Improving AEDT Noise Modeling of Mixed Ground Surfaces (2017)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

  2-1 CHAPTER 2. PROBLEM STATEMENT OBJECTIVES The focus of this research is on improving the noise prediction accuracy of the FAA’s Aviation Environmental Design Tool (AEDT) by adding the capability to account for mixed impedance surfaces in modeling noise propagation over terrain. The impedance of a surface is a measure of the opposition of a medium to wave propagation (i.e., how reflective (hard) or absorptive (soft) a surface is) and is a major factor on the propagation of sound. At present, the propagation algorithms incorporated into AEDT’s noise computation engine only account for soft ground, as in FAA’s legacy noise model, the Integrated Noise Model (INM) (Boeker et al., 2008). The need for improvement of how INM models different ground impedances has been shown (Ahearn et al., 2012) and is relevant also to AEDT. The route to improving how AEDT’s propagation algorithms account for mixed impedance surfaces must include the modification of the standards on which they are based. The propagation algorithms in AEDT and INM have been prescribed by standards created with the help of the SAE Committee A-21 “Aircraft Noise Measurement and Aircraft Noise/Aviation Emission Modeling” (A-21) that oversees standards related to aircraft noise propagation. AEDT’s method for predicting lateral attenuation of aircraft noise is currently based on the equations in SAE’s AIR 5662 “Method for Predicting Lateral Attenuation of Airplane Noise” (2006). AIR 5662 is an improvement over the previous standard it replaced, AIR 1751 “Prediction Method for Lateral Attenuation of Airplane Noise During Takeoff and Landing” (1981), in that it allowed for modeling the engine installation effects of aircraft with wing–mounted engines differently than for aircraft with fuselage–mounted engines. Similarly, the findings of this project will need to be reviewed and approved by the A-21 committee for incorporation into an update to AIR 5662 before the improvements could be accepted into AEDT. This Final Report is designed to provide a critical review of relevant scientific and industry literature on sound propagation over mixed impedance surfaces along with a summary of methods used in current noise models. Included beyond the Working Paper created as Task #2 of this project is the validation and comparison of these methods and models with measurements found in the literature. A critical review of current theory is required in order to determine the method for studying the incorporation of the necessary algorithm for handling mixed impedance surfaces in AEDT. With the history and example of how the lateral attenuation standard has been updated, the objective of this project is to use the chosen method to find the relationships necessary to model the effects of single- (other than ‘soft’ ground) and mixed- impedance surfaces on the propagation of aircraft noise with the same rigor of peer review as past updates and can be shown to improve the prediction of aircraft noise when compared to the standards for past improvements. In conducting this study it is important to isolate the effect of ground impedance from other parameters that influence the propagation of sound. Because the algorithms for lateral attenuation of aircraft noise in SAE AIR 1751 were based on empirical evidence, the effects of the atmosphere on the propagation are included. The current algorithms include refraction and turbulence as part of the empirical data. Since the standard for lateral attenuation over soft ground is acceptable and has been validated by measurements since its creation, then the objective of this project is simplified if it only seeks to find the difference in lateral attenuation over mixed impedance surfaces as compared to uniform, soft ground surfaces. Furthermore, the application of effects in AEDT are applied to aircraft noise in a piecemeal fashion. For example, the correction for atmospheric air absorption is applied separately from any other effect of propagation. It is therefore reasonable to assume that this implies that isolating the effect of propagation over hard ground compared to soft ground using valid theory or measurements would be applicable as a discrete change to the existing calculation in AEDT.

  2-2 A consequence of this approach will be how measurement data will be used to find a relationship for propagation over mixed impedance surfaces. For example, given airport measurement data undertrack and at a laterally displaced monitor with mixed impedance ground in between, one can correct the measurements at both locations for all known effects in AEDT so that the resulting difference must be a result of the additional effect of propagation over that ground combined with the effects of refraction and turbulence. Knowing the meteorological data for the time of the operation will help discern whether refraction and turbulence are the cause of the difference in the corrected levels. Using theory to calculate what the difference should be for propagation between the two locations over soft ground and the actual ground and comparing the difference with the difference in the corrected measurements will serve as validation that the theory is correct and can be used in AEDT to compensate calculated levels for propagation over mixed impedance surfaces. Once the best model that represents the effects of mixed impedance surfaces on noise propagation has been selected, sensitivity studies will be conducted to determine the aspects of theory that are most important to the desired outcome of AEDT’s modeling of aircraft noise. In the United States the metric for evaluating noise around airports is the average Day-Night sound level (Ldn) for airport operations which is based upon A-weighted noise levels. As such, a deficiency of an acoustic theory or noise model at some frequency that is not a major contributor to the Ldn may not be important. Also important to the need for running sensitivity studies is the availability of the required data to exercise a theory or noise model. If the impedance of a surface can better be represented knowing more details about its composition than are commonly available, then it is unlikely that including such required information would be acceptable if it is found that the improvement in predicting sound levels over a simpler model of the ground was negligible as it relates to the Ldn metric. Knowing the consequences of choosing one theory or model over another is important, however. Equally important to the efficacy of a theory or model to the problem at hand is how to incorporate the findings of this project into the AEDT program. The burden on the user community and the efforts required to gather the necessary information to exercise the program to take advantage of the findings of this project will be considered. For example, if a more detailed understanding of the topography around US airports than that which is publically available would be needed to correctly run the model, then it is understood that such a requirement would be too great. The final portion and start of the incorporation into AEDT will be the modification of the standard. As stated above, before incorporation into AEDT, the SAE standard governing lateral attenuation for aircraft noise, AIR 5662, must be updated, and this involves a review of the work by a Project Working Team (PWT) of the A-21. The work presented in this paper will be reviewed by the PWT. If the findings of this project are accepted by the ACRP Panel and PWT, then a draft to update the standard can be produced.