Abstract: An adaptor is disclosed for use with audiometers to enable the measurement of the masking level difference (MLD) in a person's hearing capability. The adaptor comprises a signal channel and a noise (masker) channel which are connected respectively with the audiometer outputs and the earphones. Means are provided to apply the signal voltage and the noise voltage to respective mixers in the two channels and for selectively inverting the phase of the signal voltage at one of the mixers and for selectively inverting the phase of noise voltage at one of the misers. Various MLD test conditions can be established including binaural and monaural conditions and adjustable intensity at one ear.