It's not...wait, it IS rocket science!

In an advanced acoustics laboratory course at Brigham Young University, students are introduced to ANSI measurement standards in the context of sound power. They are introduced to the anatomy of a typical acoustics standard and then plan and carry out sound power measurements of an electric leaf blower usingbothreverberationchamberandsoundintensitymethods. Thestudentsarerequiredtowriteatechnical memorandum describing a) the blower’s radiated sound power levels over an appropriate frequency range, asobtainedwiththetwomethods;b)setupdocumentationanddeviationsfromthestandards’recommended practices;andc)howanydeviationsmighthavecontributedtodiscrepanciesbetweenthesoundpowerlevels obtained with the two methods. In this paper, a description of the experience is given, along with overall impressions and plans for future improvements.

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 sound near the rocket plume are critical for direct determination of the acoustical environment both in the near and far-fields. These measurements are also crucial inputs to empirical models and utilized in validating computational aeroacoustics models. NASA’s SP-8072 acoustic load prediction model [K. M. Eldred, NASA SP-8072 (1971)] is a widely used empirical model for predicting liftoff acoustics. The model implements a Distributed Source Method, which predicts the loading as the sum of the radiated acoustic pressure field from each source distributed along the plume. In this paper, the resultant sound distributions are compared among the original core length definition in SP-8072, a modification proposed by Varnier [J. Varnier, AIAA Journal, Vol. 39, No. 10, (2001)], and four full-scale solid rocket motor tests with measurements covering a spatial region of 9 to 56 nozzle diameters downstream of the nozzle. The results of the comparisons support the original Eldred core length definition on an overall level basis. However, analysis of the measured spectra unveil shortcomings within both definitions.

Crackling signals cannot be identified using any sound level or quality metric that relies solely on the long-term spectrum as input. In order to identify sound quality metrics that might succeed in modeling human perception of crackling and non-crackling sounds a set of metrics sensitive to temporal properties of signals is applied to a set of signals with equivalent spectra but exhibiting varying degrees of crackle. Several methods for altering signals including some that remove crackling sound quality from an acoustic signal were drawn from previous work [Swift, Gee, Neilsen, 2014, Swift, Gee, Neilsen, 2017]. In this paper, an additional alteration which can partially remove crackle—randomizing the Fourier phase of a crackling signal in the frequency domain in selected frequency ranges—is considered. Variables from time-varying sound quality metrics such as loudness and sharpness, as well as roughness to signals exhibiting varying degrees of crackle are explored and relationships between them that can be exploited to identify crackling sounds are identified. Time-sensitive sound quality metrics can be used to discriminate between crackling and non-crackling signals and may help inform discussions of human perception of high-performance jet noise.

The jet sound quality “crackle” has historically been studied and quantified using the statistics of the pressure waveform. Some investigators have suggested crackle, and its associated shock content, may be better quantified using the statistics of the time derivative of the waveform. Modified waveforms are used to evaluate crackle prediction criteria based on the skewness of each variable. The resultant waveforms are provided as direct evidence that the pressure distribution does not directly predict or quantify a crackling sound quality while the statistics of the derivative appear to do so.

An understanding of jet noise source mechanisms can facilitate targeted noise reduction efforts. This understanding has been enhanced with acoustic imaging technologies, such as near-field acoustical holography (NAH). In this study, multisource statistically optimized NAH (M-SONAH) was used to image the sound field near a tethered F-35 aircraft at multiple frequencies. A linear microphone array, placed along the ground, spanned the length of the jet exhaust plume. A multisource model of the sound field was included in the algorithm to incorporate the effects of the ground reflection on the measurement. Narrowband reconstructions elucidated fine details of the radiation patterns, such as multilobe radiation patterns (which may supersede “dual-lobe” patterns shown in previous studies), and broadband shock-associated noise. [Work supported by F-35 JPO.]

A new method for the calculation of vector acoustic intensity from pressure microphone measurements has been applied to the aeroacoustic source characterization of an unheated, Mach 1.8 laboratory-scale jet. Because of the ability to unwrap the phase of the transfer functions between microphone pairs in the measurement of a radiating, broadband source, physically meaningful near-field intensity vectors are calculated up to the maximum analysis frequency of 32 kHz. The new intensity method is used to obtain a detailed description of the sound energy flow near the jet. The resulting intensity vectors have been used with a ray-tracing technique to identify the dominant source region over a broad range of frequencies. Additional aeroacoustics analyses provide insight into the frequency-dependent characteristics of jet noise radiation, including the nature of the hydrodynamic field and the transition between the principal lobe and sideline radiation.