The Coronavirus Quieted City Noise. Listen to What’s Left.

The corner of Lafayette and East Fourth Street in New York used to be a busy thoroughfare, with coffee shops, gyms, bus routes and hurried students walking between classes at N.Y.U. But since mid-March, all the usual sounds of Lower Manhattan — car horns, idle chatter and the frequent rumble of the subway down below — have been replaced by the low hum of wind and birds.

The remarkable change captured in two audio clips — each collected on a Thursday afternoon one year apart, from a microphone perched one story above the sidewalk — offers some measure of what this pandemic sounds like. Sound levels here have fallen by about five decibels, enough to make daytime sound more like a quiet night.

The coronavirus has transformed many aspects of life, closing businesses, canceling plans, confining people to home. But in this subtler way, life has changed, too: The city no longer sounds the same. And that realization is as jarring as the sight of empty streets.

Microphones listening to cities around the world have captured human-made environments suddenly stripped of human sounds. Parks and plazas across London are quieter than they were before the pandemic. Along Singapore’s Marina Bay, the sounds of human voices have faded. In suburban Nova Scotia, the noise of cars and airplanes no longer drowns out the rustle of leaves and wind. In New York, the city has been quieter than on the coldest winter days.

Whether you find this welcome or unnerving is another question.

“To me, it’s the sound of the city aching,” said Juan Pablo Bello, who leads a project at N.Y.U. studying the sounds of New York City. “It’s not a healthy sound in my mind. Even though I’ve been hoping for quiet in many ways for all these years thinking about noise, being obsessed with noise — somehow this is not quite what I was hoping for.”

The N.Y.U. project, called SONYC, recorded the audio clips above from one of 16 microphones that have been monitoring patterns in noise pollution in the city for more than three years, in research funded by the National Science Foundation. The microphones are mostly in Manhattan, with others in Downtown Brooklyn and Corona in Queens, but the yearslong audio archive gives a clear sense of a citywide rhythm over time — in normal times.

Twenty-nine of the city’s 30 quietest days during the last three years have been during the pandemic, the recordings suggest. The exception was Christmas Day in 2018.

Researchers in other parts of the world have captured the sounds of similar locations: public plazas without people, roads without cars, attractions with no tourists.

This is the sound of the plaza outside the Tate Modern museum in London, recorded midday last May as part of a soundscape survey in cities worldwide led by Jian Kang at University College London:

Researchers returned during the pandemic to 11 locations they had recorded around London, including public parks and once-busy commercial streets. Unlike the stationary sensors listening from above at N.Y.U., the head-mounted microphones used by the University College London team are meant to capture the experience of people in the middle of urban spaces. During the pandemic, those microphones have recorded consistently lower decibel levels at every London location.

Mr. Kang and a colleague, Francesco Aletta, are interested not just in the sound they can measure, but also in how people perceive what they hear. Recently, both recorded sound and perception of it are shifting. Sirens seem louder, or more common, but people are also listening for them in heightened ways. Sidewalk chatter that once made neighborhoods seem vibrant can now provoke anxiety: “Are those people practicing social distancing?”

Even as cities have grown measurably quieter, noise complaints aren’t necessarily down; the sound of neighboring televisions and leaf blowers can seem even more intrusive in quarantine.

In other ways, we’re suddenly nostalgic for noises that once annoyed us.

“People have said they miss the sounds of New York City,” said Arline Bronzaft, an environmental psychologist who has long studied noise pollution in the city. “They miss the honking horns, the crowds. And they would probably be the first people who were critical of those sounds. But it’s not that they miss them. They miss their lives.”

And then there are the birds — so many birds, who all seem so much louder. In fact, it’s likely that they’re actually quieter now than before the pandemic. They no longer have to sing louder to be heard over the racket of the city, a behavior, known as the Lombard effect, that has been observed in other animals, too.

“For me, I hear more birdsong,” said Mr. Kang, who lives in Sheffield, England. “One day I thought birds were coming through my house, and I looked for the bird and couldn’t find it.”

He finally found the source of the birdsong — outside. To him, this is the sound of the pandemic: nature so loud it seems to have entered his home.

“It’s almost like the countryside melody coming into the city,” said Carlo Ratti, the director of the Senseable City Lab at M.I.T., who described similar birdsong around Boston, a welcome sound to him.

During the pandemic, researchers in the M.I.T. lab have recorded walks through city parks in Singapore, New York and San Francisco, mimicking YouTube recordings of the same paths taken before the pandemic. In Golden Gate Park in San Francisco, they’ve found, the ambient noise of the city — cars driving by, construction work — has declined, replaced by birdsong.

The researchers at N.Y.U. have been able to measure not only the greater quiet, but also the subtleties of the sounds that remain. Machine learning software trained to identify the wail of sirens suggests that for most locations N.Y.U. has recorded, sirens have become less common as police activity has declined. But the frequency of sirens did spike for the sensor near the Horace Harding Expressway, next to a fire station. Sirens there doubled in early April, and researchers estimate that nearly two minutes of every hour recorded included the sound of sirens.

Across N.Y.U.’s microphones, the sound of human voices — recorded in snippets too brief to follow conversation — has become less common. The sound of car engines has persisted as some people like essential workers have continued to travel. But car horns, the sound of commuters at odds with each other, have disappeared.

At 7 p.m. each night, the N.Y.U. data has detected a consistent spike in loudness: the sound of thousands of residents clapping for the city’s essential workers.

Measured in decibels, many April days in New York were quieter than a typical holiday. The rhythm of the week — Mondays louder than Sundays — has disappeared. Nights have been especially silent.

Thomas Sugrue, a historian at N.Y.U., has noticed this walking his dog at night in Lower Manhattan.

“In lots of places — probably in most of America — the idea of a night being silent is a reassuring experience,” he said. “But when you live in a city, the absence of sound is profoundly unsettling.”

Mr. Sugrue recently pulled off his shelf an old paperback copy of “The Cholera Years,” Charles Rosenberg’s classic study of earlier outbreaks that ravaged New York. In 1832, Mr. Rosenberg wrote, visitors to the city were struck by the same observation: “the deathly silence of the streets.”

If all this silence seems similarly “deathly” today, that could undermine the benefits — for health, for education, for sleep, for children — that can come from quieter environments.

“The quiet means that no one’s out working,” said Erica Walker, a public health researcher at Boston University and the founder of the Community Noise Lab there. “It means that I am unemployed. It means that I have to plan my day around spotty transportation. It just means something very different.”

Mark Cartwright, one of the N.Y.U. researchers, suggested that their microphones were capturing something more hopeful: the baseline sound of the city, stripped of all the idling engines, the jackhammers, the honking, the stereos, the chatter, the arguments, the commerce. We’ve never been able to listen to this baseline before. Now, Mr. Cartwright said, we can begin to ask what we might want the city to sound like on top of it.