Mid-bass is the upper part of the low end, roughly 60 to 250 Hz. This is where most real-world bass fundamentals actually live — the acoustic bass, the cello, the body of a kick drum — and it's the seat of punch, thump, slam-you-can-hear, and body. It's also a major contributor to note weight and warmth, which is why a healthy mid-bass is so much of what makes a sound feel full and fun.
The crucial pairing is with sub-bass, because people mix them up constantly and they produce genuinely different sensations. Sub-bass (20–60 Hz) is rumble and depth, felt more than heard. Mid-bass is punch and warmth, very much heard. A headphone with lots of mid-bass but rolled-off sub-bass sounds punchy, warm, full, and thick — but it lacks rumble and depth, and can come across a little humpy or one-note. A headphone tuned the other way rumbles and extends but can feel comparatively lean in the body. So I want more bass is ambiguous: thump and body mean mid-bass, rumble and depth mean sub-bass.
Mid-bass is also the region most responsible for bass bleed. Because it sits right up against the lower midrange where vocal and instrument body live, an overcooked mid-bass masks and warms the mids — male voices get overly chesty, guitars lose bite, the whole presentation thickens and veils. Sub-bass emphasis usually stays cleaner because it sits below that vocal-body region. This is the trade at the heart of bass tuning: mid-bass gives the most satisfying thump, but it's also the easiest way to muddy everything above it.
Decay and control matter as much as quantity here. A mid-bass shelf with a clean attack and a controlled, fast decay can sound powerful and tight without bleeding; the same boost with loose, lingering decay sounds boomy and smears into the next note. Two headphones with identical mid-bass level can sound completely different depending on how quickly the driver starts and stops.
Too little mid-bass and music feels thin and punchless, robbed of its groove and body; too much and it turns boomy, thick, and muddy. The sweet spot gives kick drums their weight and basslines their drive while leaving the mids clear. For a newcomer, the easiest demonstration is a track built on a kick-and-bass groove: mid-bass is the part you feel as a thump in the rhythm — distinct from the deeper rumble underneath it, which is sub-bass.