A smooth headphone tames the frequencies that might otherwise turn sharp, so the whole presentation flows gently and nothing pokes at you. High notes arrive without shrillness, the midrange is often velvety, and the bass — present as it is — never erupts into a sudden boomy lump. The transitions between frequencies feel fluid. The payoff is a sound that's non-fatiguing and easy on the ears, the kind you can live with for hours.
Smoothness is usually the result of deliberate tuning: a slight roll-off in the upper treble, the absence of big resonance peaks, perhaps a gentle hand through the mids. It's essentially the opposite of a peaky or harsh sound — a frequency response line that runs relatively flat or mildly sloping, free of jagged jumps. Many Sennheiser models are loved for exactly this; their treble is present but never jarring. Tube amplifiers, too, lend smoothness by adding even-order harmonics and softening transients just a touch.
Crucially, smooth does not mean lacking detail — it's about how the detail is presented. A smooth headphone can be plenty detailed; it simply lays the information out in a relaxed way rather than shoving it at you. You hear everything, but nothing pokes or sizzles. It's the difference between a well-blended soup and biting into a raw chili: the soup can be complex (detail) yet smooth, while the chili is a sudden spike (a peak). This often goes hand in hand with low distortion — some high-end planars and electrostats are praised as wonderfully smooth precisely because even their abundant treble is distortion-free, and therefore never harsh. The same family of words covers it: buttery, creamy, liquid, the last defined as smooth, integrated, and coherent without being overly detailed.
There is a catch, and you'll spot it in the occasional backhanded compliment: the treble is smooth, but maybe a bit too smooth — it lacks excitement and air. That's the smooth-versus-sparkly trade-off. A smooth headphone will never offend, but a sparkly one might thrill you with cymbal shimmer (and also wince on a bad track). Pushed too far, smoothing can edge into veiled or overly polite, sanding away detail along with the rough bits.
Still, most reviewers treat a smooth frequency response as a virtue — generally a sign of careful tuning with no glaring issues — and pair it with words like refined, civilized, and polite. For anyone treble-sensitive or escaping a harsh pair of earphones, a smooth-sounding headphone is a genuine relief: it lets you focus on the melody and richness rather than on that one note that always made you cringe. Just remember that smooth should never mean dull. The best smooth tunings give you all the music, only without the rough edges — ideal for a long jam or for unwinding at the end of the day.