A number of aircraft noise models have been developed over the past several decades to estimate noise levels and assess the potential for community and environmental impacts from current and proposed flight operations near airbases, along training routes, and within special use airspaces. Classic Department of Defense (DoD) noise models are based on NOISEMAP technology, using linear acoustics and an integrated formulation. They use a common source noise database, NoiseFile, which is maintained by the Air Force Research Laboratory (AFRL), and contain basic assumptions which simplify their computational requirements while maintaining appropriate accuracy.
The acoustic environments in the vicinity of newer aircraft such as the F-35, F-22, and the F/A-18E/F differ from those of most prior aircraft, with high noise levels associated with higher thrust engines. At those high levels, acoustic propagation cannot be modeled using the same simple linear theories employed in the classic noise models. Furthermore, the F-22 has a rectangular exhaust geometry which changes the sound radiation patterns. Both the F-35 and the F-22 employ engine thrust vectoring which cannot be easily incorporated into classic models. Little reliable data had existed on the noise produced by such jets in the thrust vectoring mode. Moreover, the segmented flight path modeling approach typical of integrated noise models do not properly account for the complex operational and noise characteristics of the new aircraft.
New models, which take advantage of today’s computer computational capabilities, were needed to provide legally defensible noise assessments of current and future aircraft operations in protecting bases and airspace for training purposes, and minimizing restrictions based on noise. The objective of this project was to provide environmental specialists with tools, based on the latest technology, for assessing and mitigating the noise impact around bases and on ranges of the new generation of fighter aircraft operating under all possible weather and terrain conditions.
Laboratory experiments provided detailed information on the complex near-field characteristics of jet noise for a range of operating conditions which cannot be efficiently measured with real aircraft engines in the field. Field tests were conducted to measure noise from a carefully controlled sound source. The data from these were compared to propagation predictions of the candidate algorithms. The algorithms that provided the best agreement with the field data were then combined with the source data. The overall model was first compared with field measurements of noise from a stationary jet engine on the ground, and second with noise measured from aircraft overflights. Model scale experiments were performed to cover the required wide range of jet geometry and operating conditions. In addition, a complementary set of numerical simulations was performed to complete such components of the noise source database. These were validated by the available experimental flow measurements.
Once the characteristics of the noise and its propagation are understood, simulation modeling is necessary to properly represent noise, and as a platform which can accommodate realistic propagation models. The NoiseMap SIMulation model (NMSIM) had been developed for fixed wing aircraft and the Rotorcraft Noise Model (RNM) for rotorcraft. These two models are operational and have been validated for their current applications. Both NMSIM and RNM have the basic capability for treating an aircraft as a three dimensional (3-D) source moving along a flight track. Noise spectra are calculated at the ground, so that any desired noise metric(s) can be obtained from the outputs. Both models account for geometrical spreading of noise, molecular absorption of sound according to current standards, ground attenuation according to current impedance interface methods and non-flat terrain and varying ground impedance. It was decided to build a new model based on RNM that included NMSIM's graphics and metric capabilities. The new model's integrated outputs must be fully compatible with the NOISEMAP suite of programs, and in the long run transition to the new model must be virtually seamless as 3-D noise sources are developed for legacy aircraft.
A new aircraft noise model, the Advanced Acoustic Model (AAM), has been developed for the assessment of noise from military aircraft operations. It is a time simulation model that produces more physical realism and detail than traditional integrated model. It is designed to work with the existing NOISEMAP suite, so that its results can be merged with results from the existing model.
AAM includes the effects of nonlinear sound propagation, terrain, atmospheric gradients and 3-D aircraft noise sources. A new algorithm for nonlinear propagation of aircraft noise was developed and validated. Laboratory and numerical studies were performed to define the noise characteristics of the new generation of high performance military aircraft, including the effects of high thrust, non-round nozzles, dual engine configurations, and thrust vectoring. A protocol for in-flight noise measurements was developed and verified by flight test.
The information developed under this study represents a significant advance in the understanding of nonlinear propagation from high level noise sources and in the measurement of aircraft source noise levels. This will allow DoD to more accurately estimate the noise environment from aircraft operations and provide a scientific foundation for installation commanders in responding to criticisms from knowledgeable citizens on the appropriateness of these estimates. The tools developed will assist DoD in being responsive to the requirements of the National Environmental Policy Act of 1969 (NEPA) while protecting operational readiness from unreasonable restrictions based on prior limited knowledge of nonlinear noise effects.