Brenda M. Sullivan

Synthesis of Virtual Environments for Aircraft Community Noise Impact Studies

*† A new capability has been developed for the creation of virtual environments for the study of aircraft community noise. It is applicable for use with both recorded and synthesized aircraft noise. When using synthesized noise, a three-stage process is adopted involving non-real-time prediction and synthesis stages followed by a real-time rendering stage. Included in the prediction-based source noise synthesis are temporal variations associated with changes in operational state, and low frequency fluctuations that are present under all operating conditions. Included in the rendering stage are the effects of spreading loss, absolute delay, atmospheric absorption, ground reflections, and binaural filtering. Results of prediction, synthesis and rendering stages are presented.

Human response to low‐intensity sonic booms heard indoors and outdoors

A house on Edwards Air Force Base, CA, was exposed to low‐intensity N‐wave sonic booms during a 3‐week test period in June 2006. The house was instrumented to measure the booms both inside and out. F‐18 aircraft were flown to achieve a variety of boom overpressures from approximately 0.01 to 0.06 psf. During 4 test days, 77 test subjects heard the booms while seated inside and outside the house. Using the Magnitude Estimation methodology and artificial reference sounds, the subjects rated the annoyance of the booms. Since the same subjects heard similar booms both inside and outside the house, comparative ratings of indoor and outdoor annoyance were obtained. Preliminary results from this test will be presented.

Temporal Characterization of Aircraft Noise Sources

Current aircraft source noise prediction tools yield time-independent frequency spectra as functions of directivity angle. Realistic evaluation and human assessment of aircraft fly-over noise require the temporal characteristics of the noise signature. The purpose of the current study is to analyze empirical data from broadband jet and tonal fan noise sources and to provide the temporal information required for prediction-based synthesis. Noise sources included a one-tenth-scale engine exhaust nozzle and a one-fifthscale scale turbofan engine. A methodology was developed to characterize the low frequency fluctuations employing the Short Time Fourier Transform in a MATLAB ® computing environment. It was shown that a trade-off is necessary between frequency and time resolution in the acoustic spectrogram. The procedure requires careful evaluation and selection of the data analysis parameters, including the data sampling frequency, Fourier Transform window size, associated time period and frequency resolution, and time period window overlap. Low frequency fluctuations were applied to the synthesis of broadband noise with the resulting records sounding virtually indistinguishable from the measured data in initial subjective evaluations. Amplitude fluctuations of blade passage frequency (BPF) harmonics were successfully characterized for conditions equivalent to take-off and approach. Data demonstrated that the fifth harmonic of the BPF varied more in frequency than the BPF itself and exhibited larger amplitude fluctuations over the duration of the time record. Frequency fluctuations were found to be not perceptible in the current characterization of tonal components.

Measured Effects of Turbulence on the Loudness and Waveforms of Conventional and Shaped Minimized Sonic Booms

Turbulence has two distinctive effects on sonic booms: there is distortion in the form of random perturbations that appear behind the shock waves, and shock rise times are increased randomly. A first scattering theory by S.C. Crow in the late 1960s quantified the random distortions, and Crows theory was shown to agree with available flight test data. A variety of theories for the shock thickness have been presented, all supporting the role of turbulence in increasing rise time above that of a basic molecular-relaxation structure. The net effect of these phenomena on the loudness of shaped minimized booms is of significant interest. Initial analysis suggests that there would be no change to average loudness, but this had not been experimentally investigated. The January 2004 flight test of the Shaped Sonic Boom Demonstrator (SSBD), together with a reference unmodified F-5E, included a 12500- foot linear ground sensor array with 28 digitally recorded sensor sites. This data set provides an opportunity to re-test Crows theory for the post-shock perturbations, and to examine the net effect of turbulence on the loudness of shaped sonic booms.

Realism assessment of sonic boom simulators

Developments in small supersonic aircraft design are predicted to result in low-intensity sonic booms. Booms generated by current aircraft are similar to those that led to the ban on commercial supersonic fli ght over the US, so are unsuitable for parametric studies of psychoac oustic response to low-intensity booms. Therefore, simulators have be en used to study the impact of predicted low-intensity sonic booms. H owever, simulators have been criticized because, when simulating conv entional-level booms, the sounds were observed to be unrealistic by p eople experienced in listening to sonic booms. Thus, two studies were conducted to measure the perceived realism of three sonic boom simul ators. Experienced listeners rated the realism of conventional sonic boom signatures when played in these simulators. The effects on percei ved realism of factors such as duration of post-boom noise, exclusion of very low frequency components, inclusion of ground reflections, a nd type of simulator were examined. Duration of post-boom noise was f ound to have a strong effect on perceived realism, while type of simu lator had a weak effect. It was determined that post-boom noise had t o be at least 1.5 seconds long for the sound to be rated very realist ic. Loudness level did not affect realism for the range of sounds pla yed in the tests (80-93 dB ASEL).

Building structural acoustic response to aircraft sonic booms

As part of the NASA Low Boom/No Boom flight test project, a series of low‐amplitude sonic‐boom tests was completed over a 3‐week period in June of 2006. This series of flight tests was designed to evaluate indoor/outdoor human subjective response, structural acoustic building response, and the effects of atmospheric turbulence for low‐amplitude sonic booms characterized by overpressures in the nominal range of 0.1 to 0.6 pounds per square foot (psf). Low‐amplitude sonic booms were generated by F‐18 aircraft using dive trajectories to produce a range of overpressures at the Edwards Air Force Base housing area. In addition, straight and level supersonic flights were used to generate normal level (nominally 1.4 psf) sonic‐boom overpressures at the housing area. This report will describe the structural acoustic building response measurements obtained during this flight test project. A single‐family ranch‐style home was instrumented with nearly 300 microphone and accelerometer sensors to determine the incident...

Realism Assessment of Sonic Boom Simulators

The perceived realism of booms reproduced in three simulators, at NASA, Lockheed Martin, and Gulfstream Aerospace, was studied. The NASA and Lockheed facilities are loudspeaker‐driven, airtight concrete structures that enable reproduction of the entire spectrum, including the very lowest frequencies. The Gulfstream facility is enclosed within a trailer and consists of a folded horn as well as loudspeakers. This creates the sonic boom waveform as a traveling wave but is unable to reproduce the lowest frequencies. In the tests, subjects experienced in listening critically to real sonic booms heard reproductions in the simulators of sonic booms recorded outdoors. Tests comparing the NASA/Gulfstream simulators and the Lockheed/Gulfstream simulators used the same outdoor sonic boom waveforms, though each simulator uses its own preprocessing to equalize the waveforms for presentation. The tests showed that the presence of ground reflections, very low frequencies, or recording hiss were not factors in realism ra...

Measured Effects of Turbulence on the Waveforms and Loudness of Sonic Booms

The recent Shaped Sonic Boom Experiment yielded a large number of digital recordings of shaped and N‐wave sonic boom waveforms, all of which exhibited some degree of turbulent distortion. This digital data set provided an opportunity to test theoretical predictions of the shape of distortions, and to assess the effect of turbulence on the loudness of shaped booms. Distortions following the shocks were found to be virtually identical for the bow and tail shocks of each boom, confirming the usual explanation that the distortion is scattering of the shock waves and that turbulence may be considered to be frozen for the duration of a boom. RMS values of the distortions have been compared with the theory of S. C. Crow. The loudness reduction of shaped booms had been predicted by Plotkin to persist through turbulence, and this was found to be the case. The loudness calculations also confirmed that the benefit of reduced shock amplitude is nonlinear: there are benefits from both the reduced amplitude and from th...

Research On Subjective Response To Simulated Sonic Booms At NASA Langley Research Center

Over the past 15 years, NASA Langley Research Center has conducted many tests investigating subjective response to simulated sonic booms. Most tests have used the Sonic Boom Booth, an airtight concrete booth fitted with loudspeakers that play synthesized sonic booms pre‐processed to compensate for the response of the booth/loudspeaker system. Tests using the Booth have included investigations of shaped booms, booms with simulated ground reflections, recorded booms, outdoor and indoor booms, booms with differing loudness for bow and tail shocks, and comparisons of aircraft flyover recordings with sonic booms. Another study used loudspeakers placed inside people’s houses, so that they could experience the booms while in their own homes. This study investigated the reactions of people to different numbers of booms heard within a 24‐hour period. The most recent Booth test used predicted boom shapes from candidate low‐boom aircraft. At present, a test to compare the Booth with boom simulators constructed by Gu...

Further developments in aircraft flyover noise synthesis and propagation

Subjective assessments of the noise from aircraft flight operations require time histories of acoustic pressure at listener positions. Synthesized sound has an advantage over recordings by allowing the examination of proposed aircraft, flight procedures, and other conditions or configurations for which recordings are unavailable. A two‐stage process for synthesizing flyover noise at listener positions on the ground was previously developed, enabling the creation of an immersive test environment. The first stage entails synthesizing time histories at the flying source. Rizzi and Sullivan [J. Acoust. Soc. Am. 113, 2245 (2003); 114, 23 (2003)] presented an approach for synthesizing sound from broadband sources (e.g., jet noise) based on predicted 1/3‐octave band source spectra, with the inclusion of temporal fluctuations based on empirical data. Reported here are further developments in the synthesis of tone‐dominated source (e.g., fan noise). The second stage entails propagation of the synthesized sound fro...