Intermodulation Distortion: IM distortion test mixes 60 Hz and 7 KHz signals in a 4:1 amplitude ratio. For virtually all electronic amplifier circuits, there is an approximate relationship between THD and SMPTE IM distortion. For example, if an amplifier measured 0.1% THD at 60 Hz at a given operating level, its SMPTE IM distortion would measure about three or four times that, or 0.3% to 0.4% at an equivalent operating level. IMD is usually very low in transformers and is particullary audible.
Bibliography: “Handbook for sound engineers”, Glen Ballou. Cuarta edición. Focal Press.