Open-air type earphone

An open-air type earphone comprising:

• a driver unit including means defining a magnetic circuit, and a diaphragm, and voice coil which form a vibration system:
• a housing containing said driver unit and having a sound generation opening to the atmosphere in front of said driver unit, and at least another opening to the atmosphere from said housing behind said driver unit and which has acoustic resistance material extending thereacross;
• an elongated duct extending from said housing behind said driver unit and communicating, at one end, with the interior of said housing and, at its other end, with the atmosphere to provide an equivalent mass which is in parallel with an acoustic resistance of said acoustic resistance material and which is effective for reducing the lowest resonance frequency of the earphone;
• a casing attached to said housing behind said driver unit;
• and
• means defining a pipe communicating, at its ends, with said housing and said casing, respectively, and having a diameter and a length longer than said diameter of the pipe, a resonant frequency of a resonant circuit constituted by an equivalent mass formed by said pipe and a compliance formed by said casing being selected to be near to a resonant frequency which is determined by a mass of said vibration system and a compliance of said housing in back of said driver unit as to suppress a relatively high frequency peak that would otherwise be formed in the frequency characteristic of the earphone by said reducing of the lowest resonance frequency of the earphone.