Revision Standard - Superseded. The methods that are used to measure telephone-set transmission performance may be divided into two classes: objective methods and subjective methods. The objective methods, which are those discussed in this standard, utilize a sound source resembling the human mouth to drive the transmitter, and an acoustic coupler resembling the human ear as a termination for the receiver. The sound source comprises a source of electrical energy and a moving-coil loudspeaker unit having supplementary electrical and acoustical components which produce a sound field simulating that of a typical human voice. This special loudspeaker is referred to as an artificial mouth. The acoustic coupler comprises a microphone and a terminating air volume having supplementary acoustic components which produce a load on the receiver simulating that of a typical human ear. This coupler is referred to as an artificial ear. The artificial mouth and ear are useful in comparing the performance of telephone sets at frequencies in the significant portion of the speech range.