In the last few years, widespread access to some of the digital technologies mastering engineers use have called into question whether it’s necessary to take an album to someone like Grundman in order for it to sound good. In 2014, the Montreal-based firm MixGenius launched LANDR, a cloud-based mastering service that automates the mastering process using algorithms that detect genre, and allow you to select a specific style of mastering. The process is “instant,” and it costs between $2-20 per track, or a $9/month subscription for MP3 files; by comparison, mastering at a good studio can run as high as $500 an hour, though the fee for most mastering engineers lies somewhere in between. Users report that while mastering with LANDR might be better than nothing, the technology isn’t especially flexible or intelligent. You get what you pay for: a computer algorithm, rather than a live engineer with taste and experience.

Still, for young artists working on tight budgets, LANDR can feel like a godsend, and the technology fits neatly into a trend of DIY record making that challenges whether big studio resources are required to making great-sounding art. Even a high-profile artist like Kendrick Lamar recently released a collection called untitled unmastered. And the epilogue to Death Magnetic’s mastering controversy is its own object lesson on the question of DIY: The same day that Metallica released the record, they also released its original stems to Guitar Hero 3, and players noticed that the video game’s versions sounded much better than the originals. Wily fans extracted raw audio from the Guitar Hero renditions and remastered the album, correcting what had offended them in the original mix and master. The result is now, in some circles, the “real” Death Magnetic.

The rise of DIY mastering on the cheap hasn’t rendered expert mastering obsolete so much as it’s revealed the salutary effect a truly gifted engineer can have on a project. A great deal of the mastering engineer’s power comes from her place in the life cycle: She is the absolute final person to affect the way music sounds before it reaches the public. “For us, mastering is the last chance to resolve issues about an album to make it more coherent, cohesive, and listenable,” says Drew Daniel of Matmos. “They’re also there to protect you from certain kinds of narcissism and flashy yet hollow ways of exaggerating your art.”

Emily Lazar, one of few women of prominence in this field, and one of only two woman to ever receive a Grammy nod for mastering work, explains the last-stop quality of a masterer’s job this way: “I try to achieve what an engineer, artist, or producer can’t do on his or her own. Sometimes that’s technical, but just as often it’s conceptual. People will spend a lot of time—years, even—mixing and remixing. After all of that time, an album can sound outrageously different from track to track, with new people coming into the process with their own ideas.”

In this sense, a mastering engineer is responsible for the kind of editing that’s especially crucial to any art made by many hands. Beyoncé’s Lemonade, for example, employed over a hundred collaborators, making the record’s cohesion a tremendous challenge—and emphasizing the importance of a stopgap for aural coherence at the very end of the project.

One musician friend of mine likened mastering engineers to expediters in high-powered professional kitchens: “They aren’t doing the cooking, but they can make sure each dish is right and adjust presentation or garnishes so the whole meal comes together.” The metaphor is apt, flawed only in that it underestimates the full range of what a mastering engineer is responsible for overseeing. While an expediting chef is presented with a set menu, a masterer typically has no idea what might be walking in the door from hour to hour. In a typical day, an engineer can work on anywhere between one and eight different projects in a range of styles and genres. So it might be more apt to compare someone like Grundman or Lazar to a roving expediter, dropping in on different kitchens throughout the day, each with its own cuisine, native ingredients, and staff.

Grundman was quick to note that he doesn’t select or screen artists, which means that anyone who wants to book with him can, assuming they find him available and affordable. “The main thing is you can’t be prejudiced,” he explained. “Each artist is giving you their insight into what they feel, and it doesn’t matter what kind of music it is. You have to allow yourself to get on that wavelength and get a sense of what you could do so that it connects more directly for the listener. You’re trying to contribute to the artist’s dream.”