Christopher M. Hobbs

The role of nonlinear effects in the propagation of noise from high-power jet aircraft

To address the question of the role of nonlinear effects in the propagation of noise radiated by high-power jet aircraft, extensive measurements were made of the F-22A Raptor during static engine run-ups. Data were acquired at low-, intermediate-, and high-thrust engine settings with microphones located 23-305 m from the aircraft along several angles. Comparisons between the results of a generalized-Burgers-equation-based nonlinear propagation model and the measurements yield favorable agreement, whereas application of a linear propagation model results in spectral predictions that are much too low at high frequencies. The results and analysis show that significant nonlinear propagation effects occur for even intermediate-thrust engine conditions and at angles well away from the peak radiation angle. This suggests that these effects are likely to be common in the propagation of noise radiated by high-power aircraft.

Measurement and Prediction of Noise Propagation from a High-Power Jet Aircraft

Static engine run-up noise measurements have been made on the F-22 Raptor at low and high power settings. At afterburner, the propagation measurements reveal significant evidence of nonlinearity in that there is much greater high-frequency energy than is predicted by linear theory. The measurements have been compared against the results of a nonlinear numerical model based on the generalized Mendousse-Burgers equation. Although the model simplifies the propagation environment in that it neglects ground effects and atmospheric variability, agreement between the measured and nonlinearly predicted spectra is quite favorable. This comparison demonstrates that nonlinear effects can play a significant role in the propagation of high-amplitude noise and that prediction of these effects is possible with this type of numerical model.

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.]

Spectral and statistical analysis of noise from reusable solid rocket motors

As part of investigations into the design of next-generation launch vehicles, near and far-field data were collected during horizontal static firings of reusable solid rocket motors. In addition to spectral analysis at individual microphone locations, the spatial and temporal variation of overall and one-third octave band pressure levels at sideline and polar arc arrays is considered. Analysis of the probability density functions reveals positively skewed pressure waveforms, but extreme skewness in the first-order estimate of the time derivative because of the presence of significant acoustic shocks. However, plume impingement is the likely cause of reduced high-frequency levels and skewness at far-downstream positions.

Measurement and prediction of nonlinearity in outdoor propagation of periodic signalsa)

Far field propagation measurements of high-amplitude periodic signals generated by the U. S. Army Research Laboratory’s Mobile Acoustic Source (MOAS) have been made. The MOAS is a large horn-coupled electropneumatic loudspeaker capable of producing sound at a few hundred hertz with a maximum overall sound pressure level of 155dBre20μPa at 1m. The possible influence of nonlinear effects have been investigated because the measurements exhibit greater sound pressure levels at high harmonics than are predicted by a linear propagation model. Between 100 and 375m, nonlinearly predicted spectra obtained via a generalized Burgers equation-based model are consistently closer to measured spectra than are linear predictions, according to calculations of mean absolute error. These comparisons strengthen the assertion that nonlinearity is, in fact, the primary cause of disagreement between the measured and linearly predicted spectra at high frequencies. Comparisons between the nonlinear model and measurements, however...

SCAMP: Focused Sonic Boom Experimental Execution and Measurement Data Acquisition

This paper documents the May 2011 experimental flight test acoustic and meteorological measurements gathered as part of the Superboom Caustic Analysis and Measurement Program (SCAMP). The SCAMP team, led by Wyle and NASA includes partners from the Boeing Company, Pennsylvania State University, Gulfstream Aerospace, Eagle Aeronautics and Central Washington University and collaborators from Cessna, Northrop-Grumman and Nagoya University. The objectives of the SCAMP research program are to validate, via flight test measurements, models for sonic boom signatures in and around focal zones, and to apply these models to predict focus booms for low-boom aircraft designs. This experiment required precision flight of an F-18B executing different maneuvers to create focused sonic booms. The experiment was designed to capture concurrent F-18B onboard flight data instrumentation, high-fidelity, ground-based and elevated acoustic data, and surface and upper air meteorological measurements. The ground-based acoustic instrumentation array was located at the Cuddeback Air-to-Ground Gunnery Range, California. Primary acoustic measurements were on-track, in a plane of symmetry corresponding to the formal theory. Off-track measurements were achieved by flying the aircraft along a path laterally displaced from the linear measurement array. Under SCAMP, a process was developed whereby a trained measurement team provided immediate feedback based on aural observations to the On-Site Test Director. In real time, desired changes of the F-18B way points and flight paths were then relayed to the pilot, thereby improving focused boom measurement captures and data outcomes. During the two weeks of flight operations, 70 sonic boom events were captured by the instrumentation systems.

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...

Waveform and Sonicboom Perception and Response (WSPR) program: Low-amplitude sonic booms over small and large communities

The Waveform and Sonicboom Perception and Response (WSPR) Program conducted in California in November 2011 was designed to test and demonstrate the applicability and effectiveness of techniques to gather data relating human subjective response to multiple low-amplitude sonic booms. It was a practice session for future wider scale testing of communities, eventually using a purpose built low-boom demonstrator aircraft. The WSPR program addressed the following: design and development of an experimental design to expose people to low-amplitude sonic booms; development and implementation of methods for collecting acoustical measures of the sonic booms in the neighborhoods where people live; design and administration of social surveys to measure peoples reactions to sonic booms; and assessment of the effectiveness of various elements of the experimental design and execution to inform future, wider-scale testing. Sonic Boom data was measured during the WSPR experiment. Sonic booms were created by planned NASA F...

Objective data collection and analysis for the waveform and sonic boom perception and response program (WSPR)

The Waveform and Sonic boom Perception and Response (WSPR) program experiment was conducted in November 2011. Low-amplitude sonic booms were created by planned NASA F-18 supersonic flights executing a unique dive maneuver. The WSPR program was designed to simultaneously collect objective sonic boom acoustic data and subjective response data from residents in the Edwards Air Force Base residential community. Sonic Boom field kits were developed for the WSPR program consisting of a digital data acquisition system with networked nodes, deployable for extended periods of time. The Sonic Boom Unattended Data Acquisition System (SBUDAS) purposely developed for sonic boom community noise testing was deployed and details of the measurement system and all aspects of the objective data collection process are described. Data analysis during testing provided vital information to the flight planners for experimental execution. This paper also explains the post-experimental analysis of the objective data achieved by cr...

Effective area of acoustic lure surveys for Mexican spotted owls (Strix occidentalis lucida)

During acoustic lure surveys for birds, topography and ambient noise are likely to be important determinants of detectability. Examinations of propagation were conducted for acoustic lures (human‐made calls) and owl responses recorded during acoustic surveys for Mexican spotted owls in the Gila National Forest (2005). Lure surveys were designed based on formal agency protocols, which assumed a 0.43‐km detection range under typical conditions. A total of 558 points was called over a heavily forested, topographically complex 20×24‐km area. Real‐time measurements of owl calls and lures were made with a calibrated recording system. Ambient noise was collected using an array of 39 Larson‐Davis 820 and 824 sound‐level meters. The NMSIM (Wyle Laboratories) single‐event propagation simulator was used to model propagation of both owl and human calls. The resulting model of survey effort was compared with a simple two‐dimensional statistical model. Probability of detecting owls did not fit the expectations of the a...