The method requires a suitable Artificial Mouth. It also specifies a standard Artificial Ear, standard circuits comprising direct-current power feed and connecting loops, and a standard pressure microphone. 2.3 This Standard is intended for use in measuring the transmission characteristic over the frequency range most useful for speech, e.g. 150 to 5000 hertz. It is to be used for single-frequency measurements and for measurements with continuously varying frequencies such as slow sweeps and warble bands. 2.4 It is suitable also for measuring the transmission performance of conventional telephone sets equipped with handsets using carbon transmitters and used with 2-wire circuits. It is not intended to be applicable to special devices such as noise-exclusion transmitters, distant-talking transmitters, insert-type receivers, or noise-exclusion receivers equipped with large ear pads. 2.5 This Standard is based on a standard pressure microphone to measure the sound field produced by the Artificial Mouth and to measure the pressure developed in the Artificial Ear. 2.6 Drawings and test procedures as well as otherre pertinent information have been included as an aid to designers in setting up the tests.