A masking tone alternates with the combination of masking tone plus a stepwise decreasing test tone. First the masker is 1200Hz and the test tone is 2000Hz. Second the masker is 2000Hz and the test tone is 1200 Hz. Count how many examples of the test tone can be heard in each case.
A pure tone masks tones of higher frequency more effectively than tones of low frequency. This may be explained by reference to the simplified response of the basilar membrane for two pure tones A and B shown in the figure below. In (a), the exitations barely overlap; little masking occurs. In (b) there is appreciable overlap; tone B masks tone A more than A masks B. In (c) the more intense tone B almost completely masks the higher-frequency tone A. In (d) the more intense tone A does not mask the low frequency tone B.
This demonstration uses tones of 1200 and 2000 Hz, presented as 200-ms tone bursts separated by 100 ms (see figure above). The test tone, which appears every other pulse decreases in 10 steps of 5 dB each, except the first step which is 15 dB.