A new $1 million system for monitoring noise from National and Dulles airports won't necessarily make their neighborhoods quieter, local activists are warning, and airport officials agree.

The monitoring system, one of the cornerstones of the airport authority's efforts to reduce aircraft noise, will begin operating early next year. The system will consist of 32 listening stations -- 20 near National, 12 near Dulles -- linked to a computer that will give officials information on the noise level of each arriving and departing plane.

Activists who for years have pressured the authority to cut airport noise -- particularly at National -- say they welcome the monitoring system, but say more listening stations are needed. They also question whether airport officials have a plan to use the information to get airlines to curtail noise.

"It's nice that they're putting in a new system, but I don't think anything will really change," said Annette D. Davis, who represents Montgomery County on the Council of Governments' airport noise committee.

"I think we need more noise monitors to get a fuller picture" of the effects of aircraft passing over neighborhoods, said Davis, who lives in National's flight path in the Cabin John area. "And if no one does anything with the data, this won't mean much."

Airports officials, who have tried to balance their desire to keep National busy against residents' wishes for fewer flights and less noise, acknowledge that those who expect the monitoring system to help make an immediate difference will be disappointed.

"There's no way noise monitors alone will make things quieter," said Neal Phillips, the authority's environmental manager.

Phillips said airport officials will use the system to identify airlines that are consistently louder than competitors' that fly the same type of aircraft. The authority then will encourage those airlines to change their operating methods so their planes make less noise, a tactic anti-noise activists fear will be ignored by airlines.

"People hoping that there's going to be some magic fix or that we're going to beat airlines over the head with data . . . . That's not going to happen," Phillips said. "Airlines are very conscious of their images, and I think they'll respond if it's pointed out that they're being a problem."

Each monitor, essentially a microphone and a box of computer components atop a 24-foot pole, costs about $32,000. It will cost the authority about $150,000 a year to operate the system, which will replace a 24-site system that has not worked since 1985.

Most of the old sites are being fitted with new equipment, and a few of the old monitoring stations are being moved to nearby locations considered to be better listening posts, Phillips said. Sites for eight new stations -- five near National and three near Dulles -- soon will be determined by local jurisdictions working with the Council of Governments noise committee.

Phillips said because wind direction and other urban noises can skew a monitor's reading of any one flight, readings from the monitors must be averaged to accurately estimate aircraft noise. "If a dump truck goes by one of the monitors while {a plane} is flying over, you're going to get an unusally high reading," Phillips said.

The federal government has no noise standards for airports; instead, the Federal Aviation Administration sets standards for each type of aircraft.

About 60 percent of the aircraft at National are older, noisier aircraft such as the Boeing 727. By 1998, the airport plans to restrict National to newer, quieter planes such as the Boeing 757.

Phillips and other airport officials said the switch to quieter planes will be the most significant step against noise at National. "Once we do that, it will be as quiet as it's going to be," Phillips said.

Arlington County Board member Mary Margaret Whipple, a member of the council's noise panel, said that although more monitors are needed, the new system will give local officials an idea of the noise problems residents face. "Noise monitors in and of themselves won't solve the problem," she said. " . . . But maybe we can begin to determine how bad the problem is."