Sally A. McInerny

High-intensity rocket noise: Nonlinear propagation, atmospheric absorption, and characterization

Analyses of rocket noise data measured at far-field locations during the launch of a large rocket and a smaller rocket are presented. Weak shocks are present in all of the data sets. In order to characterize these shocks, those segments of the waveforms where the acoustic pressure is increasing are isolated and the rate of increase in pressure plotted versus magnitude of pressure rise. The plots follow a trend consistent with random noise at low values of pressure rise, then transition to the pressure-squared dependence expected for weak shocks at higher pressure rise values. Power spectral densities of the noise during the period of maximum overall sound-pressure levels display high- and low-frequency spectral slopes that are close to those predicted for shock-dominated noise. It is concluded that shocks must be included in propagation models if high frequency levels are to be estimated as a function of distance from the source. Initial shock thicknesses will have to be characterized experimentally and will require instrumentation with a bandwidth well in excess of 20 kHz. Reflection-free data are essential if meaningful assessments of the statistical properties of the noise are to be made.Analyses of rocket noise data measured at far-field locations during the launch of a large rocket and a smaller rocket are presented. Weak shocks are present in all of the data sets. In order to characterize these shocks, those segments of the waveforms where the acoustic pressure is increasing are isolated and the rate of increase in pressure plotted versus magnitude of pressure rise. The plots follow a trend consistent with random noise at low values of pressure rise, then transition to the pressure-squared dependence expected for weak shocks at higher pressure rise values. Power spectral densities of the noise during the period of maximum overall sound-pressure levels display high- and low-frequency spectral slopes that are close to those predicted for shock-dominated noise. It is concluded that shocks must be included in propagation models if high frequency levels are to be estimated as a function of distance from the source. Initial shock thicknesses will have to be characterized experimentally and will require instrumentation with a bandwidth well in excess of 20 kHz. Reflection-free data are essential if meaningful assessments of the statistical properties of the noise are to be made.

On autocorrelation analysis of jet noise

Meaningful use of the autocorrelation in jet noise analysis is examined. The effect of peak frequency on the autocorrelation function width is removed through a temporal scaling prior to making comparisons between measurements or drawing conclusions about source characteristics. In addition, a Hilbert transform-based autocorrelation envelope helps to define consistent characteristic time scales. Application of these processes to correlation functions based on large and fine-scale similarity spectra reveal that the large-scale noise radiation from an F-22A deviates from the similarity spectrum model.

Modification of directivity curves for a rocket noise model

The spatial extent and downstream origin of rocket noise sources can significantly impact the physical interpretation of directivity index measurements. Valuable updates to historical rocket noise directivity indices, based on recent measurements of Space Shuttle reusable solid rocket motor (RSRM) boosters have been published by Haynes and Kenny (AIAA paper 2009-3160). However, measurements at a radial distance of 80 nozzle diameters from the RSRM nozzle exit plane are insufficient to be called the far field at low frequencies and thus require modification to the apparent source origin prior to their use in the empirical sound pressure level prediction methodologies, such as described in NASA SP-8072 (1971). In this analysis, estimates of plume source sound power level as a function of distance along the plume axis are combined with frequency-dependent, far field directivity indices to predict sound pressure level as a function of angle and range. With geometric modifications in place, the predicted overa...

THE INFLUENCE OF LOW-FREQUENCY INSTRUMENTATION RESPONSE ON ROCKET NOISE METRICS

The influence of the low-frequency roll-off and phase distortion of sound level meters (SLMs) on rocket noise metrics based on measured data is studied. Specifically, during the take-off of a space launch vehicle (SLV) called the Taurus, environmental sound pressure was measured with a carrier microphone system. This system has a virtually flat frequency response at low frequencies. In this study, the low-frequency response of SLMs was simulated using high pass filters. Two sets of filters with the same response magnitude were used: with and without phase distortion. It was shown that, for SLVs with small engines (e.g., the Delta, Scout, and Taurus), the low-frequency distortion resulting from measurements with (simulated) SLMs is apparent only in sound pressure power spectral densities (PSDs) of high resolution at frequencies below 10 Hz. This is where PSD levels are already more than 10 dB below the peak levels. There is no significant effect on the skewness of the time domain data nor on the time-energ...

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.

Autocorrelation analysis of military jet aircraft noise

Jet noise research has seen increased use of autocorrelation analyses to glean physical insight about the source and its radiation properties. Length scales and other features have been identified in support of models incorporating large-scale (LSS) and fine-scale (FSS) turbulent structures. In this paper, the meaningful use of autocorrelation in jet noise analysis is further examined. A key finding is that the effect of the peak frequency on autocorrelation width needs to be removed prior to making conclusions about the relative LSS and FSS contributions. In addition, the Hilbert transform is applied to create an envelope of the autocorrelation function to more consistently define a characteristic time scale. These methods are first applied to the analytical LSS and FSS similarity spectra, previously developed by Tam et al. [AIAA 96-1716, 1996]. It is found that the envelope of the FSS similarity autocorrelation function is more similar to that of a delta function than the LSS envelope. These curves are used to more effectively quantify FSS and LSS features in noise spectra from the F-22A Raptor. [Work supported by ONR.]

Aircraft jet source noise measurements of a Lockheed Martin F‐22 fighter jet using a prototype near‐field acoustical holography measurement system.

Military jet aircraft generate high levels of noise which require innovative measurement and analysis methods to characterize the jet noise. A near‐field acoustic holography (NAH) system is being developed to provide model refinement and benchmarking, evaluate performance of noise control devices, and predict ground maintenance personnel and community noise exposure. A prototype NAH system was used to perform jet source noise measurements of an F‐22 at Holloman AFB, NM. This NAH system utilizes a patch and scan measurement approach using a 90 channel microphone array and over 50 reference microphones. The microphone array was used to make pressure measurements on a planar surface extending from the jet nozzle to 75 ft along the plume and over 6 ft in height. These measurements were made parallel either to the shear layer or to the nozzle center line of the jet plume at three different offset distances. Measurements were made every 6 in. on the surface which results in over 1800 measurements per offset dis...

Characterizing acoustic shocks in high-performance jet aircraft flyover noise

Acoustic shocks have been previously documented in high-amplitude jet noise, including both the near and far fields of military jet aircraft. However, previous investigations into the nature and formation of shocks have historically concentrated on stationary, ground run-up measurements, and previous attempts to connect full-scale ground run-up and flyover measurements have omitted the effect of nonlinear propagation. This paper shows evidence for nonlinear propagation and the presence of acoustic shocks in acoustical measurements of F-35 flyover operations. Pressure waveforms, derivatives, and statistics indicate nonlinear propagation, and the resulting shock formation is significant at high engine powers. Variations due to microphone size, microphone height, and sampling rate are considered, and recommendations for future measurements are made. Metrics indicating nonlinear propagation are shown to be influenced by changes in sampling rate and microphone size, and exhibit less variation due to microphone height.

Investigation of multi-lobed fighter jet noise sources using acoustical holography and partial field decomposition methods 1

Full-scale tactical aircraft noise exhibits multiple radiation lobes not seen in laboratoryscale jets. These lobes have different radiation directions yet appear to have similar, overlapping source regions. Near-field acoustical holography (NAH) source reconstructions, in conjunction with partial field decomposition (PFD) methods that produce physically meaningful partial fields, are used in the current work to investigate the nature of these radiation patterns. First, it is shown that the two main radiation lobes are highly incoherent, suggesting independent partial sources. Second, these lobes are isolated as mutually orthogonal partial fields. In this representation, the lobes seem to be generated by independent yet spatially coincident extended partial sources. Source comparisons are made between non-afterburner and afterburner engine powers to investigate whether afterburner combustion produces any sources that are fundamentally different from those of nonafterburner operations. The current results show no qualitative changes occur due to the addition of the afterburner thrust aside from minor variations in source distribution, level, and the nature of the overlap between the multiple lobes.

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.