The Fairfax County Board of supervisiors cleared the way yesterday for large-scale residential development in most of a long corridor beneath the airplane flight paths south of Dulles International Airport. The board's new policy will require builders to install acoustical insulation in thousands of new houses.

Buyers of new residences in the 6,500-acre corridor running to the Prince William County line also must be given warning statements about the airplane noise potential in the area, under the new policy.

The corridor, based on air traffic expeeted after 1995, is expected to have noise levels ranging from "generally unacceptable" to "intolerable" under U.S guidelines.

The Fairfax policy will prohibit residential construction only in the "intolerable" zone.

Thousands of homes already in the advanced planning stage are not covered by the policy, but county officials hope to expand the policy to cover this development by early next year.

Shortly after adopting the policy yesterday, the board implemented it, refusing to grant medium-density residential zoning on a 161-acre parcel in the worst noise zone. Instead, the supervisiors granted zoning for industrial uses deemed more compatible with loud noise.

Hitherto, the board has had what county officials admit is a contradictory policy on residential development in the Dulles noise corridor.

In 1972, the board adopted guidelines banning residential development from areas where the noise was considered "generally unacceptable."

In the following years, plans for residential construction around the airport increased and it also became clear that the noise corridor would expand in the future, subjecting to high noise levels.

Yesterday's action came after county planner John Price told the board the new policy would permit construction of single-family houses in an area where the noise "would begin to have a significant detrimental impact on individuals."

In a interview later, Price said that requiring installation of acoustical insulation would "improve the environment to the point where residential development can be permitted."

The vote for the new policy was 8 to 1. with Supervisior Audrey Moore (D-Annandale) the lone dissenter.

Moore said the board was retreating from the position it took in 1972 and reaffirmed last year that single-family homes should be banned from areas where there is loud aircraft noise.

Under U.S. guidelines, noise levels are considered "generally unacceptable" if they rate a "noise exposure forecast" - called a NEF - of 30 or above. An NEF rating is generated by a computer and takes into account the type of aircraft, the amount of traffic mix and the sound that places produce, particularly when at full throttle, as on take-off.

The Fairfax policy will allow construction of thousands of homes meeting certain acoustical standards in areas where the NEF rating is 30 to 35. It also permits construction in noisier areas - with NEF ratings of 35 to 40 - if there are "extenuating circumstances." Some construction might approved in these areas if it were found compatible with a subdivision that was already there.