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Beyond major and minor intervals, perfect intervals (unison, fourths, fifths, octaves) can be diminished or augmented. Each alteration creates a recognizable quality: augmented fourths and fifths sound bright and dissonant; diminished fifths sound dark and unstable. By ear, you learn these distinctive qualities.
Compare a perfect fifth with an augmented fifth to hear the bright, sharp difference. Then practice diminished versions. Use the tritone (augmented fourth/diminished fifth) as a reference, since it has the most distinctive sound.
You already know the two main interval quality categories: major/minor intervals (seconds, thirds, sixths, sevenths) and perfect intervals (unisons, fourths, fifths, octaves). The reason perfect intervals form their own category isn't arbitrary — these are the intervals whose frequency ratios are simplest (2:1 for octaves, 3:2 for fifths, 4:3 for fourths), making them acoustically very stable and pure-sounding. Because of this special stability, they don't come in major and minor varieties; instead, altering them by a half step produces either a diminished or augmented version, each with a dramatically different quality.
Start with the interval you should already know best: the tritone. A tritone is exactly three whole tones — it's both an augmented fourth (C to F#) and a diminished fifth (C to Gb), depending on the notation. By sound it's the same: a tense, unstable, restless interval with no natural resting point. It splits the octave exactly in half, which is precisely why it's so dissonant — there's no simple frequency ratio that makes it stable. The tritone is your anchor for altered perfect intervals because its sound is unmistakable. Once you can hear it reliably, you have a reference point for everything else.
From the tritone, work outward. A perfect fifth sounds hollow and open, almost ancient — like a bell or an open string on a guitar. The augmented fifth (one half step wider) sounds bright and slightly unstable, with an upward-straining quality. It appears in augmented triads and is a common chromatic color in Romantic harmony. The diminished fifth is the tritone itself in a different name. The perfect fourth sounds stable when it appears as a consonance but has a characteristic "suspended" quality — it wants to resolve down to the third. The augmented fourth is again the tritone. The diminished fourth (one half step narrower than perfect) sounds surprisingly close to a major third — its acoustic size is identical — but in context it functions and resolves differently.
The key ear-training lesson here is that acoustic size and harmonic function diverge with enharmonic equivalents. An augmented fifth (C to G#) and a minor sixth (C to Ab) span the same number of semitones, but in a musical context they sound different because our ear hears their resolution tendencies: G# wants to rise to A (upward resolution), while Ab wants to fall to G (downward resolution). Your ear needs to learn to hear both the raw intervallic quality and the directional pull. Practice by comparing the perfect version to its altered counterpart on any instrument: play a perfect fifth, then raise the top note to hear the augmented fifth, then lower it further to hear the diminished fifth. The progression makes the distinctions concrete before abstract category names attach to them.
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