Improving AEDT Noise Modeling of Mixed Ground Surfaces (2017)

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1-1 CHAPTER 1. INTRODUCTION This Draft Final Report for the Airport Cooperative Research Program (ACRP) project 02-52 presents the deliverables to meet the goal of improving the Federal Aviation Administration’s (FAA) modeling of aircraft noise propagation over soft, hard and mixed ground surfaces using the Aviation Environmental Design Tool (AEDT). This report addresses the final deliverables for this project: 1. A technical report documenting the entire research effort, including the research methodology, results, and a prioritized list of additional related research needs. 2. Detailed documentation of model improvements to address impedance variability of ground surfaces to improve the noise prediction accuracy of AEDT. 3. A supplement to the AEDT User Guide to help practitioners incorporate the effect of impedance variability of ground surfaces. Items 2 and 3 are included as appendices at the end of this report. The report begins with statement of the objectives of this research followed by a summary of current theory on noise propagation over ground. The ground is a flat surface referred to as a boundary condition in acoustics. To determine the effect that ground has on sound propagation over it one needs to know the ground’s resistance or impedance to sound propagation both within and above the surface. The term used throughout this report to characterize ground is its impedance which can be calculated from the ground’s physical characteristics. One such characteristic is the flow resistivity which is a measure of the ground’s resistance to the passage of sound. Hard ground, such as concrete or the surface of water, reflects sound with little absorption; thus, hard ground has a high impedance value. It is common to perceive sound propagated over water as being louder than if it were heard over land. Similarly, sound in a room can be louder next to a hard wall because of the near perfect reflection of sound at the wall. An example of soft ground would be grass-covered lawn or a sand beach. Soft ground is more porous, allowing sound to penetrate the surface as it propagates over the surface. This penetration can result in some of the sound being absorbed which results in less of it being reflected and allowed to travel to a receiver. The AEDT’s current modeling of aircraft noise propagation is based upon measurements over soft ground; whereas, most airports are surrounded by a mixture of soft and hard ground surfaces. Following a summary of the current theory of sound propagation and the characterization of ground impedance a review of current noise models is presented. The review details how noise models account for sound propagation over ground and how their methodologies might be employed to improve AEDT’s modeling of aircraft noise propagation over hard and mixed-impedance surfaces. In particular, the means by which current noise models employ theory to calculate the propagation of sound is used to determine how best to incorporate aspects of the theories they employ, while recognizing that AEDT must run efficiently with the type of information available for modeling airport noise. In addition, AEDT is an integrated noise model that calculates noise emissions from aircraft operations based on individual events, rather than by simulation of aircraft operations in time. As a result, the calculation of noise at a receiver must be studied for applicability to an aircraft’s entire event as opposed to a detailed time history of the event. The next section of the report is a summary of the data that has been gathered for this project. Details of the noise measurement data from aircraft operations at airports, highway measurements, and dedicated ground impedance measurements will include the method of measurement and the extent of information is available. Some of these data are used to validate the algorithms that result from the research performed in the validation section of the report. The data which is not directly used for this report is delivered as part of the Final Report at the end of this project in digital format for use in future investigations on aircraft noise propagation over varied terrain. The previous sections are a product of the work performed to produce the Interim Report for this project. The Panel reviewed that report and met with the Research Team to decide on the Go Forward

1-2 Plan. This detailed what and how theory would be used for the remainder of this project’s research efforts. Straight ray theory was chosen along with a selection of standard models of ground impedance to determine which combination would best meet the needs of this project. The Findings and Applications section of this report is the result of the efforts by the Research Team since meeting with The Panel. Exercise of the chosen theory for propagation and how the ground should be treated are demonstrated. A section lists the potential AEDT improvements that will result from this work. These improvements were developed with input from the users and developers of noise models in general and AEDT specifically as well as the requirements of utilizing the theory in the research presented. Developers of AEDT were tasked to ensure the feasibility of incorporating the results of this study into the program’s architecture and user interface. In addition, the problem of how to apply the chosen theory in AEDT’s calculations is addressed. This is termed ‘Integrated Modeling Issues’ because what is essentially a point-to-point theory of sound propagation must be used in AEDT’s noise engine to model a complete event along a flight track. Addressing this issue was paramount to modeling propagation over multi-impedance surfaces. At the end of the Findings and Applications section is the validation of the theory with airport noise data, where the efficacy of the theory as well as its limitations are demonstrated. The concluding section of this report outlines the findings and suggests further research. The appendices detail the format of the data set as well as provide a supplement for the AEDT User’s Guide and another section detailing how the theory improves AEDT’s calculation of sound over multi- impedance surfaces.