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 to be discussed in this standard, utilize a sound source resembling the human mouth to drive the transmitter, and a simple air volume resembling that of 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 that provide a sound field similar to that produced by a typical human voice. This special loudspeaker is referred to as an Artificial Mouth. The terminating air volume, in which the sound pressure generated by the receiver is measured, is provided by a device 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.