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Suggested Citation:"Appendix D - Computation of Uncertainty." National Academies of Sciences, Engineering, and Medicine. 2016. Evaluating Methods for Determining Interior Noise Levels Used in Airport Sound Insulation Programs. Washington, DC: The National Academies Press. doi: 10.17226/23473.
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Suggested Citation:"Appendix D - Computation of Uncertainty." National Academies of Sciences, Engineering, and Medicine. 2016. Evaluating Methods for Determining Interior Noise Levels Used in Airport Sound Insulation Programs. Washington, DC: The National Academies Press. doi: 10.17226/23473.
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Suggested Citation:"Appendix D - Computation of Uncertainty." National Academies of Sciences, Engineering, and Medicine. 2016. Evaluating Methods for Determining Interior Noise Levels Used in Airport Sound Insulation Programs. Washington, DC: The National Academies Press. doi: 10.17226/23473.
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Suggested Citation:"Appendix D - Computation of Uncertainty." National Academies of Sciences, Engineering, and Medicine. 2016. Evaluating Methods for Determining Interior Noise Levels Used in Airport Sound Insulation Programs. Washington, DC: The National Academies Press. doi: 10.17226/23473.
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Page 138
Suggested Citation:"Appendix D - Computation of Uncertainty." National Academies of Sciences, Engineering, and Medicine. 2016. Evaluating Methods for Determining Interior Noise Levels Used in Airport Sound Insulation Programs. Washington, DC: The National Academies Press. doi: 10.17226/23473.
×
Page 138
Page 139
Suggested Citation:"Appendix D - Computation of Uncertainty." National Academies of Sciences, Engineering, and Medicine. 2016. Evaluating Methods for Determining Interior Noise Levels Used in Airport Sound Insulation Programs. Washington, DC: The National Academies Press. doi: 10.17226/23473.
×
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Suggested Citation:"Appendix D - Computation of Uncertainty." National Academies of Sciences, Engineering, and Medicine. 2016. Evaluating Methods for Determining Interior Noise Levels Used in Airport Sound Insulation Programs. Washington, DC: The National Academies Press. doi: 10.17226/23473.
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D-1 A P P E N D I X D Current Loudspeaker Practice The basic uncertainty outlined in ISO 1996-2 is for a single source and single receiver mea- surement. In the study herein, this would be one aircraft flyover or one loudspeaker test. To reduce the uncertainty in the source or receiver regions, one can replicate the test or otherwise increase the quantity of data, such as by adding additional microphones or sweeping through an area. In the case of loudspeakers, one can increase the number of loudspeaker positions. For current practice, people have been using the +2-dB source (exterior) measurement position, measuring 1 m–2 m from the façade, for virtually every measurement predicated on the use of an outdoor loudspeaker. Also, many have been sweeping a large area of the wall or roof under test of the building’s surface. It appears that this sweeping is equivalent to using about three or four fixed-position microphones. However, the swept measurements can approach a distance of 30 cm (~1 foot) from the wall, which is considerably closer to the wall than the +2 dB region. A reverberant build-up of 5 dB at the wall surface and 2 dB at 1.5 m (5 ft.) from the wall is assumed. This suggests a build-up of 4.5 dB at 0.25 m (10 in.) from the wall and 4 dB at 0.5 m (1.6 ft.) from the wall, which is in agreement with existing standards. Even if one wants to get only A-weighted data, there are three criteria that must be met in order to use the +2 dB (1 m to 2 m) position. Although the two general methods (outdoor loudspeaker and flyover) each meet two of the three criteria required for use of A-weighting at the +2 dB position, neither meets all three. Thus, the use of the +2 dB position is deprecated for all TL measurements that do not utilize sweeping, The three criteria are essentially (1) the wall be big enough in extent from the point, O, where a line normal (perpendicular) to the wall goes through the point representing the microphone position. This perpendicular distance is termed d. The shorter of the distances from point O to the two vertical wall edges is distance b. The shorter of the distances from point O to the two horizontal wall edges is distance c. The requirements for this criterion are that b > 4d, and c > 2d. For most houses, the largest d can be is 0.5 m. See Figure D-1 for a graphic representation (from ISO 1996-2). The second criterion is for balance between the incident and reflected waves and essentially requires that d ≤ 0.05a, where a is the perpendicular distance from the loudspeaker to the wall. When one measures along a perpendicular to a house wall, the distance is 10 m and the corre- sponding requirement is d < 0.5m, again, purely by chance. The third criterion, that the measurement be in the +2 dB (1 m–2 m) region and not too near the wall, requires that d > 1 m, which is clearly impossible. But it is possible to add a number between 2 and 5 dB depending on the distance d. The +5 dB value is currently recommended, but the “correct value” could be 3.5 or 4.5 dB. It is not unlikely that the uncertainty to this +4 dB offset is 0.5 dB. Computation of Uncertainty

D-2 Evaluating Methods for Determining Interior Noise Levels Used in Airport Sound Insulation Programs Current Flyover Measurements For flyover measurements, each aircraft measured represents an independent trial. Especially on takeoff, each aircraft that flies by does so at a different climb profile and with some lateral dispersion. This is equivalent to having the measurement microphones move around during the measurement. All three of these positions, in many cases, will suffer from nearby reflecting structures. Because many other factors affect the received sound, +3 dB is being used for the basic location uncertainty. Although +3 dB is the basic airplane fly-by uncertainty, independent replications reduce that uncertainty. If one has n independent measurements, then the effective uncertainty for the aver- age of n samples is approximately the basic uncertainty divided by the square root of (n - 1). The problem is that it is not known a priori what size n will be for any given measurement or to what degree the samples will be independent. So, to be conservative, assume only 26 samples; exactly 26 were selected so that the square root of (n - 1) is 5, and then divide that 5 by 2 because degree of independence for the 26 samples is not known. So for current practice, the outdoor flyover measurements for a set of data have a value of 3 dB/2.5 = 1.2 dB for the location factor to the measurement uncertainty. For loudspeaker measurements that use fixed positions, these must be flush-mounted wall positions. However, there is a current practice of using a swept microphone, especially when two people replicate the basic swept-microphone measurement, each using his/her own sweep- ing technique. Figure D-1. Microphone position geometry.

Computation of Uncertainty D-3 For the loudspeaker method, the position of the loudspeaker itself becomes a source of addi- tional uncertainty. In particular, if the structure under test has insulated windows (dual glazed) of the type MAM, these may produce resonances that are dependent on the angle of incidence. This source of uncertainty is unrelated to other factors, and therefore will be added in as an independent source of uncertainty. This source of uncertainty is due to the change of TL of the window with angle of incidence, independent of ground dip (sound reflected off the ground). This factor is 1 dB when MAM windows are present and 0 when they are absent. Uncertainty Analysis The loudspeaker represents a large and somewhat distributed source as opposed to a micro- phone, which represents more nearly a one point source. For this reason, there is added uncer- tainty to the loudspeaker position. Largely, this latter position effect is seen in the variation of the well-known ground dip, which is basically a function of the loudspeaker and microphone heights, the distance from the loudspeaker to the wall, the measurement position on the wall, the acoustic impedance of the ground at the point of reflection, and the angle of reflection. From experience, the uncertainty to the ground dip is usually less than 1 dB, but the research team is using 1 dB to be conservative. In addition to location, the other sources of uncertainty are the effect of background noise, instru- ment error, and the effects that meteorological conditions have on sound propagation. Loudspeaker testing is done with a loud source and high indoor levels compared to most neighborhood and household backgrounds, so the research team assigns an uncertainty of 0.3 dB to these. The flyovers may or may not be loud compared to the background either indoors or outdoors. If consultants are careful to ferret out questionable data, and maintain a 10 dB signal-to-noise ratio, then the uncertainty is 0.5 dB. The instrument measurement uncertainty is given in ISO documents as 0.4 dB. The uncertainty due to meteorological conditions, as with most of these factors, is a function of the precise testing conducted and the precise circumstances of the test. In the analysis being discussed in this section, relative measurements are contemplated. That is, for measurements such as those for pre- and post-construction, most of the meteorological effects will cancel out because the fundamental TL may remain almost the same (at least when measured in one-third octave bands). However, background noise effects and the amplitude of the test signal may change between tests. Table D-2 provides the uncertainties for the current measurement methods for relative measure- ments both with and without windows that may exhibit MAM resonance. Best Practice For flyover measurements, the two changes the research team suggests are (1) to minimize net effects, the distance from the outdoor microphone(s) to the test room should be less than 5% of the nominal aircraft distance of closest approach and (2) heightened concern about the effects of nearby reflecting objects or structures with the rule that, when in doubt, one should use a ground- plane or surface-mounted microphone position, whichever is farther from grazing incidence. For loudspeaker measurements, fixed-position outdoor microphones should be flush-mounted on the surface of the structure under test. The number of microphones needed is dependent on the number of loudspeaker positions chosen. Measuring with a fixed microphone, the emission from two different loudspeaker locations is, for statistical purposes, equivalent to measuring one loudspeaker location at two different microphone positions. So for the acoustic measurements, a

D-4 Evaluating Methods for Determining Interior Noise Levels Used in Airport Sound Insulation Programs team will be interested in the number of loudspeaker positions times the number of microphones in ascertaining the uncertainty. For best practice, a minimum of four positions/microphones is suggested. For example, this might be accomplished with two microphone positions and two other loudspeaker positions. The number of loudspeaker positions depends on the presence or absence of windows that exhibit MAM resonance, so making additional measurements at various angles of incidence is suggested. In the absence of variation with angle of incidence, one loudspeaker position is sufficient, but with resonances, at least four different loudspeaker positions are sug- gested; and it is still ideal to have two microphone positions for each façade. Table D-2 gives the best practice uncertainties for the aircraft method and for the loudspeaker method using two outdoor fixed-position microphones and four outdoor loudspeaker positions with windows that exhibit MAM resonance. Just below these two cases are the best practice data for the loudspeaker method using the swept-microphone technique with two, 30-second sweep measurements made for each test, and with four outdoor loudspeaker positions with MAM windows on the façade. This method is included for accuracy (low uncertainty in the range from 1 to 1.25 dB) while still being fast and simple. The problem is that the measurements can be closer to the wall than 1 to 2 meters, and the sound level may be above the +2 dB level and closer to the +5 dB level. Using linear interpolation one can estimate the following: With respect to the table, a normal distribution with a mean of 0.5 m (1.6 ft.) and a standard deviation of 0.25 m (10 in.) should be close to the actual situation. This translates to a mean of 5 dB and a standard of 0.5 dB. Any systematic offset to the mean also should be less than 0.5 dB. What is needed is a study on the transition from the level on a test wall surface to its +2 dB distance. In the near future, further development of intensity in conjunction with an indoor loud- speaker is suggested because of the many potential benefits that this TL measurement method could provide, including: 1. Unaffected by weather. 2. Unaffected by ground dip, MAM resonance or any other resonance effect. 3. Because of the short distances involved, it is unaffected by air absorption, relative humidity, ground absorption, and reflections from nearby structures. 4. Because of the high levels, it is usually unaffected by other environmental noises. 5. Because the acoustic signals of interest are confined to the test room itself and to the outdoor surfaces of the test room, there are minimal outdoor effects; the only two are wind on the microphone and precipitation. 6. Can find “hot spots” (on low TL building elements showing high noise transmission) to a resolution of about 10 cm (4 in.). 7. Elimination of logistical problems outdoors like: “Where can I fit in a loudspeaker?” 8. No noise problems with neighbors. 9. Because of all the problems other methods have that this method does not have, this method is likely the most accurate method. The intensity method is listed on its own line in Table D-2. Wall Distance (m) 0 0.25 0.5 0.75 1.0 1.25 1.5 Level added (dB) – theorecal 6 5.5 5 4.5 4 3.5 3 Level added (dB) – real world 5 4.5 4 3.5 3 2.5 2 Table D-1. Reverberant build-up at façade.

Method Field Meas. Margin of Error Calculated Total Margin of Error Outdoor Measurement Factors Interior Measurement Factors Meteor ology Ground Dip MAM resonanceLocaon Ambient Instrument Locaon Ambient Instrument Exisng Pracce Aircra flyover ± 1.9 ± 1.9 1.2 0.5 0.4 0.4 0.5 0.4 1.0 0.5 Speaker outside ± 1.8 ± 1.9 1.3 0.3 0.4 0.4 0.3 0.4 0.5 0.3 1.0 Best Pracce Aircra flyover ± 1.4 0.2 0.5 0.4 0.2 0.5 0.4 1.0 Speaker outside ± 1.1 0.2 0.3 0.4 0.2 0.5 0.4 0.5 0.2 0.5 Intensity ± 0.9 0.4 0.3 0.4 0.4 0.3 0.4 Notes: 1 Flyover method: Assumes only aircra with a 10 dB or higher signal-to-noise ra‚o used. 2 Loudspeaker: Assumes only data with a 10 dB or higher signal-to-noise ra‚o used. 3 Loudspeaker Best Prac‚ce: Assumes four loudspeaker posi‚ons, two surface-mounted microphones (exterior), and interior spa‚al average/manual scan. Table D-2. Calculated measurement uncertainty.

Abbreviations and acronyms used without definitions in TRB publications: A4A Airlines for America AAAE American Association of Airport Executives AASHO American Association of State Highway Officials AASHTO American Association of State Highway and Transportation Officials ACI–NA Airports Council International–North America ACRP Airport Cooperative Research Program ADA Americans with Disabilities Act APTA American Public Transportation Association ASCE American Society of Civil Engineers ASME American Society of Mechanical Engineers ASTM American Society for Testing and Materials ATA American Trucking Associations CTAA Community Transportation Association of America CTBSSP Commercial Truck and Bus Safety Synthesis Program DHS Department of Homeland Security DOE Department of Energy EPA Environmental Protection Agency FAA Federal Aviation Administration FAST Fixing America’s Surface Transportation Act (2015) FHWA Federal Highway Administration FMCSA Federal Motor Carrier Safety Administration FRA Federal Railroad Administration FTA Federal Transit Administration HMCRP Hazardous Materials Cooperative Research Program IEEE Institute of Electrical and Electronics Engineers ISTEA Intermodal Surface Transportation Efficiency Act of 1991 ITE Institute of Transportation Engineers MAP-21 Moving Ahead for Progress in the 21st Century Act (2012) NASA National Aeronautics and Space Administration NASAO National Association of State Aviation Officials NCFRP National Cooperative Freight Research Program NCHRP National Cooperative Highway Research Program NHTSA National Highway Traffic Safety Administration NTSB National Transportation Safety Board PHMSA Pipeline and Hazardous Materials Safety Administration RITA Research and Innovative Technology Administration SAE Society of Automotive Engineers SAFETEA-LU Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users (2005) TCRP Transit Cooperative Research Program TDC Transit Development Corporation TEA-21 Transportation Equity Act for the 21st Century (1998) TRB Transportation Research Board TSA Transportation Security Administration U.S.DOT United States Department of Transportation

TRA N SPO RTATIO N RESEA RCH BO A RD 500 Fifth Street, N W W ashington, D C 20001 A D D RESS SERV ICE REQ U ESTED N O N -PR O FIT O R G . U .S. PO STA G E PA ID C O LU M B IA , M D PER M IT N O . 88 Evaluating M ethods for D eterm ining Interior N oise Levels U sed in A irport Sound Insulation Program s A CRP Report 152 TRB ISBN 978-0-309-37505-4 9 7 8 0 3 0 9 3 7 5 0 5 4 9 0 0 0 0

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TRB's Airport Cooperative Research Program (ACRP) Report 152: Evaluating Methods for Determining Interior Noise Levels Used in Airport Sound Insulation Programs provides guidance for selecting and implementing methods for measuring noise level reduction in dwellings associated with airport noise insulation programs. The report complements the results of ACRP Report 89: Guidelines for Airport Sound Insulation Programs.

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