The Loudoun County Board of Supervisors decided this week to prohibit residential development in a 5,000- to 6,000-acre area northwest of Dulles International Airport because of the level of airport noise projected for the area.

The plan concurs with acceptable noise levels in residential areas recommended by the Federal Aviation Administration, the Department of Housing and Urban Development and the Environmental Protection Agency, the staff report said. The plan will be in effect for five years, according to county officials.

County staff computed the noise patterns on a Loudness Day Night (LDN) index that measured noise at the airport on a daily basis for one year and dividing that figure by 365. They called the resulting maximum allowable noise level LDN 65.

The measured noise includes sharp or abrupt sound, ongoing or chronic sounds and periods of relative silence. Night noise received double weight in the calculations, according to county planner Richard Calderon. Noise from private and corporate planes was included in the computations.

Under the new plan the county generally will not approve residential development within range of the LDN 65 noise, Calderon said. Some affected areas are parts of Sterling, Sterling Park and Broad Run Park, some land that borders the Potomac River.

The board defeated a motion by Dulles Supervisor Ann Kavanagh that would have widened the affected area by lowering the allowed noise level to LDN 60. Kavanagh said she had hoped to outline a 50-year plan because noise levels will increase as use of the airport grows.

According to Henry Mahns, Metropolitan Washington Airports planner, Dulles Airport has three runways in operation with 60,000 commercial airplanes arriving and leaving annually. That number is expected to increase to 155,000 by the year 2000. Dulles has an ultimate capacity of five runways and 430,000 annual commercial air carrier operations, Mahns said.

The vote was part of the county's Dulles North Area Management Plan, which covers the 34-square-mile area northwest of the airport. Supervisors agreed on the first portion of the plan Oct. 7 when they approved a motion to allow a housing density range from 1.6 units per acre to 4.0 units per acre in some areas.

In another action, the board established an ad hoc committee to study state and county policy on drunk driving. The move came at the request of a 20-member citizens group formed after Leesburg resident Shane Williams, 16, was killed in an alcohol-related accident July 13, 1984.

According to a county staff report, there have been five known alcohol-related highway deaths in Loudoun County this year.

The new committee will seek tougher drunk-driving laws from the General Assembly, according to Kay Williams, president of the Shane Williams Committee named for her son. The group's recommendations to the new committee will include a request to lower the Blood Alcohol Content (BAC), which determines whether someone is legally drunk, from .15 percent to .10 percent.

The supervisors appropriated $2,500 for the ad hoc committee, which they said should include one member of the board, one member of the Shane Williams Committee and one member to be appointed by the board from each of the county's eight districts.

Williams said that her committee presented a petition to the board signed by more than 1,000 people who indicated they would like to see tougher drunk-driving laws in Virginia.

The board also appropriated $20,000 from the county's emergency contingency fund to be used in a stepped up chemical spray program designed to control the rapid proliferation of gypsy moths in Loudoun County.