NEW YORK, United States — Prada and Britney Spears. Versace and Nine Inch Nails. Chanel and Jay-Z. All unexpected pairs, perhaps. But each coupling — of fashion and sound — appeared on the runways of the ready-to-wear shows which wound down two weeks ago.

Fashion and music have long enjoyed a special relationship — just try and imagine a fashion show without sound. And in the approximately 12 short minutes that most designers have to tell their seasonal story, it's sound designers like Frédéric Sanchez (who paired Prada with Britney's "Work Bitch") and Michel Gaubert (who tapped Jay-Z's "Picasso Baby" for Chanel) who are charged with "synthesizing [a fashion designer's] ideas into sound," as Sanchez puts it.

"In a movie, you have someone who can push the plot along verbally. But [with fashion], music and clothes are your whole plot," says Rene Arsenault, who has worked on soundtracks for Tom Ford since his time at Gucci, on the importance of music to seasonal runway shows.

At Comme des Garçons' most recent show, every look marched to its own signature tune — whether a snippet from a Fred Astaire movie, or the sound of crackling wood fire — to communicate that each stood on its own. During the Versace finale, models paraded to Drake rapping the house's name in tight succession. At Chanel, "we used 'Picasso Baby,' all mixed with avant-garde music from the 1970s, because the show was staged in an art gallery," Gaubert explains. And at Céline, he continues, "we only used one song" — Soul II Soul's "Back to Life" — "and we just remixed it to death because it's a three and a half-minute song and [we needed it] to last longer without getting boring."

The journey to fashion soundtracks like these begins anywhere from two months to a few days before the show, Gaubert says, depending on the designer. The initial inspiration can range from mood boards to clothing samples. “Sometimes it’ll be a musical reference, sometimes it’ll be a film reference, or I’ve been shown [something] more abstract, like art or sculpture,” adds Arsenault.

“First, I meet with the designers to exchange ideas and decide where we want to go,” Gaubert says. “Sometimes, they’ll bring music I should listen to. And then I see what works. Sometimes, within one [meeting] we’ll find all the music we like; sometimes it takes two times, three times. And then we start mixing it.” The whole process — which often ends hours, if not minutes, before the show — takes about 40 hours in total, Sanchez estimates.

“The key element is to get the musical cornerstone of the collection — that one piece that really exemplifies and speaks for the designer,” Arsenault says. “That’s the piece that either opens the show, closes the show, or is the main theme of the show. Once we have that, then it’s about approaching it like a soundtrack and varying the theme a little bit. Not introducing anything that’s so drastically different from what we already agreed upon, but maybe something that takes it down, takes it a little bit to the left, a little bit to the right. You want to start with a statement that typifies what the designer is trying to say. When the first girl walks out, it’s got to be pretty obvious. Then my job is to make that all flow together and sound like a styled soundtrack.”

Many of fashion's top sound designers have longstanding relationships with the designers with whom they collaborate. Gaubert, for instance, first began working with Karl Lagerfeld in the early 1990s. "The relationship with a designer has to be personal to understand clearly the world in their head, everything that influences the collection," says Javier Peral, who creates soundtracks for Jason Wu and Carolina Herrera.

“Once you work with someone over a period of time, you know their aesthetic already, and their taste,” adds Arsenault. “So whenever you hear a song [that might work well for them], you tuck it away into a playlist.”

Indeed, the search for the right songs is a never-ending process. “I discover songs and do research on the Internet, I go to record stores, live concerts, read music magazines, watch movies,” says Mimi Xu, who works with brands like Topshop and Ostwald Helgason. “Music is literally everywhere in my life.”

“It’s keeping your ears open all the time,” says Arsenault. “Now, anyone with an Internet connection and a sense of discovery is going to be able to find music. It’s just what you filter out and what you keep and put into a playlist that is the difference between people that are successful doing this and people that aren’t successful doing this. It all comes down to taste.”

But in the search for what’s right, personal taste must be sometimes cast aside, Gaubert notes. “I listen to a lot of things, new or old, and I even listen to things I don’t really like, because it’s important to know what’s around. With [fashion] designers, they sometimes don’t like certain fabrics or colours, but they know if they do it at the right time, it’ll be nice.”

The search must also go much deeper and wider than the Top 40. “The point of a fashion show is not playing the greatest hits,” Gaubert says. “Or something that’s trendy because it’s trendy. It has to fit. Anyone can play the newest sound in a show.” Arsenault agrees: “You don’t want to walk into a show and hear the same thing you just heard on the radio.” In fact, a good fashion soundtrack must feel unique. “When you leave the show, [you want to] feel that the music was custom-made for the show,” Gaubert adds.

Part of creating a differentiated soundtrack lies in mixing. And part lies in paying attention to what songs others have used. “It’s very important for me to look at what other people do,” Sanchez says, “because it’s very important for me not to do things that have already been done. If I know somebody has used [a song], I’m not going to propose the same thing.”

Though, typically, fashion show music has a strong beat to it, that “doesn't work all the time,” Xu says. “You always hear people think [a song] is going to be good [for the runway] because models can walk to it,” Gaubert adds. “But when anyone walks… when I walk on the street, I don’t need a sounding beat, you know? What the audience gets from the music — that’s more important than the bass of the music and the beats.”

Arsenault agrees. “It doesn’t have to be boom boom boom all the time. A lot of the shows I’m doing now are much more fluid, more about the overall sonic soundscape that you’re laying out, be it two songs, three songs, four songs, and how those songs are interwoven.”

So how does a sound designer know his or her soundtrack is working?

“When you play something for a designer and you see their eyes light up,” Arsenault says. “That’s when you know it’s going to work.”

“I mean, this isn’t failproof,” Gaubert says. “Like any creation, it’s taking chances. How does a designer know their collection is going to be perfect on the runway? But on a technical level, we do test the sound in the location.”

“What I like is at the end of the show, they say the show is great,” Sanchez says. “I don’t like when they say that the music was great. I consider that [to mean] I didn’t really do my job. It’s very important when the whole thing comes together. If the music has been too important to the show, it’s not good.”

Gaubert agrees. “When a show is good, it means everything was good. The production was good, the location was good, the music was good. It all made one, and that’s the way it works the best.”