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A Porter Airlines plane take off from Toronto's Billy Bishop Airport on Wednesday April 10, 2013.

Chris Young/The Globe and Mail

It's 4:30 on a rainy afternoon near the foot of Bathurst Street, and somewhere up above the drone of turboprop engines is getting louder. The incoming plane materializes out of the clouds, heading for Toronto's island airport. Its wheels touch the runway and a few seconds later the plane reverse-thrusts to slow itself down. At that moment the noise level suddenly peaks and, even with a stiff wind blowing the sound away from shore, it's a distraction.

"The airport provides the sound backdrop of our lives," said Kathryn Exner, who lives steps away in a co-op apartment with her family. "When I get my kids up in the morning and get them ready for school, the noise in the background is the planes. When I put them to bed at night, it's the planes."

Now Ms. Exner and her neighbours have a bigger worry. Porter Airlines, whose popularity has dramatically increased the number of passengers using the island airport, wants to alter the ban that currently keeps it from flying jets there. The company maintains that the technology behind the newly available Bombardier CS100 jet makes it no louder than the turboprops Porter now uses.

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CommunityAIR, a residents' group that opposes the plan, says the jets will further degrade the sound environment near the waterfront and hasten the airport's transition into a major transportation hub, opening the door to even louder aircraft.

Porter CEO Robert Deluce told The Globe and Mail the airline is not arguing for the jet ban's full removal, saying instead that the CS100 is "is in a class of its own and should be exempted." He added that CS100s would still have to operate within the island airport's existing noise limit, which combines the perceived loudness of an aircraft with the number of flights.

But as to the question of how much aircraft noise is too much, experts who study the phenomenon say the debate over Porter's plans is just the latest to spotlight a notoriously complex area where the physics of sound meets the politics of noise.

"It's very difficult to quantitatively evaluate human perception," said Luc Mongeau, a professor of mechanical engineering at McGill University who specializes in acoustics.

Dr. Mongeau points out that even though the sound profile of an airplane can be precisely measured, how that sound is perceived in a real-world situation depends on a range of factors, including atmospheric conditions, flight path and even the way an individual pilot handles the plane. Once the listener is brought into the equation, both the level and character of a sound, as well as its frequency of occurrence, can evoke a range of responses.

"The hard thing is to decide how to measure that cumulative experience – how does that play out over time," said Patricia Davies, an engineering professor at Purdue University who specializes in noise and noise perception.

In a letter to the Toronto Port Authority, CommunityAIR notes that Porter's turboprop planes are in violation of the tripartite agreement by exceeding allowable limits on two of three parameters that are used to assess aircraft noise.

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The port authority counters that this excess is balanced by a lower noise level on the third parameter, which means the cumulative noise profile of the planes are allowed by the international standards the agreement refers to – a position that CommunityAIR rejects.

As to what impact the CS100 will have, the authority says it will need to better understand the noise profile of the new plane, which has not yet flown, before it can comment.

Bombardier says the 110-seat CS100 will be the quietest jet on the market, with a sound footprint only a quarter as large as that of competing aircraft. In practical terms, a listener would have to be standing "within the runway area" to hear the airplane sound as loud as city traffic at close quarters – roughly 70 decibels, said Robert Dewar, vice-president and general manager of the company's C-series program.

Mr. Dewar added that Bombardier felt a strong commercial pressure to make the CS100 as quiet as possible, particularly for European markets where population density is high.

The CS100 has a relatively large wing and is powerful enough to land and take off at steeper angles than is typical for most jets. That means it can spend less time generating noise near the ground. But the CS100's most important feature, as far as sound goes, are its geared turbofan engines, built by Pratt & Whitney, which Dr. Mongeau calls a "paradigm shift" in the battle against aircraft noise.

Porter has dubbed the new planes "whisper jets", a marketing-savvy nickname that was once used by Eastern airlines to usher in the Boeing 727 back in 1964. That Porter has recycled the term speaks to the tradition of using sound as a selling point in the airline industry.

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"When jets first came out, they were tremendously loud," said Victor Sparrow, who heads Pennsylvania State University's acoustics program, the largest in the United States.

What changed was the realization that air could be directed through a fan on its way into the engine, and then a portion of it directed around rather than through the engine's core where combustion takes place. This lowers air velocity, which reduces noise dramatically. As engine design evolved, gradually more and more incoming air was diverted around the core. To accommodate the air flow, engines morphed from the narrow cylinders of early passenger jets to the bulging, but far quieter, models seen today.

Now that strategy has reached its limit. "That was what you call the low-hanging fruit," Dr. Sparrow said. "These days it's much harder to get a significant noise decrease."

The engines employed by the CS100 overcome this with a system of gears that allows the fan to rotate at a different rate than the turbine inside the core. The result translates into a big reduction in noise that Bombardier says will meet new recommendations that are expected soon from the International Civil Aviation Organization, the body that national regulators look to in setting noise limits near airports.

Such recommendations have been growing stricter, keeping pace with a growing body of research investigating the impact of noise on human health.

"Noise always causes adverse effects when it interferes with activities, such as communication, concentration, relaxation, or sleep," said Wolfgang Babisch, a noise expert with Germany's Federal Environment Agency in Berlin and a co-author of a major study of airport noise. Published in 2008, the study included 4,861 individuals living near six major European airports, and found "significant exposure-response relationships" between aircraft noise and exposure to aircraft noise and hypertension, particularly where sleep interruption was involved. Yet, Dr. Babish added, "Noise effects depend very much on the situational context."

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This dependence and the ambiguity it implies means the debate around the island airport is unlikely to end soon, whether or not jets are allowed to fly there.

"I keep telling my students, acoustics is a good area," Dr. Sparrow said. "As long as people make noise, there will always be lawsuits – and there will always be a need for consultants to try to help people solve their problems."

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