Sound from Explosive Chemical Reactions

Time-domain metrics are used to investigate the nonlinearity of the sound in the vicinity of the F-35 AA-1. The first measure considered is the average steepening factor (ASF), which we define as the inverse of the wave steepening factor and is a ratio of the expectation value of the positive slopes in the waveform to the expectation value of the negative slopes. The second nonlinearity metric is the skewness of the time derivative of the pressure waveform (derivative skewness), which describes the asymmetry of the distribution of slopes in the waveform. Spatial maps of both metrics applied to the F-35 AA-1 data reveal that regions of increasing derivative skewness correspond more closely to the maximum sound radiation area, whereas the largest values for the ASF seem aligned with the regions where the waveform amplitude distributions are most asymmetric. It is proposed that these two metrics reveal different characteristics of the nonlinear propagation of jet noise. The ASF is more representative of the average slopes, which are dominated by high frequencies. Conversely, the derivative skewness identifies of large positive slopes and hence relates to the shock content in the noise.

Beamforming techniques for aeroacoustics applications have undergone significant advances over the past decade to account for difficulties that arise when traditional methods are applied to distributed sources such as those found in jet noise. Nevertheless, successful source reconstructions depend on array geometry and the assumed source model. The application of phased-array algorithms to ground array measurements of a full-scale tactical jet engine at military and afterburner engine conditions yield different source reconstructions. A deconvolution approach for the mapping of acoustic sources (DAMAS) is utilized to remove array effects seen in conventional beamforming and allows for improved interpretation of results. However, the distributed nature of the jet noise source, as well as large correlation lengths at low frequencies, can result in inaccurate source locations and/or amplitudes for both conventional beamforming and DAMAS. Results using DAMAS-C, an extension of DAMAS, indicate the degree of source correlation within the military aircraft noise. Source reconstructions on the jet centerline for different one-third octave band frequencies confirm the greater source correlation at low frequencies. These preliminary results represent the first implementation of DAMAS-C on full-scale jet noise data.

One of ASA's outreach activities is hands-on exploration workshops for local school groups and girl scouts troops held during our semiannual conferences. In general, outreach programs have three main purposes: 1) public service, 2) generate enthusiasm and interest, and 3) supplement learning. Despite the good work that has been done in the past, it is apparent that the goals of the current ASA outreach activities could be better achieved through increased efficiency. This project continues the development of the ASA outreach programs held in conjunction with our meetings to better meet the above goals. Specifically, we have developed a structure in which demonstrations are grouped into five-minute stations each pertaining to a different physical system. To increase the ease with which volunteers can assist with the stations, one-page summaries and reference posters outlining the basic principals of each station have been prepared. Volunteers are encouraged to be guided by the interests of the students in their interactive discussions.. In addition, we have created a brief introductory presentation for the workshops to explain what can be done in the field of acoustics. These improvements not only provide opportunities to excite student interest but also increase the efficacy of the outreach efforts.

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.