BYU ARG Gets Gatlinged!

For an adequate understanding of the broadband acoustic phenomena generated by a rocket exhaust jet impinging on a flame deflector, this study experimentally clarifies the factors that cause the difference in the broadband acoustic field of a supersonic ideally expanded jet impinging on an inclined flat plate for various nozzle–plate distances. According to previous studies, there are two possible factors: the Mach waves, which are radiated from the free-jet region and reflected by the plate, and the acoustic waves generated in the impingement region. To distinguish the effects of these factors, this study compares the following three results: the overall sound pressure level distribution, images extracted from the schlieren visualization movies using acoustic-triggered conditional sampling, and tracing lines of the acoustic intensity vectors of the Mach waves. The results reveal that the nozzle–plate distance affects the fraction of the Mach waves that are generated in the free-jet region and reflected by the plate, resulting in a higher overall sound pressure level in the upstream direction for larger nozzle–plate distances. It is concluded that the location of the plate relative to the source region of the Mach waves significantly affects the acoustic phenomena, owing to the variation in the nozzle–plate distances.


Read More: https://arc.aiaa.org/doi/abs/10.2514/1.J056504

Acoustic intensity measurements made with multi-microphone probes traditionally use cross-spectral processing methods to estimate pressure and particle velocity. Bias errors become significant as the microphone separation becomes comparable with the acoustic wavelength. However, it has been shown that the phase and gradient estimator (PAGE) method increases probe bandwidth without modifying microphone spacing [Thomas et al. J. Acoust. Soc. Am. 137, 3366-3376 (2015)]. In this study, acoustic intensity is estimated by both the PAGE method and the traditional method across two three-dimensional (3D) intensity probes and three 2D intensity probes. Probe performance is compared in the far field of a broadband noise-radiating loudspeaker located in an anechoic chamber. The results show increased frequency bandwidth using the PAGE method across all probe designs. For 3D probes, intensity level errors were least with a spherical probe. For the 2D probes, the accuracy of intensity level and direction estimates increased with the separation distance of the microphones.

Voice quality changes with age. In many cases, these voice changes result in a lower quality of life. Because one way of identifying these voice quality changes is through perceptually estimating talker age, correlations made between estimated talker age and acoustic analysis can provide insight to the possible physiological degeneration related to vocal function. While most perceptual studies investigating estimated talker age are cross-sectional, a longitudinal study of single speakers could provide additional details in the progressive degeneration of the voice quality. Nevertheless, one limitation of these studies is that perceptual ratings of voice quality or talker age in a longitudinal study could be biased by recording quality. Further, the spectral qualities of recordings from earlier decades are limited by the technology used. In this paper, a spectral-normalization filter was developed and applied to a corpus of recordings from an individual spanning about 50 years (1959 - 2007) to reduce this impact of these limitations. The filter was shown to be effective in normalizing the autospectra of the recordings and the fundamental frequency was unaffected by the filter. Preliminary subjective analysis suggests that the recording quality of all the files were perceptually similar.

In acoustic intensity estimation, adding a microphone at the probe center removes errors associated with pressure averaging. Analytical bias errors are presented for a one-dimensional, three-microphone probe for active intensity, reactive intensity, and specific acoustic impedance in a monopole field. Traditional estimation is compared with the Phase and Amplitude Gradient Estimator (PAGE) method; the PAGE method shows an increased bandwidth for all three quantities. The two- and three-microphone methods are compared experimentally, showing reduced bias errors with three-microphone PAGE for active and reactive intensity, whereas using two microphones is preferred for specific acoustic impedance.

The five-segment Space Launch System solid rocket motor was recently tested at Orbital ATK. Far-field acoustical measurements were performed at angles between 80° and 120° relative to the rocket exhaust at a distance of roughly 2500 m from the rocket, approximately 800 nozzle diameters. The angular aperture allows for evaluating spatial variation in acoustic properties and a comparison with similar tests in the past, including the 2015 test of the same rocket motor. Although terrain variations introduce uncertainty, an approximate 10 dB change in level is seen throughout the aperture, consistent with previous studies. In addition, at low frequencies a high degree of correlation is seen. Near the peak radiation direction high levels of derivative skewness indicate significant shock content and crackle. This dataset also presents the opportunity to test a new method for processing acoustic vector intensity. [Thomas et al., JASA 137, 3366-3376 (2015)] Comparison with the traditional method shows an increase in usable bandwidth of more than an order of magnitude.