LONDON – Pity the poor schoolchildren, at least when the jets scream overhead, which they do about once a minute.

So Heathrow Airport will spend about $2.9 million to build "super adobe" domes, designed for earthquake zones in Asia and Africa, at 21 British schools to protect children from the noise of the third-busiest airport in the world.

Four of the domes, consisting of plaster walls and coiled bags of earth, are already in use at the Hounslow Heath Infant and Nursery School, where airplanes pass less than 200 yards overhead and in busy times, arrive or leave every 60 seconds, ­crisscrossing the sky.

The domes look like hobbit homes, but the school's head teacher, Kathryn Harper-Quinn, said they make it much easier for students to concentrate, especially for those whose first ­language is not English.

"You need to listen to the teacher, but as you need to refocus each time after a plane flies over, you start losing energy," she said.

With the domes, said Caroline Macgill, another teacher, the school is able to hold twice as many outdoor classes and is seeing better results from students — 580 in total, ages 3 to 7.

The domes, which cut the noise by 19 decibels, are just the latest example of the absurdities and complexities surrounding the long debate about the expansion of Heathrow, which desperately needs a third runway to meet increasing demand and to keep London competitive with such other European hubs — or so says Heathrow Airport Holdings, the owner.

But there is enormous resistance from the suburban boroughs that have been built up since the airport was first established in 1929 in the fields and orchards of the hamlet of ­Heathrow. The argument is over noise, to be sure, but also over safety, environmental damage, carbon footprints and the right of the government to seize the land that would be necessary for another runway.

The borough of Hounslow, itself, is opposed to a third ­runway, citing noise and environmental concerns. Still, with so many residents working at Heathrow, no one here wants the airport to shut down, either.

The fight over Heathrow has stymied successive governments. Yet another commission is scheduled to emerge by year's end with an "interim report" ordered up by the current coalition government, which halted Heathrow expansion plans originally approved in 2010.

The most recent alternative, by the mayor of London, Boris Johnson, would close Heathrow, turning the site into another suburb for up to 300,000 houses and call for a large new airport with multiple runways on recovered land in the Thames estuary, about 50 miles east of central London. Not only could more aircraft land there, they could do so 24 hours a day.

Gatwick, which is 30 miles south of London and wants a second runway, is about half as busy as Heathrow — which this year will handle about 71.6 million passengers, 31.8 percent of the passengers at all British airports. But there are significant problems involved in expanding any of the airports close to London.

While an estuary airport seems attractive, the problem is, as ever, money. The cost of this latest proposal is estimated about $77.6 billion. Estimates for a third runway at Heathrow are $22.9 billion to $29.5 billion.

By the end of this year, the commission is supposed to give its recommendations for immediate improvements of existing infrastructure, and by mid 2015 it is to devise its longer-term recommendations — safely after the next general election.