Sound from Explosive Chemical Reactions

Transient waves, like all other acoustic waves, will diffract around solid objects, such as measurement instrumentation. A derivation of an impulse response function on the surface of a rigid sphere, based on linear, classical scattering theory, is presented. The theoretical impulse response function is validated using an experiment with blast noise. An application of the impulse response function to a rocket noise measurement is discussed. The impulse response function shows that the presence of the rigid sphere significantly affects the measurement and estimation of rocket-noise waveforms, power spectral densities, and statistical measures. 

Three multimicrophone probe arrangements used to measure acoustic intensity are the four-microphone regular tetrahedral, the four-microphone orthogonal, and the six-microphone designs. Finite-sum and finite-difference processing methods can be used with such probes to estimate pressure and particle velocity, respectively. A numerical analysis is performed to investigate the bias inherent in each combination of probe design and processing method. Probes consisting of matched point sensor microphones both embedded and not embedded on the surface of a rigid sphere are considered. Results are given for plane wave fields in terms of root-mean-square average bias and maximum bias as a function of angle of incidence. An experimental verification of the analysis model is described. Of the combinations considered and under the stated conditions, the orthogonal probe using the origin microphone for the pressure estimate is shown to have the lowest amount of intensity magnitude bias. Lowest intensity direction bias comes from the six-microphone probe using an average of the 15 intensity components calculated using all microphone pairs. Also discussed are how multimicrophone probes can advantageously use correction factors calculated from a numerical analysis and how the results of such an analysis depend on the chosen definition of the dimensionless frequency. (C) 2014 Acoustical Society of America.

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 overall sound directivity more closely matches that estimated by convective Mach number alone. These improved, more physically-based directivity indices will aid in the accurate prediction of full-scale launch vehicle noise, which is a key factor in estimating both vibroacoustic loading and environmental noise concerns.

Development of the next-generation space flight vehicles has prompted a renewed focus on rocket sound source characterization and near-field propagation modeling. Improved measurements of the noise near the rocket plume are critical for direct determination of the noise environment. They are also crucial in providing inputs to empirical models and in validating computational aeroacoustics models. NASA's SP-8072 acoustic load prediction model (1971) is a widely used method for predicting liftoff acoustics. The method implements two Distributed Source Methods (DSM-1 and DSM-2), which predict the loading as the sum of the radiated field from each source distributed along the plume. In this paper, measurements of a static horizontal firing of an Alliant Techsystem (ATK) Orion 50S XLG are analyzed with respect to the historical data that drive the SP-8072 prediction models. Comparisons include total sound power and sound power spectrum, and the distribution of the sound power and sound power spectrum along the length of the plume. Scalar pressure measurements yield reasonable agreement between the Orion-50S XLG data and both methods for undeflected plumes in the original SP-8072. However, development of these comparisons has prompted significant questions regarding the underlying physics of the two methods.

An important consideration in characterizing noise from heated, supersonic jets is the crest factor (CF). The large CF in high-speed jet noise is the result of a positively skewed probability density function for the waveform, which translates into infrequently occurring, large-amplitude positive peak pressures. Sufficient system headroom is required in the data acquisition system to provide an accurate representation of these peak pressures and thus avoid clipping or microphone saturation/distortion. But the question remains as to the importance of capturing the single largest pressure out of potentially millions of waveform samples or if a percentile-based CF is adequate. Measurements near a static tactical aircraft reveal CF increases with engine power, with the maximum CF directed upstream of the overall sound pressure level, and a maximum CF of 20 dB at full afterburner. Second, clipping of measured waveforms at different thresholds reveals that a CF definition based on the 99.99 percentile is sufficient to represent overall and band pressure levels to within 0.1 dB and waveform and time-derivative skewnesses to within ~1%. If an estimate of the time-derivative kurtosis is needed within 1% accuracy, then the 99.999 percentile CF is required for headroom estimates.

Vortex cannons have been used by physics teachers for years, mostly to teach the continuity principle. In its simplest form, a vortex cannon is an empty coffee can with a hole cut in the bottom and the lid replaced. ¹ More elaborate models can be purchased through various scientific suppliers under names such as “Air Cannon” ² and “Airzooka.” ³ We will briefly discuss the uses of a vortex cannon in teaching and a new type of vortex cannon for teaching.