What Mastering Actually Does
Mastering is the final step of audio post-production. A mastering engineer listens to your mix on calibrated, high-end monitoring in an acoustically perfect room and makes final adjustments to: overall tonal balance, loudness and dynamic range, stereo width, and consistency across a multi-track release. They also prepare files for digital and physical distribution in the correct formats.
When You Need a Professional
You need a professional mastering engineer if: you're releasing physical media (vinyl especially — vinyl mastering is a genuine specialist skill); your release will be pitched to blogs, press, or labels where first impression is critical; you're releasing an album where track-to-track consistency matters; or you simply want objective ears that haven't been staring at the session for weeks.
AI Mastering — Useful But Limited
Services like LANDR and eMastered have become competent for basic streaming release mastering. They won't hear the emotional context of your music or make creative decisions a human would. For a quick single release with a limited budget, AI mastering is a reasonable choice. For anything you care deeply about, a human engineer is worth the cost ($50–$200 for an independent release master is typical).
Self-Mastering
If your mixing environment is well-treated and your monitoring is reliable, self-mastering with reference tracks and a loudness meter is achievable. The risk is the same ears that mixed it are mastering it — you'll be blind to problems you've become accustomed to. At minimum, take a full 24-hour break between mixing and mastering.