Apparatus comprising: an earphone for an active noise reduction earphone, comprising: structure for engaging an outer ear so that the earphone is positioned and retained in an ear of a user; a nozzle providing a passageway that is adapted to acoustically seal the earphone with an ear canal of the user at the transition between the bowl of a concha of the user and the entrance to the ear canal to form a cavity; active noise reduction circuitry comprising a feedback microphone adapted to be acoustically coupled to the ear canal, for detecting noise in the earphone; feedback circuitry responsive to the feedback microphone for providing a feedback noise cancelling audio signal; and an acoustic driver for transducing an output noise canceling audio signal comprising the feedback noise canceling audio signal to noise canceling acoustic energy; and wherein the nozzle is arranged such as to control an acoustic impedance of the nozzle, by choosing a cross sectional area of the nozzle, a ratio of the nozzle length to the nozzle cross sectional area, and an acoustic mass of the nozzle, in such a way as to cause an absolute value of the acoustic impedance of the nozzle to be both below 8 × 10 5 kg m 4 × sec at 100 Hz and below 8 × 10 5 kg m 4 × sec at 1 kHz. an earphone for an active noise reduction earphone, comprising: structure for engaging an outer ear so that the earphone is positioned and retained in an ear of a user; a nozzle providing a passageway that is adapted to acoustically seal the earphone with an ear canal of the user at the transition between the bowl of a concha of the user and the entrance to the ear canal to form a cavity; active noise reduction circuitry comprising a feedback microphone adapted to be acoustically coupled to the ear canal, for detecting noise in the earphone; feedback circuitry responsive to the feedback microphone for providing a feedback noise cancelling audio signal; and an acoustic driver for transducing an output noise canceling audio signal comprising the feedback noise canceling audio signal to noise canceling acoustic energy; and structure for engaging an outer ear so that the earphone is positioned and retained in an ear of a user; a nozzle providing a passageway that is adapted to acoustically seal the earphone with an ear canal of the user at the transition between the bowl of a concha of the user and the entrance to the ear canal to form a cavity; active noise reduction circuitry comprising a feedback microphone adapted to be acoustically coupled to the ear canal, for detecting noise in the earphone; feedback circuitry responsive to the feedback microphone for providing a feedback noise cancelling audio signal; and an acoustic driver for transducing an output noise canceling audio signal comprising the feedback noise canceling audio signal to noise canceling acoustic energy; and wherein the nozzle is arranged such as to control an acoustic impedance of the nozzle, by choosing a cross sectional area of the nozzle, a ratio of the nozzle length to the nozzle cross sectional area, and an acoustic mass of the nozzle, in such a way as to cause an absolute value of the acoustic impedance of the nozzle to be both below 8 × 10 5 kg m 4 × sec at 100 Hz and below 8 × 10 5 kg m 4 × sec at 1 kHz.