidirectional condenser microphone unit comprising:

• a cylindrical unit case including:
• a front acoustic terminal in a front end portion of the unit case;
• and a rear acoustic terminal in a peripheral wall portion of the unit case that is away by a predetermined distance from the front acoustic terminal;
• an electrostatic type electroacoustic transducer including a diaphragm and a fixed pole that are opposedly placed with a predetermined distance therebetween, the electroacoustic transducer being housed between the front acoustic terminal and the rear acoustic terminal of the unit case;
• and
• a sealing member fitted to a rear end portion of the unit case, the sealing member forming an air chamber having a predetermined volume on a back surface side of the electroacoustic transducer, wherein
• a velocity component taken in from the rear acoustic terminal acts on a back surface of the diaphragm through the air chamber and a sound hole included in the electroacoustic transducer,
• a portion of the sealing member is formed in a convex spherical shape, the portion ranging from an apex part adjacent to a central portion on the back surface side of the electroacoustic transducer to a hem part adjacent to the rear acoustic terminal,
• the rear acoustic terminal has a predetermined acoustic mass component, and
• the sealing member forms an acoustic distributed constant circuit including:
• a first portion in which a cross-sectional area of a sound path continuously increases, the sound path running from an acoustic resistance part behind the fixed pole of the electroacoustic transducer to reach the rear acoustic terminal through the air chamber;
• and a second portion on the rear acoustic terminal side in which a cross-sectional area continuously decreases toward a direction farther from a sound pickup source.