This document is applicable to active acoustic noise cancelling headphones and earphones which have the function of reducing the noise heard by the user by the output sound from the transducer generated by the environment noise detection microphone and the noise reduction signal processing circuit. This document specifies the terms and definitions of this type of headphones or earphones, the characteristics to be specified, and the measurement and evaluation methods. The noise detection microphones are mounted in the body, on the surface, or on an accessory of the headphones or earphones. Signal processing circuits are analogue and digital electronic circuits. This document does not deal with equipment intended for hearing protection. The noise cancelling characteristic measurement methods may be applied to headphones and earphones having no active noise cancelling function.