“Noise exposure can increase stress, can disturb sleep, and can create general annoyance, and all those things are known to contribute to hypertension,” says Jonathan Levy, a public health researcher at Boston University who has worked on multiple studies of the health effects of airport noise.

To understand how noise affects health, you need to first know how much of it there is. From early 2015 to February 2016, Walker took noise samples in positions around Boston at all times of day and night. Conventional techniques for measuring noise (known as A-weighting) typically capture only mid-level frequencies, but Walker also included low-frequency sounds (a bus rumbling) and high-frequency sounds (the whine of a highway), which she says can be just as disruptive. From these measurements, she created neighborhood sound models, which, she says, “should be able to tell people the ambient sound level during the night, day, and weekend.”

According to Walker’s models, the only period of real quiet in Boston each day is from 1 a.m. to 5 a.m. “At 5 a.m., you start to get pickup and delivery, trucks are slamming and opening doors, throwing things, unloading things . . . once the noise starts, it’s consistently loud throughout the rest of the day,” she says. The noisiest

neighborhoods in Boston are Chinatown, the North End, and Dorchester. The quietest areas are West Roxbury, Mattapan, and some parts of Jamaica Plain. Moreover, Walker found that existing city noise ordinance thresholds are routinely broken.

An important finding of her work is that when it comes to noise, perception matters as much as any objective measurement. Walker’s team asked 1,200 people to rate their sensitivity to noise and to assess how annoyed they were by sounds like car horns, leaf blowers, tourism, crying babies, and loud parties. She found that people’s perceptions of the noise around them sometimes differed substantially from her measurements.

“In West Roxbury, a neighbor talking loudly may be a sound, but in Dorchester they might say I don’t feel like I’m in the city unless I hear my neighbors fighting,” she says. “People have different perceptions about what is noise and what’s not noise.”

Walker found this perception gap to be especially wide in East Boston, where residents reported being disproportionately sensitive to airport noise. “Noise levels in East Boston are not nearly as bad as in Dudley or Dorchester, but the perception of loudness in East Boston, it’s through the roof,” she says.

For city officials, Walker’s work is a starting point — a baseline for noise mapping the city. Even with more data, better noise control won’t be easy to come by. Already the City of Boston states, “Anything louder than 70 decibels is considered too much at any time.” But if perception counts for so much when it comes to noise and health, inevitably you have to ask, whose perception counts more?

Kevin Hartnett is a writer in South Carolina. He can be reached at kshartnett18@gmail.com.