As reported on Wired.
BY NATHAN HURST
The trek to the bottom of Bay Bridge pillar number 7 (westbound) starts down a ladder below a squeaky metal trap door, continues downwards 80 feet or so on narrow metal stairs sandwiched in a column tight enough to touch either side at once, and ends by passing through a crawl space in about 3 inches of salt water.
Once there, in a round concrete chasm, sunk beneath the San Francisco Bay, there’s enough room to stand up, which is what Benny Burtt does. And in the dim light of a little flashlight, he pulls out a pistol.
It sounds like a scene from a Hollywood thriller, but it’s not — though the sounds involved might someday show up on the silver screen. Burtt, an assistant sound effects editor for Skywalker Sound, the audio arm of Lucasfilm, is inside the new Bay Bridge on a rainy Friday with three other Skywalkers. They’re firing blanks with the gun to help record the sonic impulse response — an audio impression — of the bridge’s interior before the bridge opens and Homeland Security closes it off.
The project is mutually beneficial; the Skywalker team came to the bridge in search of new sounds, looking for audio effects that could appear in a Lucasfilm — or now Disney — movie. And the California Department of Transportation invited Skywalker Sound as a way to record and preserve the distinct impression of the bridge’s interior, through sound.
“The first things you think about showing people are visual,” says Bart Ney, communications manager for the Bay Bridge. “Everything’s visual. But when Skywalker comes out … they really come out and look at the project in a different kind of way.”
When you’re a recording team, the bridge is something more than architecture, and sonic imaging is an alternative to photographing.
“It’s kind of another way to preserve the legacy of the bridge, because the acoustics will be out there and somebody will be using them,” says Ney.
The new eastern span of the San Francisco Bay Bridge is one of the biggest engineering projects in recent California history. Scheduled to open next Labor Day, construction on the $7 billion bridge began in 2002. The process has featured innovations from directional lighting to weight transfer toearthquake resistance.
“So this would be a bad place to be during an earthquake?” Burtt asked as he descended below the water line on his way to the bottom of the column.
“This would be a good place to be during an earthquake,” Ney responded.
A few minutes earlier, the gang had hiked along the inside of the bridge itself, just under where the cars will go. The tunnel, about 25 feet wide and 25 feet tall, with a metal walkway suspended near the middle, stretches for more than a mile, 100 feet above the bay. Ducking through a hole in one of the diaphragms that break up the tunnel, the Skywalker team emerged into the E6W-4E section and began to set up their equipment.
Casey Langfelder adjusts his recorder. Photo: Ariel Zambelich/Wired
Burtt and Langfelder, along with assistant editor John Borland and intern Baihui Yang, had carried four stereo microphones, in Pelican cases, as well as several tripods and four 4-channel recorders. They began setting up to record, stationing one mic at the spot they’d fire the pistol, and one every 50 feet or so beyond.
In the bowels of the bridge it’s dark and cool and smells like concrete dust. Every footstep, every word, every sound echoes off the concrete walls, floor, and ceiling, making it hard to understand anyone.
“You guys ready?” says Burtt, then waits for the echoes to die. He fires the gun, with a pop and a spark.
The pistol gives off a “full frequency event” — that is, the sound covers the full range of audible frequencies, giving a complete impulse response. Back at Skywalker, the editors will use Altiverb to digitally remove the sound of the shot.
“Then we can run whatever sound we want through that program, and it’ll sound like we’re in here,” says Langfelder.
Each microphone they have, called mid-side mics, houses two units — a front facing element to capture the event, and a figure-eight shaped one that records stereo. Because the sounds reaching the side mic have bounced off the surroundings, they helps give a sense of ambient space, says Burtt. Together, they allow the sound engineers to adjust the width of the sound, making it project a sense of space. The microphones Skywalker brought all cost around $2,000 each, and, paired with $4,500, 24-bit recorders, capture sound at 192 kilohertz, around five to six times the quality of a CD.
“Now that we’ve recorded the Bay Bridge tunnel span, we can create an impulse response for it at each distance we recorded,” says Burtt. “Any time we need some sort of subterranean cave or super-echoey location, we can plug an effect into this and have the response without actually going back out to the bridge.”
Skywalker recorded a few more sounds as well. They’re always on the lookout for interesting audio impressions. Earlier in the bridge’s construction, Langfelder recorded a construction elevator, a jackhammer, and some other construction noises.
“You never know where these sounds might wind up,” he says. “They could wind up on Star Wars, or they could end up on other things … our library is always growing.”
While inside the bridge this time, they capture the creak of one of the trapdoors as Ney opens it, and the clang as he slams it shut. Burtt suggests getting some screams, so a few of the bridge spelunkers take turns — a primal yell, a cry for help, an animal roar. There aren’t any plans yet for what movie, if any, the sounds will appear in. But if you hear a unique echo in the theater someday, it’s possible it was recorded inside the Bay Bridge.
