“I was like, ‘man - this is the energy’”: Billie Eilish’s mix engineers reveal the star's “favourite part” of her smash hit, Birds Of A Feather

It’s nice to imagine that Billie Eilish and her brother Finneas complete all their albums - from start to finish - in their home studio. The truth is that there are other people involved though, and when it came to mixing Eilish’s third full-length record, Hit Me Hard And Soft, two of those people were Jon Castelli and his co-mixer Aron Forbes.

One of the standout tracks from the record, of course, was smash hit not-a-single Birds Of A Feather, and in a new multipart video series for Mix with the Masters, Castelli and Forbes have been explaining how they made its component parts gel together.

Jon Castelli and Aron Forbes mixing ‘BIRDS OF A FEATHER’ by Billie Eilish | Trailer - YouTube

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Step one, it turns out, was making sure that they were hearing exactly what Billie and Finneas had created. “The most sensitive part was getting the audio files from Logic to come into Pro Tools sounding exactly the same, because there’s so much inherently baked-in to Finneas’s arrangement and rough mix,” says Castelli.

This involved a bit of gain staging, but it was crucial preparation for the task ahead, because it meant that, if Finneas was ever unhappy with part of a mix, they could get back to the sound of his original.

With this done, it was time to dive into arguably the most crucial part of the mix: the vocals.

“This whole album, Billie went so deep on vocals,” says Forbes, adding that it was his and Castelli’s job to make sure the character of these vocals remained intact. “If you alter it, it starts to become, you’re like, ‘well, that's not the person I know,’” he says.

This meant that the compressor choice was crucial, and in the case of Birds Of A Feather, they chose a Brainworx plugin emulation of a Solid State Logic classic.

“I assume we have the [SSL 4000] G on this one for her,” says Forbes, with Castelli adding: “that’s what’s holding her forward, with a little bit of top-end boost and low-end body on the 200.”

Finding the right tool for the job, though, sometimes required a bit of trial and error.

“We would just A/B different boxes, like, SSL G channel versus J channel; Ozone Tape versus maybe like a Kramer tape - just these like emulation boxes that give tonal shifts and character to things without necessarily changing it.”

It turns out that it isn’t her vocals that Eilish herself likes most about Birds Of A Feather, though: “Billie’s favorite part is when that bass comes in,” confirms Forbes. “John gassed it in the best way.”

“I was like, ‘man - this is the energy,’” says Castelli of that bassline, and it certainly jumps out of the final mix. There was a balance to be struck, though, and Forbes says that they had to ask themselves “between the top end of the bass, how loud can it be without competing with her?”

Billie Eilish - BIRDS OF A FEATHER (Official Music Video) - YouTube

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Elsewhere in the series, Castelli talks about the value of having a co-mixer on a project like this, where neither the artist or producer is there with him.

“Historically you would have the producer in the room, the artist in the room,” he says. “You'd have four hands on the console - six hands on the console. We don't have that anymore.”

Of Forbes - who is also Eilish's musical director - Castelli goes on to say that he really appreciated “having him on the sofa and trading places with me at the desk,” and his co-mixer was happy to take this step back. Sitting on that sofa, Forbes says: “I instantly fell in love with this seat because the low-end made sense to me.”

It was the meeting of minds and perspectives that ultimately made the magic happen though: “there's things you can hear up there I can't hear back here and vice versa,” says Forbes.

Subscribers can watch the full video series on the Mix with the Masters website.