Hang out anywhere there are people — Amtrak, Burger King, the I.C.U. — and it can feel as if there’s no escape from their noise. A Mets game, the Beyoncé discography, FaceTime conversations about nothing: Thanks to the unfettered use of speakerphones, other people’s business is now yours too, loudly.

To be fair, not using headphones may have nothing to do with disrespect. Many smartphones don’t have traditional headphone jacks. Bluetooth headphones can be pricey. Sometimes you just forgot to bring earbuds and it’s your mom’s birthday.

But on Reddit recently, the second most popular answer to the question “What do you secretly judge people for?” was “when they watch TikToks loud in a quiet room without headphones.”

Cellphones make it easy to aggravate an entire subway car, but this isn’t a 21st-century issue; ask anyone who remembers boombox-era New York City. But there’s a difference between having a conversation on the phone and having your speaker on. For some people, going sans headphones doesn’t register as a problem. For others, it’s an affront — in some cases, debilitatingly so.

According to the etiquette expert Myka Meier, the issue is a clash over what constitutes civic pollution.

“On our phones, we selfishly have interests that we are able to tap into any second of any day, and we are so used to it that we forget other people are around us,” said Ms. Meier, who shares tips on removing tea bags and answering text messages with her 640,000 Instagram followers. “I get embarrassed if I cause somebody to feel uncomfortable or if I take up someone else’s space. A lot of people don’t feel that anymore.”

Who’s to blame for this frequently transgressed violation of our shared social contract? The answer comes in three.

Explanation No. 1

It’s Not Me, It’s You

Christine McBurney, an actor and director, was recently at a cafe in Montreal when a family with children plopped down at a table next to hers.

“They have an iPad, and they’re watching a game show — a game show — at full blast,” said Ms. McBurney, 58. “Because it was Montreal and I wasn’t afraid of getting shot, I said very nicely to the grandmother — I figured I’m almost her age, she’ll understand — ‘Do you have headphones for the kids?’”

Wrong question.

“She went ballistic,” Ms. McBurney recalled. “She was like, ‘No, you put on headphones.’ I said: ‘This is a public space, all the more reason you should be respectful. People are here to have their own conversations or work quietly.’ She kept mumbling under her breath and saying people have the right to do what they want in public.”

If this had been New York, Ms. McBurney said, she might not have said anything at all, out of fear that even someone’s granny might respond violently.

“Your life isn’t worth a temporary inconvenience,” she said.

Jay Van Bavel, a professor of psychology at New York University, calls this a Covid-era “norm erosion” that can only change if there’s sufficient norm enforcement.

“People have to be comfortable to say, ‘Please put your headphones on,’” said Professor Van Bavel, the director of N.Y.U.’s Social Identity & Morality Lab.

But for many people, that’s hard to do. He pointed to one famous demonstration from the 1970s, when the social psychologist Stanley Milgram had his students ask New York City subway riders to give them their seat. Most of the students had a lot of trouble doing it.

“It seems easy to ask,” Professor Van Bavel said, “but at the moment, you are violating a norm, and it’s hard.”

Absent relief at the cafe, Ms. McBurney sat and suffered. (She had headphones of her own, but she was writing and didn’t want to listen to music, nor did she feel like being bullied into putting them on.) The incident still nags, she said — an ominous sign “that our boundaries must be deteriorating.”

“We can’t solve the jackhammer or noisy neighbors, usually,” she said. “But you can put on a pair of headphones. There are so many social contracts that we all agree to, and this should be one of them.”

Explanation No. 2

It’s Me (but Also You)

Cris Edwards doesn’t go to the movies anymore. Not because of the price of popcorn, but because of the sound of it.

“Eating sounds — gum smacking, dinner scenes with loud eating — it drives me insane,” he said. “Pen clicking, keyboard typing, people talking with wet mouth sounds or smacking lips: Those are triggering for me.”

Mr. Edwards is the founder of soQuiet, a nonprofit advocacy group for those with misophonia, a sensory disorder in which people experience an unusually strong aversion to everyday sounds.

“It overwhelms you, and it’s hard to talk about when you’re in this angry panicked state,” Mr. Edwards said. “It overwhelms your nervous system. It’s maddening.”

M. Zachary Rosenthal, director of the Center for Misophonia and Emotion Regulation at Duke University, recalled a time when he and a family member who has misophonia were at an airport lounge when “this bro gets a big bowl of pita chips.”

“He’s 10 feet away and he’s, like, opening up his mouth to the sky as wide as possible, and crunching as loud as humanly possible — the whole place could hear it,” Professor Rosenthal said. “It’s like he was trying to generate the most abrasive sound.” His relative “had a fight-flight reaction,” so they moved to another part of the lounge.

Professor Rosenthal said that almost everyone is bothered by sounds of some kind, but what bothers me may bother you less, or not at all. Misophonia, which is on the extreme end of what he calls the “sound sensitivity spectrum,” has no official diagnosis for misophonia, but there are clinical treatments available.

Roughly 5 percent of Americans have moderate or severe impairment caused by misophonia, he said, but almost everybody has the technological capability to bug others within earshot.

“Maybe we’re not more rude than ever,” he said. “We just have new ways of being rude.”

But as Mr. Edwards noted, one solution costs nothing.

“Do people not realize they can put the phone to their ear?” he said.

Explanation No. 3

It’s Something (or Someone) Else

About three decades ago, Cristina Bicchieri, a professor of philosophy and psychology at the University of Pennsylvania, was on a plane with smokers.

“I remember telling my friend, ‘Oh my god, can we tell them to stop, please?’” she said on a video call from Tuscany. “He said: ‘Cristina, we are in America. No.’”

Professor Bicchieri, the director of Penn’s Center for Social Norms and Behavioral Dynamics, said that in Italy she felt “a greater sense of kindness” rooted in a strong and enforced social contract that forbids uncivil behavior toward strangers.

Contrast that with America, she said, and its more “extreme idea of freedom from constraint.”

Money may have something to do with it, too. Most people would never listen to a phone at a Broadway show.

“In a theater, you pay for the fruition of something, and somebody would be impinging on that,” Professor Bicchieri said. “That’s well understood. But on the train, you don’t pay for the fruition of quiet time.”

In Italy, she added, “the rule is, ‘Wait, you are impinging on my wanting to read quietly.’ People would understand that.”

So what to do? Ms. Meier, the etiquette expert, said compassionate negotiation might work.

“If I’m making something a big deal by embarrassing someone, that person may become instantly embarrassed and offended,” she said. “Maybe they were unaware and they will apologize.”

If that doesn’t do the trick, bring your kid. Or a doll.

“You could always say, ‘My baby is sleeping,” Ms. Meier added. “Do you by chance have headphones?”