The noise spectrum in group links is usually uniformly distributed, i.e. all parts of the frequency band are equally disturbed by the noise signal. Sound-programme signals, on the other hand,are not of uniform distribution.The mean power density of the signals tends to decrease towards higher frequencies.Furthermore,the sensitivity of the receiving part (consisting of the radio receiver, the loudspeaker and the human ear) in respect of noise is very dependent on the frequency.(This can be seen from the psophometric weighting curve which is a measure of the sensitivity of the complete receiving part.)
ITU-T J.17-1988 history
1988ITU-T J.17-1988 PRE - EMPHASIS USED ON SOUND - PROGRAMME CIRCUITS