Bass is the foundation that the kick drum sits on. When the relationship between bass and kick is right, the low end of the mix feels solid and powerful. When it is wrong, the mix either lacks low end entirely or it is so muddy that no amount of EQ can save it. Mixing bass is mostly about managing the kick-bass relationship, with secondary attention to making the bass audible on small speakers (where most listeners actually hear music).
Bass has three frequency zones you need to think about separately. The sub-bass (40 to 80 Hz) is what you feel in your chest in a club. The body (100 to 250 Hz) is what makes the note recognizable as a bass note. The grit (700 Hz to 3 kHz) is the pick attack and string definition that lets the bass cut through on laptop speakers and earbuds. A pro bass mix handles all three intentionally; an amateur bass mix usually handles only the body, which is why amateur mixes feel weak on big systems and inaudible on small ones.
High-pass filter the bass at 30 to 40 Hz. Below that, you are just adding rumble that wastes headroom and mostly cannot be heard or felt. Boost the sub region by 1 to 3 dB around 60 Hz if it needs more weight. Cut the boxy mud region around 250 to 400 Hz by 2 to 4 dB — almost every bass benefits from this cut. Boost the grit region around 1 kHz by 2 to 4 dB to bring out the pick or finger attack.
The kick-bass relationship is the most important interaction in any mix. Two approaches dominate. First: complementary EQ. If your kick has its energy at 60 Hz, scoop a few dB out of the bass at exactly 60 Hz so they do not collide. Boost the bass at 100 to 150 Hz where the kick is weaker. Result: kick lives in the sub, bass lives in the lower mids, and they take turns being the dominant low-end element.
Second approach: sidechain compression. Route the kick to the sidechain input of a compressor on the bass. Set the compressor to duck the bass by 2 to 4 dB every time the kick hits. Fast attack (1 to 5 ms), fast release (50 to 100 ms). Result: the bass gets out of the kick's way for a fraction of a second, then comes back. This is the sound of every dance, EDM, and pop record made since 2008. It is also how Massive Attack and Portishead made the late-90s trip-hop sound.
Mono-sum the low end. Frequencies below 120 Hz should be in mono, not stereo. Stereo low end causes phase cancellation on club systems, mono playback systems (phone speakers, AM radio, single-driver Bluetooth speakers), and vinyl pressings. Use a stereo-imager plugin to force everything below 120 Hz to mono. The bass will sound the same on stereo systems and dramatically better on mono systems.
Bass distortion (parallel) is a lesser-known but powerful technique. Send the bass to an aux, run it through a distortion plugin (saturation, fuzz, or amp sim), then blend the distorted signal back in at -15 to -25 dB below the dry bass. The distortion adds harmonics in the upper mids that make the bass audible on speakers that cannot reproduce the fundamental. This is how a 60 Hz fundamental becomes audible on a laptop speaker that has no woofer at all.
Compression on bass should be musical, not aggressive. 2:1 to 4:1 ratio, slow attack (10 to 30 ms) so the pluck transient survives, slow release that pumps with the song. 3 to 6 dB of gain reduction on average. The goal is to even out the dynamic range so that no note disappears in the mix and no note jumps out painfully. Aggressive bass compression (8:1, fast attack, heavy gain reduction) kills the groove.
Synth bass and sub-bass require slightly different treatment. Synth bass usually comes pre-compressed and pre-EQd from the synth, so it needs less surgical work. The 808 sub-bass that dominates modern hip-hop and pop is essentially a sine wave at 40 to 60 Hz; the only thing it needs is a fast saturator to add upper harmonics, plus pitch-locking sidechain compression keyed to the kick.
Common mistakes: boosting the sub without high-passing first (rumble fills the mix), no kick-bass relationship management (mud), stereo bass below 120 Hz (phase issues on club systems), too much body in the 200 to 300 Hz range (the boom problem), no upper-mid presence (the bass disappears on phones).
Reference tracks: Billie Eilish's "bad guy" for sub-led pop, Royal Blood's "Figure It Out" for distorted rock bass, Jamiroquai's "Virtual Insanity" for finger-funk presence, Daft Punk's "Get Lucky" for dance-pop bass with sidechain. Each represents a different approach to the same problem of being audible everywhere.