One of the classical experiments of psychoacoustics is the measurement of the lowest sound level at which a tone is heard in the presence of a white noise masker. This sound level is called the 'masked threshold'. In the following example, the masker is played at a fixed level, and the level of the tone can be adjusted. On every repeat of the stimulus, the masker is started first, and a short time later the tone comes in.
Set the tone to a level at which you can hear it clearly. Introspectively, even though the noise and the tone sum together to a single sound wave, the two sounds sound differently - it is as if two 'objects' were playing at the same time.
Next, find your masked threshold by lowering the tone until you can no longer hear it. What happens for tone levels that are very close to masked threshold? Do you still hear the tone as a 'different thing'?
Als, is your masked threshold the same if you start with a loud tone and decrease it until it disappears or if you start with an inaudibly quiet tone and increase it until you can hear it?