"Inside the noise model: A tool to predict airport's impact"

Monday, June 16, 2003

Inside the noise model: A tool to predict airport's impact
By Dan Wascoe Jr.
The Minneapolis (MN) Star Tribune


The Metropolitan Airports Commission (MAC) is expected to vote today on
assumptions about future changes in the aviation industry that, with the
help of a complicated bit of computer alchemy, eventually will help decide
which metro-area homes next receive airport noise insulation.

The key to unlocking noise forecasts is the Integrated Noise Model (INM),
which digests the MAC's assumptions along with huge amounts of data ranging
from noise characteristics of 237 types of planes to temperature, humidity
and local terrain.

It applies weighted values to some data -- notably, penalties for night
flights -- and uses mathematical sequences called algorithms to spit out
noise contours on a map.

"It's safe to say that in the calculation of a contour it could involve tens
of thousands of calculations," said Tom Connor, noise manager for the FAA's
Office of Environment and Energy in Washington, D.C.

The assumptions that the MAC might vote on today, such as the number of
future takeoffs and landings at Minneapolis-St. Paul International Airport
and the types of airplanes that will fly there, will help consultants
forecast future airport noise.

Those forecasts, in turn, will help the MAC decide how many thousands more
homes it needs to insulate, at a cost of perhaps hundreds of millions of
dollars.

The commission is expected to vote later this summer on the forecasts before
sending an updated noise plan to the Federal Aviation Administration (FAA).

In the end, the Airports Commission will get maps that show a range of noise
contours. It has committed to insulating houses exposed to an average of 65
decibels, and has said it also intends to insulate homes exposed to 60 to 64
decibels.

Experts say that only one other airport -- in Cleveland -- has received FAA
approval to insulate houses below the 65-decibel threshold, and even there
the FAA has approved no federal funds for such work.

That means the commission will need to consider how much of its other
resources to spend on expanding its noise-insulation program -- particularly
in the face of opposition from Northwest and other airlines.

The noise model

The guts of the noise model is a 28-year-old FAA procedure designed to
analyze noise around civilian airports. It has been updated over the
years -- the latest version is numbered 6.1 -- and is still considered the
bible of noise forecasting by experts such as Connor.

But even bibles are open to interpretation.

Jim Spensley, an electronics engineer who heads an anti-noise organization
called the South Metro Airport Action Council, said the FAA model is "quite
insensitive" to the total number of flights.

Spensley, a persistent MAC critic, also said that night-flight estimates
should be linked to hourly temperature readings, instead of annual averages,
because noise is more noticeable in cold weather. Such links would expand
the noise contour and make more homes potentially eligible for insulation,
he said.

Connor does not claim that the model is perfect. He said it does not
adequately calculate how temperature and wind affect the spread of sound
waves, or account for how tall buildings or hills might temper jet noise.

But Connor said the model's equations have been refined by engineers for
nearly three decades. Another strength, he said, is its database of noise
generated by different kinds of planes under different conditions.

Whatever the model's strengths and flaws, Spensley said commission members
are not delving into how it will be applied. He also questioned whether
assumptions of future takeoffs and landings are sound, especially during
peak periods.

A new noise oversight committee of community and aviation representatives
has been briefed about the forecasts but has not yet offered its views to
the commission.

Connor said that while the FAA does ascertain that all affected parties have
helped prepare the plan, the agency does not check the accuracy or
reasonableness of assumptions used by an airport agency or its consultants,
"not beyond confirming that they used official sources such as operating and
flight manuals."

"We only get involved on the modeling side," he said -- the proper use of
the computer program that generates the forecasts.

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