A mastering engineer receives the final stereo mix and applies subtle processing to ensure it translates well across all playback systems: streaming platforms, radio, vinyl, club sound systems, and earbuds.
Mastering involves EQ adjustments, stereo enhancement, compression, limiting, and loudness optimization. The engineer ensures the track meets loudness standards (typically -14 LUFS for streaming) without sacrificing dynamic range.
For albums or EPs, the mastering engineer also ensures sonic consistency across all tracks — matching levels, tonal balance, and spacing between songs.