Micah Downing

Acoustical Nonlinearities in Aircraft Flyover Data

Numerous analyses techniques have been proposed as means of characterizing acoustical nonlinearities in high-thrust engine noise. These include probability distributions for the pressure and the time derivative of the pressure (i.e., the gradient), the skewness and kurtosis coefficients of the pressure and its time derivative, and Howell-Morfey nonlinear indicators. In this paper, a number of these analyses techniques are applied to acoustic data recorded during a series of military jet flyovers. The analysis examines these different measures as a function of microphone height above the ground. This analysis provides strong indications that microphone should be mounted well above the ground to properly measure nonlinearities in high-thrust engine noise.

Metrics That Characterize Nonlinearity in Jet Noise

Analyses of acoustic data recorded during a series of high thrust military jet flyovers indicate that the skewness of the pressure is not well correlated with the sound pressure level, but the skewness of the gradient is. The influence of propagation distance and engine exhaust velocity on the results requires further examination. The results presented here emphasize the need for high bandwidth, high signal to noise ratio measurements. [Work supported by the Strategic Environmental Research and Development Program.]

Measurement of Near-Field and Far-Field Noise From Full Scale High Performance Jet Engines

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.Copyright

USAF single event sonic boom prediction model: PCBOOM

The United States Air Force is required to analyze the environmental impact from sonic booms produced by supersonic maneuvering aircraft. To meet this need, a single event prediction model PCBOOM3 has been developed. This model uses full ray tracing propagation theory and includes the effects of aircraft maneuvers, non‐standard atmospheric profiles, and winds. Calculations include focused superbooms when they occur. Input parameters are flexible to allow general supersonic maneuvers. The program accepts user‐defined F functions, and has F functions built in for current USAF aircraft. The model predicts the location and waveforms of sonic booms on the ground. Predicted sonic boom footprints are in the form of contours (psf, CSEL, etc.) and isopemps, or as tabular values at specific points. Signature output is provided in either graphical or tabular format for arbitrary points within the footprint. This program will enhance the ability to predict sonic booms for operational planning or for incident investigation. It will also be used as a research tool to investigate sonic boom generation and propagation. [Work supported by USAF AL/OEBN.]

Do recent findings on jet noise answer aspects of the Schultz curve

Recent research efforts on nonlinear propagation from high performance jet aircraft have revealed an interesting challenge to predicting community response. This challenge focuses on receiver perception of these unique acoustical signals, which contain acoustical shocks that appear to increase their relative loudness and/or noisiness. This current finding suggests a need for an improved description of a receiver perception of the loudness of these signals in order to improve the assessment of noise impacts from these aircraft. Looking backwards, an interesting question emerges: did the earlier low bypass jet engines on commercial and transport aircraft also include these acoustical shocks? If they did contain these features, then the perceptual differences observed between aircraft and other transportation noise sources may be partially explained.

Aeroacoustic research complex for aircraft source noise characterization

Aircraft noise has been traditionally measured with either a few ground-based microphones or a linear groundplane 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.

Measurement of the natural soundscapes in south Florida National Parks

The National Park Service is in the process of developing noise management plans for one or more of its parks in south Florida—Everglades National Park, Biscayne National Park, and Big Cypress National Preserve. A central concept is the definition of the natural (ambient) soundscapes as a resource to be managed as authorized by the NPS Organic Act of 1916 and other relevant mandates. The key to this concept is the development of a credible and defensible description of that resource. Previous studies have involved sound level monitoring with manned observations over relatively short time periods of 1–3 h in which all natural and intruding sounds were identified. This study evaluated the use of unmanned monitors to extend the measurement of the natural soundscapes to time periods of several days duration. Results from unmanned measurements will be compared to previous manned studies and methods for quantifying the natural soundscape and the effect of intrusions will be discussed. [Work supported by The Nat...

Spectral decomposition of turbulent mixing and broadband shock-associated noise from a high-performance military aircraft

Sound from high-performance military aircraft originates primarily from the turbulent mixing noise, but at smaller inlet angles, broadband shock-associated noise (BBSAN) is present. The similarity spectra of the two components of turbulent mixing noise developed by Tam et al. [AIAA Paper 96–1716 (1996)] represent noise associated with fine and large-scale turbulent structures and provide reasonable fits for ideally expanded, supersonic jet noise. For non-ideally expanded jet flow, BBSAN contributions to the spectral shape need to be included in spectral decompositions in the sideline and forward directions. A model proposed by Tam et al. [J. Sound Vib. 140, 55–71 (1990)] and later simplified by Kuo et al. [AIAA Paper 2011–1032 (2011)] provides a spectral function that models the BBSAN spectral shape. The ability of the BBSAN and similarity spectra shapes to account for the measured spectra is evaluated for ground-based microphones that covered a spatial aperture from 35 to 152 degrees. Spectral decomposit...

Spatiotemporal analysis of high-performance military aircraft noise during ground run-up

Recent measurements of high-performance military aircraft noise have revealed that full-scale jet noise has features and structures that are still only partly understood, such as the presence of multiple acoustic radiation lobes in the aft direction at certain frequencies. Spatiotemporal analyses of a ground-based microphone array measurement of the noise from a tethered F-35 at various engine conditions are used to investigate these features of the sound field. The ground array covered an angular aperture of 35–152 degrees relative to the front of the aircraft. The large angular aperture allows for a detailed investigation of the correlation and coherence at frequencies exhibiting multi-lobe behavior. This spatiotemporal analysis yields further evidence of the characteristics of multi-lobe behavior in high-performance, full-scale jet noise. [Work supported by an Office of Naval Research grant, a USAFRL SBIR, and the F-35 JPO. Distribution A: Approved for public release; distribution unlimited. Cleared 07...

Characterizing nonlinearity in jet aircraft flyover data.

This paper examines evidence of nonlinear propagation in noise data collected during flyover measurements of three different military jet aircraft. The measure used to examine the waveform data for nonlinearity is the skewness of the time derivative of the pressure waveform during the maximum amplitude portion of the flyover. The derivative skewness has been used in the past to identify nonlinear steepening and shock formation, which causes large positive derivative values. A plot of the maximum 0.5 s equivalent level with the derivative skewness calculated from the 6 dB-down portion of the recorded waveforms shows a clear correlation between the two quantities for all three flyovers, irrespective of engine condition, altitude, or microphone location. Although the trends differ somewhat between the aircraft and the effects of propagation distance and merit further consideration, these preliminary results could point toward a simple model for establishing bounds on nonlinear propagation in flyover data.