The noises coming out of the whirring, blinking underground computer at Dulles International Airport sounded remarkably like birds. Electronics engineer Monty Montgomery turned up the volume. "Yup. Birds. They're sitting next to the mike at Station 12 near National (Airport) . . . that's on a pole in Langley Forest . . . but here comes a jet."

The birdsong, which registered 56 decibels dB(A) on the noise-monitoring computer and made the underground computer room sound pleasantly like an aviary, disappeared in a crescendo of jet blast that alternately flashed 69, 68, 70 decibels.

Montgomery turned down the volume. "That's not very loud for a jet, but it appears that it's about average for that spot (about 10 miles north of the airport). Of course it's only been in operation a few weeks and the mike isn't calibrated correctly yet," he said.

The first three of 15 noise monitors to be installed around National Airport are now operational, open microphones on top of utility poles that are recording everything from whistling wind and chirping birds - they are spiked to prevent birds from perching on and singing directly into them - to passing trucks and the overriding roar of jets.

The first was installed March 1 in Rosslyn, near Spout Run, where jets frequently exceed 90 decibels on takeoff, according to Montgomery. The Langley Forest monitor was installed just north of CIA headquarters and the third was installed last week at Marlan Forest, south of Alexandria near the George Washington Memorial Parkway. The FAA hopes all 15 monitors will be in place by the end of this month and fully operational by May 1.

Two of the 15 monitors are far from National flight paths, in Springfield and Chevy Chase. They were suggested by the Council of Governments in response to citizen complaints, FAA officials say, and may be moved to closer-in locations if aircraft noise levels there are as low as expected.

The new noise monitors are providing the Federal Aviation Administration with the first live recordings of what it's like to live along the Potomac River and the path of National's 600 daily commercial jet flights.

The two- and three-engine jets using National are not as loud - at least as they pass Rosslyn - as the four-engined jets using Dulles, which frequently hit 110 decibels on take-off, or the supersonic Concordes which are about twice again as loud, sometimes more than 119 decibels. The nine Dulles noise monitors have been in place for almost two years.

National is a much busier airport than Dulles and is in a more populated area, which means the sound of jets is heard more frequently by more people. ANd because the Potomac is narrow and winding, jets occasionally fly close or directly over residential areas.

How often this happens and the noise made by the planes - as measured by the monitors - will be available in monthly, or more frequent, reports that the FAA will begin publishing this summer. The format and frequency of the reports have not been determined.

The weighed decibel scale dB(A) is calculated on a logarithmic basis, with a 10-decibel increase representing a doubling of noise as perceived by the human ear. The noise level of a normal conversation is about 70 decibels, someone shouting at two feet or heavy city traffic is about 90 decibels and the loudest noise the human voice can make is about 115 decibels, comparable to listening to a trumpet three feet from your ear in a discotheque.

When the computerized noise levels of National's jets are compared with tape-recorded radar tracks of all planes coming and going at the airport, it will provide the exact course, height and identity of every plane using the airport as well as a record of just how much noise it made when passing the monitors.

This detailed flight information probably will be more crucial than the noise-monitoring alone, FAA officials say, because there are no limits or regulations on how much noise planes are allowed to make at National. But FAA regulations do require planes to "follow the river" and not fly inland over homes until having flown 10 miles north of the airport or 5 miles south - roughly beyond the Beltway.

In the future when residents of Old Town Alexandria, Rosslyn or Georgetown complain that jets are too noisy or actually are flying over their houses, the FAA noise-complaint center will be able to pinpoint where the plane flew and instantly retrieve from the computer the noise it made passing the monitors.

The present plan is that callers to the complaint centers (at both National and Dulles) will be told the noise levels recorded by the plane they're complaining about and if the plane is allegedly off-course they will be informed as soon as the FAA receives the computer information - usually the following day - where it went.