Generally, the more consonant the interval, the more it sounds to the average listener like a single tone, rather than two. Take guitar power chords, for example, which are made up of the root and fifth of the chord, with no third present. Since the perfect fifth is a clear contender for the most consonant interval in all the land, power chords sound like just one note to the untrained ear. A minor second, however, probably the most dissonant interval, is very definitely perceivable as two notes.
The minor ninth (the interval one semitone higher than the octave) is known as an ‘avoid’ note – it’s considered just too dissonant. This can be got around by inverting the interval and playing a major seventh instead. So, the interval between the E and F in a Cadd11 chord is a minor ninth, but playing the F below the E, not above it, turns the interval into a more palatable major seventh.