SOUND-SOUND INTERACTION

 Answer 

Backward masking. When the gap is 100 ms (case A), you can probably hear all steps of the ramp clearly. When it narrows to 20 ms (case B), the last 2 or 3 steps show signs of being masked. With a 0 gap (case C) it is virtually impossible to hear the tones in the ramp at all. About 10 ms between tone and noise would be necessary to hear the tone separately.