THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

336 13. Criteria and Regulations for Noise Controlroads, where environmental impacts are likely to occur and federal funding is usedto finance all or part of the project. The Federal Highway Administration (FHWA;not to be confused with FHA, the Federal Housing Administration) delegates theresponsibility for preparation of environmental impact statements to the individualdepartments of transportation of the affected states. Moreover, the U.S. Departmentof Housing and Urban Development (HUD) also requires that when a developerplans residential property with the aid of construction funds or a guarantee of suchfunds from HUD, the developer must assess the potential impact of transportationnoise upon that development.Assessments of traffic noise are usually made in terms of the overall A-weightedsound levels. Octave-band and one-third octave-band levels are generally used onlyfor the purpose of developing vehicle noise abatement measures such as acousticbarriers. Traffic noise tends to vary greatly over time, so methods are required todeal with this variation and its resultant impact on people.Time PeriodThe time period used as a basis by FHWA is the 1-h period when the trafficis at its heaviest. This period is called the worst noise hour. The Departmentof Housing and Urban Development and certain states, California for example,require assessment over a 24-h period. Two types of descriptors are in general use:statistical descriptors and time-averaging descriptors.Statistical Descriptors. The 10-percentile-exceeded level L 10 encompasses theacoustic magnitude of individual traffic noise events, such as passages of heavytrucks, and also the number of such events. Originally the FHWA noise abatementcriteria specified only in terms of a 1-h L 10 . This descriptor is clearly inadequatefor situations where (1) the hourly traffic rates are low, (2) vehicles are not evenlyspaced along a road, and (3) the values of L 10 could not be combined mathematicallyon the basis of calculations for separate events.Time-Average Descriptors. Time-average descriptors are now widely used forassessing traffic. The most common such descriptor is the 1-h average sound level(abbreviated 1HL), which is essentially the A-weighted equivalent continuoussound level L eq taken over a 1-h period. This carries the advantage of (1) easycomputation through the use of integrating meters, (2) assumptions regardingvehicle spacing are rendered unnecessary, and (3) the average levels for separatecategories of sources may readily be combined. The principal disadvantage is thatit can be extremely sensitive to isolated events having a high sound level, but whichdo not necessarily provoke a correspondingly high human response.FHWA Assessment ProceduresThe Federal Highway Administration (FHWA, 1976) requires that expected trafficimpacts be determined and analyzed. Highway projects are classified byFHWA into two categories: Type I, which is a proposed federal or federal-aided