Midge Costin has had it with people forgetting the difference between sound editing and sound mixing every time the Oscars roll around. Once and for all: “Sound mixing brings all the elements together, all the various recorded tracks, and synthesizes them into one soundscape. Editing covers voice, music, effects, ADR – how do they sound?” So, that should be the end of that.

Of course, the professor and audio-biz legend had more on her mind than settling cocktail party disputes when she set out to create her new documentary Making Waves: The Art of Cinematic Sound. Her career’s emphasis on education gave her the idea to condense a semester’s worth of introductory material into one compact package, breaking down the essentials of her craft for an audience of laypeople. She introduces and defines key terms – foley work, automated dialogue replacement, mono v stereo v surround sound – in order to render visible labor that generally goes unseen and unnoticed when done well.

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“I went to film school, and I hated sound,” Costin tells the Guardian by phone. “It scared me, I had panic attacks doing it, so I wanted to be a picture editor when I graduated. I was doing my thesis, and a friend called me up to tell me that none of their union guys would touch 16mm sound. I fell into it by mistake, because I needed the money. I took the sound job, and then went, ‘Oh, crap. I’m responsible for tone and mood.’ I had a conversion, once I realized that sound wasn’t just technical. It adds so much to the story.”

She took to the job like a fish in water, quickly establishing herself as a formidable talent in her field and a reliable resource for big-league productions. She later grew disillusioned with the action-adventure gigs offered to her (she mentions getting jaded after working on a Michael Bay film – The Rock, judging from her IMDb page – before adding, “Maybe I shouldn’t use any names?”) and decided to go into academia. It was at USC that she first made plans with colleague-turned-producer Bobette Buster to assemble a documentary adapting her lectures for a single bite-sized package. They floated some plans in the early 2000s, but had to wait a decade and change for the protections on fair use law to beef up so that they could excerpt major movie clips without paying an arm and a leg.

The courts have since ruled that sampling footage will be acceptable so long as it’s done in the spirit of public edification, and just like that, Costin was off. Between the connections she’d made in the industry and favors called in from fellow sound people, she put together an all-star lineup of commentators. From her former student Ryan Coogler to Steven Spielberg, who named her the Kay Rose chair at USC’s School of Cinematic Arts, the deep bench of experts dissect scenes classic and contemporary to illustrate the great quantities of work that go into creating and fine-tuning a soundscape. Spielberg, for example, goes into the subtly expressionistic quality of the shellshocking beach invasion that opens Saving Private Ryan.

Facebook Twitter Pinterest Walter Murch mixing Apocalypse Now. Photograph: Matson Films

Just about everyone takes good audio work for granted, from the casual viewer to the budget-balancers at the studios. “They say sound is 50% of the story, but on the films I was doing, the post-production sound budget would be 1% to 1.5% of the total,” Costin recalls. “The average moviegoer thinks that when you turn on the camera, it starts also recording sound. They don’t even get that they’re recorded separately and synced. How much work going into sound, when done correctly, isn’t even perceptible.”

She organized the many different roles in the audio side of the film world within a “circle of talent” likening the conjunction of voice, sound effects and music to the cooperating sections of an orchestra. Each role requires precision, patience and perseverance. Smashing watermelons to approximate the noise of splintering femurs is all fun and games, but some tasks can be more tedious.

“When you think about doing footsteps, you’ve got to do them for everyone. Everybody has their own unique footsteps,” Costin says. “So many American films go abroad, and when you take out the production track, you lose everything. If someone’s tapping their fingers while talking, you’ll lose the tapping when they dump the dialogue for redubbing. The rerecording is extensive. They even have what’s called a ‘cloth track’, where they just rub various fabrics to get the sound of people shifting in their chair or getting up … Sometimes, the most important sound can be a creak or a breath.”

Facebook Twitter Pinterest Midge Costin at the 2019 London film festival. Photograph: John Phillips/Getty Images for BFI

Her film concludes with an eye toward the future, focusing on technology as the driving force for sound’s advancement in the theater and the home. Unlike the film purists that continue to keep mechanical methods alive, the audio set has fully embraced the computerized era. “Some people will say it just sounds better with analog equipment,” Costin concedes. “But the variety of things you can do, what you can see on the digital readouts, it gives you so many more options. Aside from the difference, which is hard to hear for a lot of people, we’re all pretty happy to be on digital.”

On the consumer end, she’s of two minds. She’s gladly watched equipment affordable to the average family grow more sophisticated, but that’s come with its own issues. “The problem sometimes comes down to how people have their home systems set up. Theaters get carefully calibrated, but listening at home leaves a lot of room for error. I’d love for people to see movies in theaters.”

However the tools of its creation or display may change, Costin knows that her life’s passion will be secure. Cinema needs sound to be its fullest, most exhilarating self, and sound needs artists to bring it to life. “As we watch things on smaller screens, sound gets even more important. The smaller someone’s face gets, the more reliant we are on audio cues for emotion and tone of a scene.” With a laugh, she adds, “You all need us!”