Accurate measurement of the noise fields emitted by a full scale high performance jet engine and jet plume (with supersonic jet flow) requires detailed planning and careful execution. The apparent acoustic source can be very large, more than 50 feet long and 20 feet high and wide. The jet plume contains many noise generating sources, the main two being shock (broad band and shock cells) and turbulent mixing. This paper is an initial description of a detailed method to accurately measure and describe the near-field noise while simultaneously measuring the far-field noise. For a large high performance jet engine, the acoustic far-field may not be formed until more than 1000 ft away from the plume. The paper also describes proposed methods to measure the non-linear propagation of the noise from the near-field to the far-field. The proposed methodology described with vetting will be considered as an US military standard (MILSTD) with possible later consideration as American standard measurement technique to describe noise fields for personnel noise exposure and for measuring the performance of jet engine noise reduction technologies.
Small caliber firearm (SCF) noise sources are typically impulsive in nature, possess a large amount of acoustic energy (consequently a large hearing damage risk), and are not omnidirectional. These sources are often operated in indoor shooting ranges where the potential for noise exposure risk is greater due to the reflective surfaces in the room. Indoor sound propagation models require inputs such as geometry, wall material properties, and some quantified source level description. One such room acoustic modeling technique is the Image-Source Method (ISM). ISM typically assumes specular reflections off the walls and represents those reflections as image sources. Many ISM algorithms can operate with high computational efficiency for simple omnidirectional source models, usually represented by a single quantity: sound power. However, ISM models using an omnidirectional source assumption can produce high errors in some scenarios involving highly directional sources, such as SCFs. In this work, an ISM algorithm has been modified to predict listener exposure levels from non-omnidirectional sources in complicated room designs, and has been validated against measured data from an SCF on an indoor Air Force shooting range.
UAS (unmanned aircraft systems) are used in a variety of commercial and military enterprises. In military settings, being able to identify location as well as number of enemy UAS can lead to stronger strategic positioning or appropriate countermeasures. In community settings, UAS can be a source of annoyance. Recently, AFRL measured three different UAS flown by pilots from Sinclair Community College. These UAS included various sizes and blade amounts. They were flown back and forth through an array of 20 acoustic sensors at nominal thrust. In addition to flying through the array, they were also flown at heights of 15 feet and 100 ft above the array. Using both telemetry and acoustic data, spherical source characterizations of the systems can be derived. These three-dimensional characterizations can lead to a better understanding of UAS acoustics as well as improved measurement design. This study describes the recent measurement in detail, some preliminary analysis, and the lessons learned. Future analysis and testing will promote acoustic identification of a larger variety of UAS.UAS (unmanned aircraft systems) are used in a variety of commercial and military enterprises. In military settings, being able to identify location as well as number of enemy UAS can lead to stronger strategic positioning or appropriate countermeasures. In community settings, UAS
Aircraft noise has been traditionally measured with either a few ground-based microphones or a linear ground-plane array of microphones. These techniques capture one-dimensional and/or two-dimensional characteristics of aircraft flight noise. The US Air Force Research Laboratory has started the construction of a 3-dimensional measurement facility at White Sands Missile Range in New Mexico. This facility, the Aeroacoustic Research Complex (ARC), will allow aircraft to fly through the array, collecting fully 3D acoustic data. ARC is initially being developed in two phases The first phase includes two 91.4 m tall towers separated by 244 m and will focus on noise from rotary wing and UAV aircraft. The second phase will add two 366 m tall towers separated by 610 m and will focus on large and high performance fixed wing aircraft. This facility will allow more accurate characterization of in-flight noise directivity by providing synchronized 3-dimensional magnitude & spectral acoustical signatures from 50+ microphones. ARC responds to a critical need for validation of existing predictive acoustic models. Such models are used for aircraft design, survivability, nonlinear acoustic propagation research and assessing noise exposure to residents living adjacent to airfields.
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