Prediction of jet mixing noise with Lighthill's Acoustic Analogy and geometrical acoustics.

A computational aeroacoustics prediction tool based on the application of Lighthill's theory is presented to compute noise from subsonic turbulent jets. The sources of sound are modeled by expressing Lighthill's source term as two-point correlations of the velocity fluctuations and the sound refraction effects are taken into account by a ray tracing methodology. Both the source and refraction models use the flow information collected from a solution of the Reynolds Averaged Navier-Stokes equations with a standard k-epsilon turbulence model. By adopting the ray tracing method to compute the refraction effects a high-frequency approximation is implied, while no assumption about the mean flow is needed, enabling the application of the method to jet noise problems with inherently three-dimensional propagation effects. Predictions show good agreement with narrowband measurements for the overall sound pressure levels and spectrum shape in polar angles between 60° and 110° for isothermal and hot jets with acoustic Mach number ranging from 0.5 to 1.0. The method presented herein can be applied as a relatively low cost and robust engineering tool for industrial optimization purposes.

A model for the rapid assessment of the impact of aviation noise near airports.

This paper introduces a simplified model [Rapid Aviation Noise Evaluator (RANE)] for the calculation of aviation noise within the context of multi-disciplinary strategic environmental assessment where input data are both limited and constrained by compatibility requirements against other disciplines. RANE relies upon the concept of noise cylinders around defined flight-tracks with the Noise Radius determined from publicly available Noise-Power-Distance curves rather than the computationally intensive multiple point-to-point grid calculation with subsequent ISO-contour interpolation methods adopted in the FAA's Integrated Noise Model (INM) and similar models. Preliminary results indicate that for simple single runway scenarios, changes in airport noise contour areas can be estimated with minimal uncertainty compared against grid-point calculation methods such as INM. In situations where such outputs are all that is required for preliminary strategic environmental assessment, there are considerable benefits in reduced input data and computation requirements. Further development of the noise-cylinder-based model (such as the incorporation of lateral attenuation, engine-installation-effects or horizontal track dispersion via the assumption of more complex noise surfaces formed around the flight-track) will allow for more complex assessment to be carried out. RANE is intended to be incorporated into technology evaluators for the noise impact assessment of novel aircraft concepts.

Subjective dominance as a basis for selecting frequency weightings.

The objective of this paper is to propose and illustrate a simple approach for the selection of frequency weightings for the assessment of environmental and transportation noise. In recent years, the A-frequency weighting has become almost universal except where existing standards and regulations mandate the use of alternative weightings and/or frequency summation procedures, but even where this has been based on extensive research, no real consensus has been achieved. The proposed approach is based on the concept of subjective dominance, which does not always conform to the physically dominant frequencies identified by the A- or other frequency weightings and summation procedures used in measurements and/or predictions. The proposed approach is illustrated by the results of a limited series of five listening tests that clearly demonstrate that no single objective frequency weighting or summation procedure is capable of providing the best-fit to subjective responses across a range of different contexts. Subjective dominance varies across different listening contexts and situations, and should, therefore, be considered whenever noise management and control decisions are being made. The proposed approach will naturally require further research because of the wide range of different contexts and situations in which it might need to be applied.