The United Airlines site, for example, says, "As a courtesy to other travellers, please use headphones when listening to any device."
Courtesy to other travellers, however, seems to be in short supply.
As cabins become more crowded and seats shrink, passengers are complaining about the noise. A 2015 survey on airline etiquette by GfK Global, a market research firm, found that half of all airline passengers were annoyed by "audio-insensitive" seatmates who talked loudly, engaged in noisy video games or played music - roughly the same as the previous year.
Anecdotal evidence suggests the problem is getting worse, and solutions are scarce and not always effective.
You can try asking a flight attendant for help. That's what Jarrett Sorko did on a recent flight after his seatmate engaged in a video game and chat session on his laptop. He discreetly excused himself and asked a flight attendant if she could have a word with the unruly gamer. The crew member complied.
"This seemed to work for a short time, but within 30 minutes, he was back at it again," says Sorko, who works for an Internet company in Mountain View, California. "Talking to his friend as if this guy was in the seat next to him. I became incredibly frustrated. I was forced to put up with this for the remainder of the flight."
You can also ask the passenger directly. Kerwin McKenzie, a former airline employee, has seen this happen often, and it usually works. "Most people don't like confrontation," he says. Often, they'll ask a flight attendant to remind a fellow passenger to use a headset, but just as often, a passenger will politely ask another passenger.
Benét Wilson, an aviation journalist, found herself in a similar situation on her way from Washington to Barcelona last month: seated near a mother whose child was playing a loud computer game with no headphones.
"I politely asked her to have him turn it down," she says. At first the child refused, but with some encouragement from other passengers and his mother, finally agreed. There was just one problem: He didn't have a headset to use.
"Long story short, I gave him an extra pair I always keep for situations like this, and all was well," Wilson says.
All of which brings us to the topic of your own headset. You should have one, and not just any headset, but a pair of reliable noise-canceling headphones, which can also cut the hum of the aircraft engines. Hogni Kamban, a headphone reviewer for the site Picky Ear, says these headphones are perhaps the biggest development in noise management for airline passengers. They're more effective and affordable than ever.
"By blocking out a large part of the cabin noise, they can actually be used without listening to volumes that are dangerously high," he says.
Bottom line: If you're listening to anything that a seatmate might not want to hear, use a headset. Jodi Smith says it's the right thing to do, and she should know: She's an etiquette consultant from Marblehead, Massachusetts.
"Be sure your actions do not negatively impact those around you when you're on a plane," she says. "This means wearing headphones when listening to music, viewing a movie or watching a show. This also means you should restrain yourself from singing, humming or whistling along to music only you can hear. And the volume should be low enough that your seatmates can't hear the words, dialogue or beat."
But could airlines require headset usage? Sure, but such a rule would be as difficult to enforce as a seat-belt requirement. Although it might make sense for safety reasons, and while the cabin crew can strongly suggest you keep your device plugged in, there might not be enough flight attendants on board to turn the crew into a headset patrol. It's better for passengers to do the right thing of their own free will - and maybe with a nudge or two from their fellow travellers.
• Christopher Elliott is a consumer advocate, journalist and co-founder of the advocacy group Travelers United.
