The Rotorcraft Noise Model (RNM) is a suite of computer models that predicts far-field noise for single or multiple flight vehicle operations. RNM calculates the effects of sound propagation over varying ground terrain for acoustic sources using geometrical theory of diffraction algorithms, and through a horizontally stratified atmosphere over uniform terrain with winds. RNM calculates the noise levels in the time domain and with a variety of integrated metrics at receiver positions on or above the ground at specific points of interest and over a uniform grid.

Vehicle operations are defined as either single flight tracks or as multiple flight tracks with varying vehicle types and flight profiles. Acoustic properties of the noise source(s) are defined as sound spheres and may be obtained from flight test or wind tunnel measurements, or theoretical predictions. The vehicle source characteristics may be described in any combination of broadband (in the form of one-third octave band levels), narrow band (user-defined bandwidth and spacing with arbitrary bands permitted), or pure-tone data (in the form of specific frequency sound pressure levels and phase). Vehicle source characteristics are prescribed as a function of vehicle operating state along a defined flight trajectory. This acoustic source characteristic database lookup procedure allows the RNM code to be utilized for numerous flight vehicles, such as rotorcraft, tiltrotor vehicles, and fixed-wing aircraft, as well as ground-based vehicles.

RNM is capable of presenting the time history of a noise event at a single observer position, the noise footprint on the ground at a given time, or the noise contours for many different noise metrics. RNM is also capable of output ting the results in a file format that can be imported into a Geographical Information System (GIS). The noise contours that are generated can be overlaid to scale on a background map — a utility ideal for performing noise abatement studies, evaluating noise impacts at airports and vertiports, and performing land-use noise studies. Multiple sources may be modeled, allowing for rotorcraft and tiltrotor operations. Multiple vehicles, such as those in formation flight, may also be modeled using RNM.

Additionally, RNM can be used to generate acoustic animations of operational procedures. Demonstrating the effects of atmospheric propagation in the surrounding area by means of a sound visualization video is a powerful tool for providing an inherent understanding of the vehicle source characteristics and the effect of weather and intervening terrain.

This work was done by Juliet A. Page, Clif Wilmer, and Kenneth J. Plotkin of Wyle Laboratories for Langley Research Center. This software is available for use. To request a copy, please visit https://software.nasa.gov/software/LAR-17753-1 .


NASA Tech Briefs Magazine

This article first appeared in the May, 2016 issue of NASA Tech Briefs Magazine.

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