If you’ve ever stood in the Spring Street subway station in Manhattan and asked yourself, “Hmm, I wonder if standing here is a terrible idea?” then a quick consultation of the noise exposure standards promulgated in June 1998 by the National Institute for Occupational Safety and Health will show that, indeed, standing there is a terrible idea. During the lull between trains, the station is a quiet enough place. You can hear a woman’s boot heel clacking against the concrete platform, the MetroCard reader’s piercing electronic beep, the click-click-click of spinning turnstiles, a train in an adjacent tunnel shaking the walls with tectonic rumble.
This is the rush hour’s quietest moment, when noise levels inside the station drop to a low of seventy-one decibels. The hush, on a Thursday morning in January, lasts eight seconds.
An uptown 6 local train arrives. Its doors open with a staggered “thunk-unk-unk”; the fans inside the train’s twenty roof-mounted, six-and-a-half-ton Bombardier heating and cooling units spool up; and the station’s noise level jumps to ninety decibels. That’s as loud as an idling semi truck. According to National Institute for Occupational Safety and Health (NIOSH) rules, workers exposed to ninety decibels of noise for eight hours a day must wear protective gear to prevent ear damage and eventual deafness.
From there, the sound piles on. Spring Street station is built on a convex curve of subway track shaped like the tail of a parenthesis, and when the downtown 4 express rolls over the middle track, its hard-fixed wheels shriek and bark as they struggle to make the turn, emitting this horrible EE!eeEEE!ee!EEEE!!! that stabs your brain.
A man on the platform scrunches up his face and sticks fingers in both ears. The doors on the uptown 6 whack closed and the train’s three-phase, 147.5-horsepower traction motors cycle up as the cars accelerate. Now someone’s pushed the crash bar on the emergency door, which screeches its bi-tonal alarm like a British police siren on helium, and there’s some loud electrical bzzzzzzzz from someplace you can’t even tell. Above all this noise is a little recorded announcement piped through the loudspeakers to inform you in a friendly but all-business female voice that “The next…uptown…6 train…will arrive…” But you know this more from memory than from hearing, because you can’t actually hear any of it. The noise in the station climbs to 106 decibels, and since you’ve already consulted the handy chart on page two of the NIOSH noise exposure report, you know for certain that your inner ears can take only three minutes and forty-five seconds of this hell before they start to suffer permanent damage, and the noise burrows past your eardrums into that part of your brain where you escape fearful things and hide, it’s invading even that soft peaceful space, yes, oh God it’s terrible why did I ever leave St. Louis where it’s quiet I hate New York make it stop—
Which is to say that, acoustically speaking, the New York City subway system is a crazy-making hellscape. The Metropolitan Transit Authority knows this, and perhaps even more surprising, they’re about to spend $26.6 million dollars to fix it.
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When the MTA started planning the Second Avenue subway line, the first major extension of New York City’s subway grid in fifty-three years, its engineers thought about ways to reduce sound levels inside the new stations. The goal the authority set for itself was modest. It would not try to make the new stations whisper quiet, like D.C.’s Metro. It would not consider how people “respond to the acoustics holistically as they come into the subway from the urban street,” says Joe Solway, the MTA’s lead acoustical engineering contractor on the project, who prefers to think in such terms. Nor would it bother fitting the sounds of the subway into what Solway calls New York’s “amazing urban soundtrack.”
The MTA does not care about amazing urban soundtracks. The main thing the authority does care about when it comes to sounds is making sure people can hear the public address loudspeakers. This might make the agency seem unimaginative, but there’s a good reason for the agency’s acoustic conservatism: thirty-one people died in a fire in the London Underground’s King’s Cross station in 1987, partially because they couldn’t hear evacuation instructions. No deaths have been directly attributed to subway noise in New York in recent years, although new tracks along the D train in Bensonhurst, Brooklyn, prompted noise complaints in 2010 from business owners underneath the elevated line. Last year, City Council member Peter F. Vallone Jr. mailed a letter to the MTA about the sound of air brakes along the Q and N lines near the Ditmars Avenue stop in Astoria, Queens, which he compared to standing “next to a NASA launch site.”
Besides, making a sound system people can actually hear is quite a bit more complicated than it seems. It requires engineers to isolate and capture two different kinds of vibration. The first comes from the train’s wheels spinning on the steel track, which reverberates for blocks and even miles and turns our buildings, sidewalks and streets into speakers. This is called structure-borne vibration. The other kind is called airborne vibration, which includes all the higher frequency stuff like electrical equipment hums and the echoing sound of human voices (which, even with seven million daily riders, plays a surprisingly modest role in the field of subway acoustics.)
For the MTA, figuring out how to tame all these unwanted noises and amplify the only sound they really want started with one basic question: What’s the best way to float a train?
The Problem in Harvey Weinstein’s Basement
Sometime after Harvey Weinstein, the movie producer, bought his eight-bedroom, five-story West Village townhouse for $14.9 million in 2006, he discovered a little problem. He wanted to build a movie theater in the basement, but his house sits so close to the 1/2/3 subway line that every passing train shakes the foundation like a teacup on the San Andreas Fault.
So Weinstein hired Bonnie Schnitta. Five-foot-two with girlish, white-blond bangs that cover her forehead and direct attention to her bright blue eyes, Schnitta is the kind of genius-type who makes her incredibly complex work sound fun. As founder and president of SoundSense, an acoustical consulting and engineering company, she’s helped clients including Ralph Lauren and Peter Jennings with their airborne and structure-borne vibration abatement needs. She once helped a baker who tried to stop his equipment from vibrating by placing a giant mixing machine on springs.
“So the neighbor in the adjacent building was trying to sleep, and he heard this ca-tunk ca-tunk ca-tunk of the mixing machine walking around the floor,” says Schnitta, 61, who has a bachelor’s degree in math, a bachelor’s and a master’s degree in mechanical engineering, and a doctorate in math, engineering and ocean physics with a specialty in signal processing. She solved the problem by replacing the baker’s springs with squishy synthetic rubber pads. “It’s amazing what a little neoprene can accomplish!”
Schnitta’s fix for Weinstein’s basement was rather more involved. What she built was a theater that floats, with the interior space suspended from the rest of the house using springs along the ceiling, neoprene decouplers along the walls, and a combination of neoprene and decoupler feet under the floor. Working together, these tools isolate, or decouple, the theater from the foundation so that energy waves radiating down the track, through the ground and into the foundation get absorbed by the squishy barrier, leaving the screening room quiet and motionless.
“Then it didn’t have a direct connection to the rest of the building so that everything was acting as a shock absorber,” Schnitta said.
The MTA’s new subway line will operate along the same principle. When most of New York’s lines were built in the early twentieth century, the engineers focused on securing track tightly to the ground. This made the system hardy, but it also created a direct pathway for vibrations to radiate in every direction.
Unlike the last time New York built a major subway line, however, the T line train that the MTA plans to build underneath Second Avenue is being constructed in a place that’s already heavily populated. Engineering it the old way would cause quivers and shakes in hundreds of existing buildings.
“If it’s a deep foundation that’s sitting on the rock, you’ve got a very efficient transfer of vibrations into your building,” Solway says.
So the MTA’s new train tracks will float. There are different ways to accomplish this. One way is to spread a layer of neoprene and springs along the tunnel floor, so the entire track slab is decoupled from the ground. This system is the best in terms of isolating vibrations, since it spreads the weight of the train across a long, spongy bed.
It’s also the most expensive. Keeping with its mission to build something better, but not perfect, the transit authority went with a Low Vibration Track (LVT) system. Like existing tracks, this new one will be secured with steel plates bolted to concrete ties. But each tie will sit on a rubber boot embedded into the concrete track bed like a tooth in a jaw. Trains traveling the track will send vibrations down into the rubber boots, which will isolate some (but not all) of the energy before it escapes into the subway’s concrete foundation and then out into the surrounding ground. Compared to current trains, the result will be weaker vibrations radiating shorter distances.
During the project’s first phase, the two-mile section from East 63rd to 96th Streets that is scheduled to be completed by 2016, the floating track system will cost the transit authority $25 million.
“You can either solve the problem at the tracks or you can float all the apartments,” says Schnitta, who also consulted with the MTA about its track choice. “Obviously it’s better to stop it at the source.”
The floating track will reduce the really long-wavelength, low-frequency vibrations that subway trains send radiating out through the ground. Energy waves with higher frequencies and shorter wavelengths can’t travel through the walls, so they get stuck inside subway stations, where they bounce around and create the aforementioned acoustic hellscape—which engenders the MTA’s second acoustic sticky wicket: How to make a big subway train sound like a little subway train?
The Train Sandwich
Joe Solway is an engineer who likes his toys, and right now he’s commanding a very expensive toy. It’s the listening lab inside the new Water Street offices of Arup, the global engineering company hired as the MTA’s consultant on the acoustics and vibrations projects along the Second Avenue line. The room’s walls are padded in black sound-absorbing materials, which make it sound like a radio studio. There’s a big curved projection screen in front, and a circle of eighteen speakers at various heights plus four subwoofers arranged in a circle facing the middle of the room.
Solway, a reserved character with a British accent, wire rim glasses, buttoned-down shirt collar and blond hair combed into a classic news anchor wave, can barely contain his delight.
“The listening lab was completely a game-changer in the way that we do our design,” he says, giving clients “the subjective feel of how you will experience a space.”
Sitting at a bank of computer screens in the back of the room, Solway proceeds to layer sound as one might place slices of turkey breast on a piece of bread. He starts by playing the individual sound that his client, the MTA, really cares about:
“A Brooklyn-bound R train is approaching the station. All express trains are currently running on local tracks.”
The female announcer’s voice is loud, professional and precisely annunciated. In his methodical way, Solway proceeds to screw it up. By changing the settings on his computer, he filters the original recording through a crappy MTA microphone, which constrains the woman’s voice to its highest frequencies and makes it sound scratchy.
Since most subway announcements are recorded in open booths without soundproofing, Solway layers typical station sounds in the background. Suddenly, the announcer’s voice fades into the middle distance. After that comes the buzz from unshielded electrical cables, which makes the woman sound as if she’s using a hair dryer. In his next slice, Solway plays this whole recorded mess through the station’s band-limited speakers, turning the voice brittle and tinny, and then he factors in the station’s lousy acoustics. Lost in sonic backwash, the announcer’s voice is almost entirely swallowed up.
“Now, of course there’s one thing we’re missing,” Solway says.
A final keyboard tap brings in the whooshing air rush of a train. The perky-but-professional public address announcer? Please. Her voice might as well not exist.
Since he started working in the Second Avenue project in 2002, Solway’s job has been to fix this war of sounds, or, more accurately, to arrange a negotiated truce.
His first step was to build a computer model of a hollowed-out tube with all the dimensions of a Second Avenue subway stop. Into this virtual space he sent a sound called an electronic handclap, but which sounds like a gun fired in an empty warehouse. By measuring the speed, strength and direction of this noise as it bounced through the space, Solway created a sonic snapshot. Like a computer-generated photo that shows how a planned subway station will appear to the eye, this one uses acoustic data to recreate how it will sound to the ear.
“This is like an acoustic fingerprint that has all the elements of all the different reflections of all the surfaces,” he says.
The fingerprint allowed Solway to experiment, placing different types of soundproofing in different places inside the virtual station to see which variations do the best job quelling the noise. He started by thinking up ways to seal the trains acoustically from the rest of the station. Many airport trams accomplish this by lining the platform edge with sliding doors. This won’t work along Second Avenue, however, because Q and T trains with different car lengths will use the same track, making it impossible for them both to align with a single set of entrances.
So what Solway and the MTA created is a sonic enclosure that wraps around the train like a submarine sandwich roll. The ceiling over the tracks will curve down toward the train. A separate tube running the length of the station that contains speakers, security cameras and air ducts, and which is called the service carrier, will hang from the ceiling beside the tracks. Its shape will have a similar arc as the ceiling, and both will be lined with sound-absorbing materials. Working together, the service carrier and the ceiling will reflect noise back at the train and shrink the gap between train and platform, limiting how much noise can escape.
“Sound can be shaped. It can be tuned,” says Solway. “Compared to current stations, this is going to be a much more controlled environment.”
Trains will still make lots of noise, of course, and so will electrical equipment and crowds of people. To limit these sounds, many interior station walls will be covered with sound-absorbing panels that resemble modern house insulation. The stuff Solway recommended has a noise reduction coefficient of 0.8, meaning it soaks up eighty percent of the sounds that strike it. (That efficiency will be lessened somewhat by an outside layer of grey perforated ceramic tiles that will be exposed to the public and used to protect the soft sound absorber from damage.)
If everything works the way Solway’s computer model suggests, most sounds inside the station will get one bounce off a reflective surface, like the floor, before getting trapped by the absorptive panels.
“So the public gets the direct sound of the speakers firing down, and then they get an early reflection off the floor,” he says. “But then once that energy is gone, you want to absorb it as quickly as you can.”
The last sound Solway will try to control is fan noise. Many existing New York subway stations sit so close to street level that they receive ventilation through grates built directly into the sidewalk. Construction on the Second Avenue line is boring through bedrock one hundred feet below ground, however, so it requires fans to push air into the stations and tunnels and suck it out the other side. Each station will have four fans at its south end and another four at the north.
These fans are big. Together with their blast shafts, they require so much space that the MTA is building separate townhouse-sized buildings at either end of some stations to house them. On both sides of every fan will be a silencer that functions like a muffler on a car. Built with sound-absorbing mineral wool or fiberglass clad in a wall of perforated metal, each silencer will soak up twenty to thirty decibels of noise. Some fans will also be enclosed in separate soundproofed rooms. By the time the Second Avenue subway is complete in 2025, fan roar from these shaft buildings will be “insignificant and imperceptible” to neighbors and people walking by on the street, according to the MTA’s environmental impact statement for the project.
Back in Arup’s sound lab, Solway replays the original recording: “A Brooklyn-bound A train…” This time, he layers in everything the MTA plans to improve: a good microphone, a soundproof booth, shielded cables, decent speakers, fan splitters, and a station with $400,000 worth of sound-absorbing acoustic panels. The result is…significantly less crappy. Honestly, that’s the best one can say for it. There’s still plenty of background noise. The announcer sounds echo-y and distant. But she is audible. On the speech intelligibility index, which ranges from inaudible at zero to perfect clarity at one, the final result lands perfectly in the middle, at 0.5.
“We didn’t want to over design and make it too quiet,” Solway says. “We could have made it quieter. But we didn’t because it would’ve been more cost, and really we just wanted it to work.”
It’s comforting to know that in New York even the Metropolitan Transit Authority, which moves seven million people every day, rips giant holes in Second Avenue and channels enough force to shake us in our homes—even this powerful entity struggles mightily to be just barely heard.