Rehabilitation of Highway 62 West, Crater Lake National Park, Klamath County, Oregon
METHODS FOR ASSESSING IMPACTS
Soundscapes and Noise Quality
The Federal Highway Administration has adopted noise abatement criteria (23 CFR 772) that establish hourly A-weighted decibel (dBA) levels for various land-use activity categories. Noise levels were considered to have an impact when they approached (within 1 decibel) or exceeded the criteria, or when the predicted noise levels substantially exceeded the existing noise levels. A substantial increase is defined as a predicted noise level of 10 to 15 dBA greater than the existing noise level. Table 5 summarizes the noise levels for various land-use categories. Due to the types of activities and serene quality of the park, Activity Category A was used with a criterion of 57 dBA Leq(h).
TABLE 5. FEDERAL HIGHWAY ADMINISTRATION NOISE ABATEMENT CRITERIA
|Activity Category||Criteria dBA L eq(h)1||Description of Activity|
|A||57 (exterior)||Lands on which serenity and quiet are of extraordinary significance and serve an important public need and where the preservation of those qualities is essential if the area is to continue to serve its intended purpose|
|B||67 (exterior)||Picnic areas, recreation areas, playgrounds, active sports areas, parks, residences, motels, hotels, schools, churches, libraries, and hospitals|
|C||72 (exterior)||Developed lands, properties, or activities not included in Categories A or B, above|
|E||52 (interior)||Residences, motels, hotels, public meeting rooms, schools, churches, libraries, hospitals, and auditoriums|
1 dBA Leq(h) = A-weighted average noise level over a 1-hour period.
Source: 23 CFR 772.
Noise levels are described by a logarithmic scale in units of decibels. The human ear perceives noises of different frequencies in different ways. The dBA approximates human perception of the overall noise spectrum and is, therefore, used in most noise studies. Small changes in noise levels of 3 dBA or less are not noticeable by the average person. Because the dBA scale is logarithmic, a 10 dBA increase in noise level is generally perceived as a doubling of the sound.
The only known noise receptors in the project study area would be potential hikers along the Pacific Crest Trail (which crosses the roadway near the east terminus of the project), Annie Creek Canyon Trail (located 1,320 feet (402 meters) from the east terminus of the project), and a northern spotted owl activity center near the west boundary (located 2,952 feet (900 meters) from the roadway). Park staff presumes that the wilderness area around the project corridor is not heavily used by visitors (Mac Brock pers. comm. 2002). For the purposes of this study, reference locations of 50 and 100 feet from the roadway centerline were used in estimating noise levels.
Construction projects would be accomplished under the implementation of either alternative B or C. Equipment and vehicles involved in milling the pavement, road base preparation, paving, and finishing activities would generate the primary source of noise. Construction noise would be intermittent and short term in duration.
- Negligible – an action that could change the ambient noise environment, but the change would be slight and result in an increase of 3 dBA or less.
- Minor – an action that would result in readily apparent changes in the noise environment with an increase or decrease of 4 to 9 dBA and affect few sensitive receptors.
- Moderate – an action that would result in readily apparent changes in the noise environment with an increase or decrease of 10 or higher dBA and affect few sensitive receptors.
- Major – an action that would result in readily apparent changes in the noise environment with an increase or decrease of 10 or higher dBA and affects most of the sensitive receptors.