ADDRESSING THE NOISE FROM U.S.TRANSPORTATION SYSTEMS Measures and Countermeasures NICHOLAS P. MILLER ransportation systems, vital to the U.S. waterway sources are not included. Sound levels should The author is Senior Vice economy, are the predominant source of be associated with a location on the ground or with a President, Harris Miller sounds outdoors. How detrimental these distance from the source; the levels in Table 1 are asso- Miller & Hanson, sounds are to human health and well- ciated with specific distances and operating conditions. Burlington, Tbeing is a subject of continuing debate. When loud The aircraft sound levels are for distances of 1,000 Massachusetts. enough and frequent enough, however, the sounds feet, and the ground transportation sources are for 50 diminish the quality of life (see box, page 6). feet—the distances at which the source sound levels can be measured reliably. The derivation of sound Transportation Sound Levels levels at other distances requires information about Estimates of the maximum sound levels produced by factors that affect the propagation of sound—such as common transportation sources are shown in Table 1; the terrain, the location of buildings or other shield- P HOTO : L ARRY L EVINE , WMATA Metrorail and automobile traffic on Interstate 66 in suburban Washington, D.C., pass through residential neighborhoods protected TR NEWS 240 SEPTEMBER–OCTOBER 2005 TR NEWS 240 SEPTEMBER–OCTOBER from the noise by barrier 4 walls and landscaping. ing structures, the meteorological conditions, the air- TABLE 1 Approximate Maximum Sound Levels for Transportation Sources craft engine mounting, the aircraft elevation, and the direction of flight in relation to the listener. The max- Approximate imum levels at a distance of 50 feet would be at least Maximum 25 decibels (dB) lower if heard at 1,000 feet. Estimated Speed/ A-Weighted Sound Distance, The levels for the surface transportation sources are Source Type Operating Condition Level, dB(A) feet from federal agency models. The aircraft levels derive Aircraft from the Federal Aviation Administration’s (FAA’s) Commercial jet Takeoff 85 1,000 aircraft noise model data and from field measure- Commercial jet High Altitude Cruise 85 1,000 ments. The sound levels should be considered typical Corporate jet Takeoff 85 1,000 or average; actual levels will vary above and below Propeller aircraft Takeoff 70–80 1,000 those indicated in the table. Helicopter Cruise 70 1,000 No Escape Roadway Vehicles Heavy truck 50 mph 83 50 Stand outside almost anywhere in the continental United States, and within a short time—probably Medium truck 50 mph 79 50 less than 1 hour—the sound of a truck, automobile, Automobile 50 mph 72 50 airplane, or train will be audible. The percentage of Rail Vehicles land in each county in which the various transporta- Diesel locomotive 50 mph 88 50 tion sources are likely to be heard during the daytime Rail cars 50 mph 80 50 can be estimated from standard transportation sound Locomotive horns - 96–110 50 levels, the routes followed by each mode, and esti- dB(A) = A-weighted decibels, a summation of sound levels across frequencies. mates of the background sound levels throughout the continental United States. Figures 1, 2, and 3 show the percentages of land Aircraft Sound Metric in each county in which the sounds of roadway FAA has identified a day–night average sound level traffic, rail traffic, and high-altitude jets may be (DNL) of 65 dB from aircraft operations as the limit heard (1). of acceptability for residential housing (see Figure A, The roadway results represent the network of page 6). Federal funding is available to assist with limited access, primary, and secondary roads; local sound insulation and property acquisition in areas roads are not included. The rail results are for with noise above the acceptable level. The federal freight lines only, and the aircraft results are for government, however, does not set land use policies; high-altitude intercity jet traffic only—general avi- local authorities determine the relationship between ation operations or departures and arrivals in the land use and sound levels. FIGURE 1 Percentages of vicinity of airports are not included. The road and DNL is a measure of total sound energy in 24 county areas in which the jet results, therefore, are likely to be underestimates, hours and therefore may include many combina- sounds of road traffic are particularly for densely populated areas and for the tions of aircraft sound levels and events. Three noticeable during the day. vicinity of major airports. Aircraft Noise Aircraft noise is an issue for people who live near air- ports. Noise is the reason most often cited for public resistance to increases in runway capacity or to alter- 2005 TR NEWS 240 SEPTEMBER–OCTOBER ations in the use of airspace. Aircraft-produced sound can reach levels that inter- fere with speech outdoors in communities some dis- tance from an airport. For example, a modern commercial jet can produce sound levels of 70 dB(A)1 to 80 dB(A) up to 3 miles from a runway, loud enough to interfere with speech outdoors for about 20 to 35 seconds. A moderately busy commercial airport with 200 to 300 daily departures could interfere with speech 10 to 20 times per hour. 1 dB(A) = A-weighted decibels. See box, page 6. 5 example combinations of sound levels and opera- tions that would produce a DNL of approximately 65 dB—assuming that all operations occur between 7 a.m. and 10 p.m.—are shown in Table 2 (page 9). Levels that exceed 60 dB(A) would begin to inter- fere with normal conversations outdoors. Aircraft Noise Issues Because commercial jet aircraft departures and arrivals are the primary sources of noise exposure in the vicinity of airports, any proposal that may increase the number of flights or alter the neighbor- hoods over which the aircraft fly is likely to arouse public concern. Lengthening runways, building run- ways, or adding gates—any plan that could be per- ceived to increase the sound exposure from aircraft—can generate public resistance. FIGURE 2 Percentages of county areas in which the sounds of rail traffic are Around airports with no scheduled commercial noticeable during the day. operations, loud jet or loud propeller operations are a Introduction to Sound Metrics and Criteria ound is quantified either as a total accumulation of sound Senergy for a period of time or as a measure of a single event. Almost all environmental sound is measured in A-weighted deci- bels [dB(A) or dBA]. A-weighting is a summation of the sound levels across frequencies; this summation de-emphasizes the lev- els at different frequencies and corresponds to the way people hear. The total accumulation metrics are called equivalent levels and represent the sound levels for either 1 hour, symbolized as LAeq,H, Leq(H), or Leq—if the time period is defined—or a 24-hour period, FIGURE B A 24-hour measurement. called the day-night average sound level (DNL or Ldn). DNL includes a weighting or penalty of 10 dB for sound that occurs between 10 p.m. and 7 a.m. Single events are quantified as an accumulation of sound energy over the duration of the event, termed the sound exposure level (SEL); or as a maximum level, Lmax; or as the length of time that the sound was above a specified threshold, known as time above (TA). According to the U.S. Environmental Protection Agency (EPA), cumulative sound exposure below 55 dB DNL poses minimal risk of adverse effects on human health, as well as minimal annoyance. Figure A shows typical values of DNL for various locations and identifies the levels established by EPA, the Department of Hous- TR NEWS 240 SEPTEMBER–OCTOBER 2005 TR NEWS 240 SEPTEMBER–OCTOBER ing and Urban Development, and the Federal Aviation Adminis- 6 FIGURE A Typical values of DNL. tration in making decisions about funding assistance. source of noise exposure and, if frequent or at night, a LOYD likely cause of complaints. A few loud corporate jets a L RIKA week, for example, can raise public concern, regardless : E HOTO of DNL values. In areas with no loud jet operations, P the sound of propeller aircraft flying over quiet neigh- borhoods, if frequent enough—particularly on week- ends—may raise concern. Helicopters are not usually louder than other air- craft (see Table 1) but can be identified easily by sound, especially when “blade slap” occurs, and can cause complaints. Helicopters also travel slower than fixed-wing aircraft and can be heard for a longer time. Other aviation-related sources of sound include engine testing for maintenance or before a flight; aux- iliary power units that provide electricity while the aircraft is at a gate; taxiing aircraft; and the low-fre- quency rumble from jets at takeoff. This last source is difficult to assess and control because the A-weighted sound level does not represent low-frequency Figure C charts the sound level of a single event, showing the maximum level and the TA. Figure D compares several typical max- imum sound levels. As the summation of all the sound energy in a single event, the SEL is generally 5 to 10 dB higher than the maximum. The SEL reflects the duration of a sound and provides a more complete estimate of the disruptive or annoying quality of an event. FIGURE C Single-event time history. The federal agencies responsible for managing transportation noise have established criteria, standards, or guidelines to identify when an impact occurs and when an increase in sound is substan- tial.