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An ocean acoustics experiment in 2017 near a shipping lane on the New England continental shelf in about 75 m of water provided an opportunity to evaluate a methodology to extract source signatures of merchant ships in a bottom-limited environment. The data of interest are the received acoustic levels during approximately 20 min time intervals centered at the closest position of approach (CPA) time for each channel on two 16-element vertical line arrays. At the CPA ranges, the received levels exhibit a frequency-dependent peak and null structure, which possesses information about the geophysical properties of the seabed, such as the porosity and sediment thickness, and the characterization of the source, such as an effective source depth. The modeled seabed is represented by two sediment layers, parameterized with the viscous grain shearing (VGS) model, which satisfies causality, over a fixed deep layered structure. Inferred estimates of the implicit source levels require averaging an error function over the full 20 min time intervals. Within the 200–700 Hz band, the Wales–Heitmeyer model captures the inferred frequency dependence of the source levels. [ABSTRACT FROM AUTHOR]

Sensitivity analysis is a powerful tool for analyzing multi-parameter models. For example, the Fisher information matrix (FIM) and the Cramér–Rao bound (CRB) involve derivatives of a forward model with respect to parameters. However, these derivatives are difficult to estimate in ocean acoustic models. This work presents a frequency-agnostic methodology for accurately estimating numerical derivatives using physics-based parameter preconditioning and Richardson extrapolation. The methodology is validated on a case study of transmission loss in the 50–400 Hz band from a range-independent normal mode model for parameters of the sediment. Results demonstrate the utility of this methodology for obtaining Cramér–Rao bound (CRB) related to both model sensitivities and parameter uncertainties, which reveal parameter correlation in the model. This methodology is a general tool that can inform model selection and experimental design for inverse problems in different applications. [ABSTRACT FROM AUTHOR]

Merchant ship-radiated noise, recorded on a single receiver in the 360–1100 Hz frequency band over 20 min, is employed for seabed classification using an ensemble of deep learning (DL) algorithms. Five different convolutional neural network architectures and one residual neural network are trained on synthetic data generated using 34 seabed types, which span from soft-muddy to hard-sandy environments. The accuracy of all of the networks using fivefold cross-validation was above 97%. Furthermore, the impact of the sound speed and water depth mismatch on the predictions is evaluated using five simulated test cases, where the deeper and more complex architectures proved to be more robust against this variability. In addition, to assess the generalizability performance of the ensemble DL, the networks were tested on data measured on three vertical line arrays in the Seabed Characterization Experiment in 2017, where 94% of the predictions indicated that mud over sand environments inferred in previous geoacoustic inversions for the same area were the most likely sediments. This work presents evidence that the ensemble of DL algorithms has learned how the signature of the sediments is encoded in the ship-radiated noise, providing a unified classification result when tested on data collected at-sea. [ABSTRACT FROM AUTHOR]

Broadband spectrograms from surface ships are employed in convolutional neural networks (CNNs) to predict the seabed type, ship speed, and closest point of approach (CPA) range. Three CNN architectures of differing size and depth are trained on different representations of the spectrograms. Multitask learning is employed; the seabed type prediction comes from classification, and the ship speed and CPA range are estimated via regression. Due to the lack of labeled field data, the CNNs are trained on synthetic data generated using measured sound speed profiles, four seabed types, and a random distribution of source parameters. Additional synthetic datasets are used to evaluate the ability of the trained CNNs to interpolate and extrapolate source parameters. The trained models are then applied to a measured data sample from the 2017 Seabed Characterization Experiment (SBCEX 2017). While the largest network provides slightly more accurate predictions on tests with synthetic data, the smallest network generalized better to the measured data sample. With regard to the input data type, complex pressure spectral values gave the most accurate and consistent results for the ship speed and CPA predictions with the smallest network, whereas using absolute values of the pressure provided more accurate results compared to the expected seabed types. [ABSTRACT FROM AUTHOR]

In ocean acoustics, many types of optimizations have been employed to locate acoustic sources and estimate the properties of the seabed. How these tasks can take advantage of recent advances in deep learning remains as open questions, especially due to the lack of labeled field data. In this work, a Convolutional Neural Network (CNN) is used to find seabed type and source range simultaneously from 1 s pressure time series from impulsive sounds. Simulated data are used to train the CNN before application to signals from a single hydrophone signal during the 2017 Seabed Characterization Experiment. The training data includes four seabeds representing deep mud, mud over sand, sandy silt, and sand, and a wide range of source parameters. When applied to measured data, the trained CNN predicts expected seabed types and obtains ranges within 0.5 km when the source-receiver range is greater than 5 km, showing the potential for such algorithms to address these problems. [ABSTRACT FROM AUTHOR]

A broadband equivalent acoustic source distribution can be used to model the sound field near a high-speed jet. Such models must account for the spatiospectral variation of the sound levels. This work presents a technique for obtaining such a model using a spectral decomposition method associated with large and fine-scale turbulent mixing noise to create broadband equivalent source distributions for each noise type. The large-scale turbulent mixing noise is represented by frequency-dependent wavepackets, while the fine-scale turbulent mixing noise is modeled as a frequency-dependent incoherent, extended source distribution. This technique is applied to acoustical measurements from an ideally expanded, unheated Mach 1.8 jet. The wavepackets model the sound field levels in the maximum radiation region, but the second incoherent source distribution is required to obtain the levels at the other locations. The combination of the incoherent source distribution and the wavepacket provides a broadband, equivalent acoustic source representation that adequately models the sound field for Strouhal numbers between 0.04 and 0.25. At higher Strouhal numbers, better agreement is obtained when accounting for a frequency-dependent shift in the apparent acoustic source region. This frequency-dependent source region is more important closer to the jet than in the far field. [ABSTRACT FROM AUTHOR]

High-performance military aircraft noise contains large- and fine-scale turbulent mixing noise and broadband shock-associated noise. A three-way spectral decomposition quantifies the contribution from each noise type in the sound of a tied-down F-35B aircraft on a linear ground-based array spanning 35-152 deg. This large spatial aperture allows for detailed investigation into the spatial variation in broadband shock-associated noise and fine- and large-scale turbulent mixing noise. The spectral models used in the decomposition capture the main features of the measured spectra with three exceptions: 1) that the F-35B engine noise contains multiple spectral peaks in the maximum radiation region, 2) that the nonlinear propagation increases the high-frequency spectral levels, and 3) that the low-frequency levels in the maximum radiation region are less than those predicted by the large-scale similarity spectrum. The F-35B broadband shock-associated noise has the same characteristic shape and variation in peak frequency as overexpanded, laboratory-scale jets. However, the peak level and width exhibit different trends than laboratory-scale broadband shock-associated noise and those recently reported for the F/A-18E aircraft. The strengths and limitations of current models to represent the spatial variation in the spectral content of F-35B noise can guide research efforts to more fully understand the sound radiation. [ABSTRACT FROM AUTHOR]

A risk factor shared by all musicians is a potential for noise-induced hearing loss. A study was conducted to explore what factors determine noise dosage during musical events experienced by college student musicians. First, noise exposure during specific activities was explored, including during ensemble rehearsal and personal practice. Next, full-day noise exposure was investigated by measuring levels experienced by student musicians during a typical daily routine. Factors such as instrument played, type of activity, location within ensemble, and room environment were related to noise dosage. Disparities in results using different standards to calculate noise dosage were also explored. Risk of noise overexposure was found to be greater in some instrument classes, such as wind instruments, than in others, such as string instruments, and can yield vastly different results depending on the metric used for noise dosage calculation. [ABSTRACT FROM AUTHOR]

Broadband shock-associated noise (BBSAN) is a prominent noise component from nonideally expanded jets in the forward and sideline directions. BBSAN from laboratory-scale jets has been studied extensively, and spatial trends in BBSAN spectral peak characteristics—frequency, level, and width—have been established. These laboratory-scale trends are compared to those for BBSAN from a tied-down F-35B operated at four engine conditions. While the peak frequency varies as expected, both spatially and across engine condition, the peak level and width do not, pointing to the need for additional research into BBSAN for high-performance military aircraft. [ABSTRACT FROM AUTHOR]

The phase and amplitude gradient estimator (PAGE) method for active acoustic intensity uses pairwise microphone transfer functions to obtain the phase gradient, which improves the calculation bandwidth over the traditional weighted quadspectral method. Additionally, for broadband sources, the PAGE theory indicates that the transfer function phase can be unwrapped to further extend the usable frequency range to beyond the spatial Nyquist frequency. Here, two experiments demonstrate intensity bandwidth extension by more than an order of magnitude using phase unwrapping. First, plane-wave tube results show accurate one-dimensional intensity calculations with the microphones separated by five wavelengths, 30 times the traditional limit. Second, two-dimensional measurements of a laboratory-scale jet with a four-microphone probe yield physically reasonable calculations at frequencies 15 times the traditional limit. [ABSTRACT FROM AUTHOR]

Correlation analyses of ground-based acoustic-pressure measurements of noise from a tethered F-22A provide insights into the sound-field characteristics with position and engine condition. Time-scaled single-point (auto)correlation functions show that, to the side of the nozzle exit, the temporal-correlation envelope decays rapidly, whereas the envelope decays more slowly in the maximum radiation region and farther downstream. This type of spatial variation has been previously attributed to a transition from fine- to large-scale mixing noise in laboratory-scale jets. Two-point space-time (cross) correlation functions demonstrate that noise from a single engine operating at intermediate power is similar to that from a heated, convectively subsonic laboratory-scale jet, whereas additional features are seen at afterburner, relative to supersonic laboratory jets. A complementary coherence analysis provides estimates of coherence lengths as a function of frequency and location. Acoustic coherence lengths across the ground microphone array are used to analyze one-dimensional, equivalent-source-coherence lengths obtained from the DAMAS-C beamforming algorithm. The source coherence reaches its maximum downstream of the maximum source level, suggesting that uncorrelated sources meaningfully contribute to the dominant source region. In addition to revealing further the nature of the sound field near an advanced tactical engine, the characteristics seen should be useful as a phenomenological comparison point for those trying to model military-scale results both experimentally and numerically. [ABSTRACT FROM AUTHOR]

This Resource Letter provides a guide to the use of acoustics in physics pedagogy, whether in stand-alone courses, or as examples, analogies, or demonstrations in other contexts. Included are the principal journals and conference proceedings in the field, references to descriptions of existing acoustics courses, textbooks at different levels, and myriad online resources appropriate for courses at both the introductory and advanced undergraduate levels and for outreach. Also provided are topic-specific references that are divided into source, resonance, and traveling-wave phenomena. Because of the ability of sound to bridge the gap between mathematical understanding and everyday human experience, acoustical examples and demonstrations may deepen understanding of physical phenomena at all levels. [ABSTRACT FROM AUTHOR]

Spatial properties of noise statistics near unheated, laboratory-scale supersonic jets yield insights into source characteristics and near-field shock formation. Primary findings are (1) waveforms with positive pressure skewness radiate from the source with a directivity upstream of maximum overall level and (2) skewness of the time derivative of the pressure waveforms increases significantly with range, indicating formation of shocks during propagation. These results corroborate findings of a previous study involving full-scale engine data. Further, a comparison of ideally and over-expanded laboratory data show that while derivative skewness maps are similar, waveform skewness maps are substantially different for the two cases. [ABSTRACT FROM AUTHOR]

Noise measured in the vicinity of an F-22A Raptor has been compared to similarity spectra found previously to represent mixing noise from large-scale and fine-scale turbulent structures in laboratory-scale jet plumes. Comparisons have been made for three engine conditions using ground-based sideline microphones, which covered a large angular aperture. Even though the nozzle geometry is complex and the jet is nonideally expanded, the similarity spectra do agree with large portions of the measured spectra. Toward the sideline, the fine-scale similarity spectrum is used, while the large-scale similarity spectrum provides a good fit to the area of maximum radiation. Combinations of the two similarity spectra are shown to match the data in between those regions. Surprisingly, a combination of the two is also shown to match the data at the farthest aft angle. However, at high frequencies the degree of congruity between the similarity and the measured spectra changes with engine condition and angle. At the higher engine conditions, there is a systematically shallower measured high-frequency slope, with the largest discrepancy occurring in the regions of maximum radiation. [ABSTRACT FROM AUTHOR]

Noise measurements near the F-35A Joint Strike Fighter at military power are analyzed via spatial maps of overall and band pressure levels and skewness. Relative constancy of the pressure waveform skewness reveals that waveform asymmetry, characteristic of supersonic jets, is a source phenomenon originating farther upstream than the maximum overall level. Conversely, growth of the skewness of the time derivative with distance indicates that acoustic shocks largely form through the course of near-field propagation and are not generated explicitly by a source mechanism. These results potentially counter previous arguments that jet 'crackle' is a source phenomenon. [ABSTRACT FROM AUTHOR]

The presence of internal waves (IWs) in the ocean alters the isotropic properties of sound speed profiles (SSPs) in the water column. Changes in the SSPs affect underwater acoustics since most of the energy is dissipated into the seabed due to the downward refraction of sound waves. In consequence, variations in the SSP must be considered when modeling acoustic propagation in the ocean. Empirical orthogonal functions (EOFs) are regularly employed to model and represent SSPs using a linear combination of basis functions that capture the sound speed variability. A different approach is to use dictionary learning to obtain a learned dictionary (LD) that generates a nonorthogonal set of basis functions (atoms) that generate a better sparse representation. In this paper, the performance of EOFs and LDs are evaluated for sparse representation of SSPs affected by the passing of IWs. In addition, an LD-based supervised framework is presented for SSP classification and is compared with classical learning models. The algorithms presented in this work are trained and tested on data collected from the Shallow Water Experiment 2006. Results show that LDs yield lower reconstruction error than EOFs when using the same number of bases. In addition, overcomplete LDs are demonstrated to be a robust method to classify SSPs during low, medium, and high IW activity, reporting accuracy that is comparable to and sometimes higher than that of standard supervised classification methods. [ABSTRACT FROM AUTHOR]

Research in physics education has indicated that the traditional lecture-style class is not the most efficient way to teach introductory physical science courses at the university level. Current best teaching practices focus on creating an active-learning environment and emphasize the students' role in the learning process. Several of the recommended techniques have recently been applied to Brigham Young University's introductory acoustics course, which has been taught for more than 40 years. Adjustments have been built on a foundation of establishing student-based learning outcomes and attempting to align these objectives with assessments and course activities. Improvements have been made to nearly every aspect of the course including use of class time, assessment materials, and time the students spend out of the classroom. A description of the progress made in improving the course offers suggestions for those seeking to modernize or create a similar course at their institution. In addition, many of the principles can be similarly applied to acoustics education at other academic levels. [ABSTRACT FROM AUTHOR]

The viability of using reflection loss measurements for the determination of sediment parameters and the predictions of broadband acoustic behavior is studied as a function of frequency for a dispersive ocean bottom. For this study, a deterministic set of reflection loss values from an idealized dispersive sediment is calculated as a function of grazing angle over a large frequency band 100 Hz–1 MHz. In each of the four decades of frequency a simulated annealing optimization process is used to invert for the sediment parameters. In addition, a set of rotated coordinates is calculated that reveals the the relative sensitivities of the reflection data to each of the sediment parameters and therefore the ability of the inversion process to converge. The accuracy and precision of each estimate is analyzed. The predicted broadband acoustic behavior from the inverted parameters from each decade band was compared to the original model. Only the estimates obtained from data that include frequencies in the transition region can successfully predict broadband behavior. [ABSTRACT FROM AUTHOR]

An iterative, rotated coordinates inversion technique is applied in conjunction with spatial narrowband filters in matrix form to localize multiple acoustic sources in the shallow ocean. When the iterative, rotated coordinates inversion technique is applied to broadband horizontal line array data designed to simulate a group of moving ships, the bearing of the loudest source is quickly identified. Both passband and stop band matrix filters are constructed for an angular sector centered on the identified bearing. Data filtered with the passband filters are used in the inversion to obtain estimates of the remaining source parameters, range, depth, course angle and speed, and prominent environmental parameters. Additional inversions applied to data modified by the stop band filters yield the bearing of the next loudest source. The procedure is repeated until all detectable sources have been localized. Results of applying this strategy to several simulated cases are presented. [ABSTRACT FROM AUTHOR]

A generalized inversion method is presented that uses a rotated coordinates technique [Collins and Fishman, J. Acoust. Soc. Am. 98, 1637-1644 (1995)] in simulated annealing to invert for both the location of an acoustic source and parameters that describe the ocean seabed. The rotated coordinates technique not only aids in the inversion process but also indicates the coupling of the source and environmental parameters and the relative sensitivities of the cost function to changes in the various parameters. The information obtained from the rotated coordinates provides insights into how the inversion problem can be effectively decoupled. An iterative process consisting of multiple simulated annealing runs that each use a different set of rotated coordinates is demonstrated. This multistep algorithm is called systematic decoupling using rotated coordinates and is especially helpful when inverting for a large number of unknown parameters. The cost function minimized in the inversion algorithm is model-data cross-hydrophone spectra summed coherently over frequency and receiver pairs. The results of applying this inversion method to simulated data are presented in this paper. [ABSTRACT FROM AUTHOR]

Blasting experiments were performed that investigate multiple explosions that occur in quick succession in unconsolidated ground and their effects on host material and atmosphere. Such processes are known to occur during phreatomagmatic eruptions at various depths, lateral locations, and energies. The experiments follow a multi‐instrument approach in order to observe phenomena in the atmosphere and in the ground, and measure the respective energy partitioning. The experiments show significant coupling of atmospheric (acoustic)‐ and ground (seismic) signal over a large range of (scaled) distances (30–330 m, 1–10 m J−1/3). The distribution of ejected material strongly depends on the sequence of how the explosions occur. The overall crater sizes are in the expected range of a maximum size for many explosions and a minimum for one explosion at a given lateral location. As previous research showed before, peak atmospheric over‐pressure decays exponentially with scaled depth. An exponential decay rate of d¯0=6.47×10−4mJ−1/3 ${\bar{d}}_{0}=6.47\times 1{0}^{-4}\,\mathrm{m}\,{\mathrm{J}}^{-1/3}$ was measured. At a scaled explosion depth of 4 × 10−3 m J−1/3 ca. 1% of the blast energy is responsible for the formation of the atmospheric pressure pulse; at a more shallow scaled depth of 2.75 × 10−3 m J−1/3 this ratio lies at ca. 5.5%–7.5%. A first order consideration of seismic energy estimates the sum of radiated airborne and seismic energy to be up to 20% of blast energy. Finally, the transient cavity formation during a blast leads to an effectively reduced explosion depth that was determined. Depth reductions of up to 65% were measured. Plain Language Summary: Blasting experiments using six successive explosions were performed. Explosives were detonated in unconsolidated ground material in four different geometric setups (linear and triangular). We monitored the experiments using geophysical instruments. The instruments measured explosive energy, and how much of that energy escapes the crater. The experiments help to understand volcanic (phreatomagmatic) and other subsurface explosive processes. Exact measurements of the resulting craters, together with known explosive energies allow the interpretation of real volcanic craters. The experimental results show initial time developments of crater sizes, which occurs on the order of 1 s for crater sizes of the order of 1 m. Up to 8% of an explosion's energy was detected as airborne signal. Up to 20% of the explosion's energy was detected as seismic (elastic) energy in the ground. Key Points: Airborne energy of an underground blast decays exponentially with scaled depth and is in agreement with previous measurementsTransient crater unloading leads to eruption at the surface from depths that were thought to be deep enough to be contained in the groundCrater sizes correlate with measured seismo‐acoustic and high‐frequency atmospheric signals [ABSTRACT FROM AUTHOR]

A distinctive feature of many high-amplitude jet noise waveforms is the presence of acoustic shocks. Metrics indicative of shock presence, specifically the skewness of the time derivative of the waveform, the average steepening factor, and a new wavelet-based metric called the shock energy fraction, are used to quantify the strength and prevalence of acoustic shocks within waveforms recorded 10-305 m from a tethered military aircraft. The derivative skewness is more sensitive to the presence of the largest and steepest shocks, whereas the average steepening factor and shock energy fraction tend to emphasize aggregate behavior of the entire waveform. These metrics are applied at various engine conditions, over a wide range of angles and distances, to assess the growth and decay of shock waves. This paper represents the first time that the development of these metrics is shown from the near field to the far field, out to 305 m. The responses of these metrics point to significant shock formation occurring through nonlinear propagation out to 76 m from the microphone array reference position. Although these strongest shocks decay past 35 m, continued shock formation and atmospheric absorption can make the steepened nature of the waveform more prominent out to 305 m. [ABSTRACT FROM AUTHOR]

A search for magnetised quark nuggets (MQN) is reported using acoustic signals from hydrophones placed in the Great Salt Lake (GSL) in the USA. No events satisfying the expected signature were seen. This observation allows limits to be set on the flux of MQNs penetrating the Earth's atmosphere and depositing energy in the GSL. The expected signature of the events was ​derived from pressure pulses caused by high-explosive cords between the lake surface and bottom at various locations in the GSL. The limits obtained from this search are compared with those obtained from previous searches and are compared to models for the formation of MQNs. [ABSTRACT FROM AUTHOR]

An examination of the received spectrogram levels of about twenty merchant ship recordings on two vertical line arrays deployed on the New England continental shelf during the Seabed Characterization Experiment 2017 has identified an acoustic feature that can be attributed to the group velocities of modes 1 and 2 being equal at a frequency f = F. The observation of such a feature is a result of β n m (2 π F) = ∞ , where βnm is the waveguide invariant for modes n and m. For the New England Mudpatch, the average value of F is about 24.5 Hz. An effective seabed model is inferred from a feature inversion method that has a deep sediment layer which lies between 190 m and 290 m beneath the seafloor with sound speeds on the order of 1810 m/s. This effective sediment model appears to be consistent with a previous seismic survey on the New England shelf that identified a deep low speed layer about 250 m beneath the water sediment interface. [ABSTRACT FROM AUTHOR]

The traditional method for intensity-based sound power estimates often used in engineering applications is limited in bandwidth by microphone phase mismatch at low frequencies and by microphone spacing at high frequencies. To overcome these limitations, the Phase and Amplitude Gradient Estimator (PAGE) method [Gee, Neilsen, Sommerfeldt, Akamine, and Okamoto, J. Acoust. Soc. Am. 141(4), EL357–EL362 (2017)] is applied to sound power for a reference sound source, a blender, and a vacuum cleaner. Sound power measurements taken according to ISO 3741:2010 (2010) are compared against traditional- and PAGE-processed intensity-based sound power estimates measured according to ANSI S12.12-1992 (R2017). While the traditional method underestimates the sound power at the spatial Nyquist frequency by 7–10 dB, the PAGE-based sound power is accurate up to the spatial Nyquist frequency, and above when phase unwrapping is successful. [ABSTRACT FROM AUTHOR]

Noise from a tactical aircraft can impact operations due to concerns regarding military personnel noise exposure and community annoyance and disturbance. The efficacy of mission planning can increase when the distinct, complex acoustic source mechanisms creating the noise are better understood. For each type of noise, equivalent acoustic source distributions are obtained from a tied-down F-35B operating at various engine conditions using the hybrid method for acoustic source imaging of Padois, Gauthier, and Berry [J. Sound Vib. 333, 6858–6868 (2014)]. The source distributions for the distinct noise types are obtained using different sections of a 71 element, ground-based linear array. Using a subarray close to the nozzle exit plane, source distributions are obtained for fine-scale turbulent mixing noise and broadband shock-associated noise, although grating lobes complicate interpretations at higher frequencies. Results for a subarray spanning the maximum sound region show that the multiple frequency peaks in tactical aircraft noise appear to originate from overlapping source regions. The observation of overlapping spatial extent of competing noise sources is supported by the coherence properties of the source distributions for the different subarrays. [ABSTRACT FROM AUTHOR]

Furthermore, just-in-time teaching questions[9] require students to practice communicating with new vocabulary and are used for in-class small-group discussions: Students generally have an easier time discussing something they've already thought about. Student perceptions Students were asked in a midcourse survey this semester to rank how well the assessments have helped them achieve each learning outcome (Table III). To promote an engaging class discussion, students are called on during class to stimulate discussion and asked to give presentations to the class a few times during the semester. In the first two semesters offered, approximately half the enrolled students dropped the class after seeing the first assignment, even though extra resources, including video tutorials, were available. [Extracted from the article]

Application of phased-array algorithms to acoustic measurements in the vicinity of a high-performance military aircraft yields equivalent source reconstructions over a range of engine conditions. 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. The hybrid method, an inverse method approached via beamforming, is applied to jet noise measured along a 50 element, 30 m linear array to obtain equivalent source distributions. The source distribution extent decreases with increasing frequency or with a decrease in engine condition. A source coherence analysis along the axial dimension of the jet plume reveals that the source coherence lengths scale inversely with increasing engine condition. In addition, a method for extending the array bandwidth to frequencies beyond the spatial Nyquist frequency limit is also implemented. A directivity analysis of the beamforming results reveals that sources near the nozzle radiate to the sideline from a relatively stationary point irrespective of frequency, while the noise source origin of downstream radiating noise varies significantly with frequency. [ABSTRACT FROM AUTHOR]

Bias errors for two-dimensional active acoustic intensity using multi-microphone probes have been previously calculated for both the traditional cross-spectral and the Phase and Amplitude Gradient Estimator (PAGE) methods [Whiting, Lawrence, Gee, Neilsen, and Sommerfeldt, J. Acoust. Soc. Am. 142, 2208–2218 (2017)]. Here, these calculations are expanded to include errors due to contaminating noise, as well as probe orientation. The noise can either be uncorrelated at each microphone location or self-correlated; the self-correlated noise is modeled as a plane-wave with a varying angle of incidence. The intensity errors in both magnitude and direction are dependent on the signal-to-noise ratio (SNR), frequency, source properties, incidence angles, probe configuration, and processing method. The PAGE method is generally found to give more accurate results, especially in direction; however, uncorrelated noise with a low SNR (below 10–15 dB) and low frequency (wavelengths more than 1/4 the microphone spacing) can yield larger errors in magnitude than the traditional method—though a correction for this is possible. Additionally, contaminating noise does not necessarily impact the possibility of using the PAGE method for broadband signals beyond a probe's spatial Nyquist frequency. [ABSTRACT FROM AUTHOR]

Many methods of two-microphone directional sensing have limited bandwidth. For active intensity, finite-difference error can be removed by using the phase and amplitude gradient estimator method. Using similar principles, a directional pressure sensor based on the phase gradient is developed that is accurate up to the spatial Nyquist frequency, and beyond if phase unwrapping is applied. A highly directional frequency-independent array response of arbitrary order can be achieved with two microphones. The method is compared against beamforming and traditional gradient sensing for single and multiple sources and is found to have improved localization capabilities and increased bandwidth. [ABSTRACT FROM AUTHOR]

Results of the first formal perceptual study of jet crackle are presented. Prior studies examined noise waveform properties believed to be linked to the jet crackle percept from a physics perspective or using signal processing and informal subjective evaluation. This investigation involves 31 listeners that rated 15 jet noise waveforms with a category subdivision scaling test. Results reveal a strong log-linear correlation between the pressure waveform time derivative's skewness and crackle rating. A regression analysis establishes practical derivative skewness bounds for a five-point categorical crackle scale and results in the suggested definition of the crepit as the unit of crackliness. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

A method is presented to suppress grating lobes in beamforming using phase unwrapping and array interpolation. When the phase of each cross spectrum is successfully unwrapped, the magnitude and phase of the cross spectral matrix may be interpolated; for cases where these quantities vary smoothly, interpolation is straightforward, even above the spatial Nyquist frequency. Two applications are presented: localization of a broadband source and characterization of a source with frequencydependent location. In both cases, grating lobes are suppressed and the source is localized at frequencies up to at least 8 times the spatial Nyquist frequency. [ABSTRACT FROM AUTHOR]

The Biot-Stoll poro-elastic model for sediments is parameterized by 13 properties. Initial investigations indicate that the individual Biot parameters influence reflection loss differently depending on the frequency range of interest. A more extensive study of the frequency dependence of the parameter sensitivities is performed by computing posterior probability distributions (PPDs) [S. E. Dosso, J. Acoust. Soc. Am. 111, 129–142 (2002)] to determine which parameters most influence reflection loss for each frequency decade from 100 Hz to 1 MHz. The sampling for the PPDs is based on the least-squares error between simulated reflection coefficient data and values modeled with the reflection module of OASES. [H. Schmidt, ‘‘OASES Version 2.1 User Guide and Reference Manual,’’ Department of Ocean Engineering, Massachusetts Institute of Technology, 1997.] The PPDs provide a quantitative value of the uncertainty in the optimum parameter estimates obtained by sampling or inversion techniques. A good value of the uncertainty is critical if one wants to use parameter estimates obtained with data from one frequency range to predict acoustic response for a different frequency range. [Work supported by the ONR.] [ABSTRACT FROM AUTHOR]

Efforts to mentor undergraduates in research require time, energy, and resources but can yield significant benefits. A review of education research on undergraduate research is presented followed by a case study of how technical and educational principles from the literature have been implemented and practiced in a new underwater acoustics lab. This case study involves the students who have joined this lab over its first three years. The results highlight the importance of several key factors that mentors can implement to improve their students' research experience. In order to meet the steep learning curve that new students face, faculty mentors should establish a culture of research (i.e., well established norms, expectations, and practices associated with being a part of a research group) through clear expectations, open communication, and student-led peer mentorship supported by carefully selected or designed resources (i.e., scaffolding). This paper seeks to share these education research resources with the acoustics community while providing a case study for review as a specific application of these key principles into an acoustics laboratory setting. [ABSTRACT FROM AUTHOR]

In this paper, quantitative understanding of a frequency-domain nonlinearity indicator is developed. The indicator is derived from an ensemble-averaged, frequency-domain version of the generalized Burgers equation, which can be rearranged in order to directly compare the effects of nonlinearity, absorption, and geometric spreading on the pressure spectrum level with frequency and distance. The nonlinear effect is calculated using pressure-squared-pressure quadspectrum. Further theoretical development has given an expression for the role of the normalized quadspectrum, referred to as Q/S by Morfey and Howell [AIAA J. 19, 986-992 (1981)], in the spatial rate of change of the pressure spectrum level. To explore this finding, an investigation of the change in level for initial sinusoids propagating as plane waves through inviscid and thermoviscous media has been conducted. The decibel change with distance, calculated through Q/S, captures the growth and decay of the harmonics and indicates that the most significant changes in level occur prior to sawtooth formation. At large distances, the inviscid case results in a spatial rate of change that is uniform across all harmonics. For thermoviscous media, large positive nonlinear gains are observed but offset by absorption, which leads to a greater overall negative spatial rate of change for higher harmonics. [ABSTRACT FROM AUTHOR]

The identification of acoustic sources is critical to targeted noise reduction efforts for jets on highperformance tactical aircraft. This paper describes the imaging of acoustic sources from a tactical jet using near-field acoustical holography techniques. The measurement consists of a series of scans over the hologram with a dense microphone array. Partial field decomposition methods are performed to generate coherent holograms. Numerical extrapolation of data beyond the measurement aperture mitigates artifacts near the aperture edges. A multisource equivalent wave model is used that includes the effects of the ground reflection on the measurement. Multisource statistically optimized near-field acoustical holography (M-SONAH) is used to reconstruct apparent source distributions between 20 and 1250 Hz at four engine powers. It is shown that M-SONAH produces accurate field reconstructions for both inward and outward propagation in the region spanned by the physical hologram measurement. Reconstructions across the set of engine powers and frequencies suggests that directivity depends mainly on estimated source location; sources farther downstream radiate at a higher angle relative to the inlet axis. At some frequencies and engine powers, reconstructed fields exhibit multiple radiation lobes originating from overlapped source regions, which is a phenomenon relatively recently reported for full-scale jets. [ABSTRACT FROM AUTHOR]

Mach stem formation during outdoor acoustic shock propagation is investigated using spherical oxyacetylene balloons exploded above pavement. The location of the transition point from regular to irregular reflection and the path of the triple point are experimentally resolved using microphone arrays and a high-speed camera. The transition point falls between recent analytical work for weak irregular reflections and an empirical relationship derived from large explosions. [ABSTRACT FROM AUTHOR]

Sounds to Astound is an acoustics demonstration show, produced for the community twice yearly by the Brigham Young University Student Chapter of the Acoustical Society of America. The free, interactive demonstration show explores the science of sound for a target audience of fifth- to eighth-grade students. Introductory acoustics concepts, such as longitudinal wave motion, wave properties, propagation effects, and standing waves, are taught with live demonstrations, animations, and videos. The goal of this paper is to inspire and encourage readers in their outreach efforts by describing the purposes of Sounds to Astound and technical details of several entertaining and educational demonstrations. Lessons learned from a decade of these student-produced shows serve as an aid for future efforts and highlight the benefits of outreach efforts, particularly for the students involved. [ABSTRACT FROM AUTHOR]

Prior anechoic measurements of a small acetylene-oxygen balloon explosion were used to study spherical weak-shock decay over short ranges [Muhlestein et al., J. Acoust. Soc. Am. 131, 2422-2430 (2012)]. Here, longer-range measurements conducted at the Bonneville Salt Flats with a larger balloon are described. Waveform and spectral characteristics and comparisons of the peak pressure decay with an analytical weak-shock model are presented. Weak shocks persist to at least 305 m, with an amplitude decay that is predicted reasonably well using the model. Deviations are discussed in the context of atmospheric effects and nonlinear ground reflections. [ABSTRACT FROM AUTHOR]

The spatial variation in vector acoustic intensity has been calculated between 100 and 3000 Hz near a high-performance military aircraft. With one engine of a tethered F-22A Raptor operating at military power, a tetrahedral intensity probe was moved to 27 locations in the geometric near and mid-fields to obtain the frequency-dependent intensity vector field. The angles of the maximum intensity region rotate from aft to sideline with increasing frequency, becoming less directional above 800 Hz. Between 100 and 400 Hz, which are principal radiation frequencies, the ray-traced dominant source region rapidly contracts and moves upstream, approaching nearly constant behavior by 1000 Hz. [ABSTRACT FROM AUTHOR]

This paper presents a reduced-order approach to near-field acoustical holography (NAH) that allows the user to account for sound fields generated by multiple spatially separated sources. In this method, an equivalent wave model (EWM) of a given field is formulated to include combinations of planar, cylindrical, spherical, or other elementary wave functions in contrast to an EWM restricted to a single separable coordinate system. This can alleviate the need for higher-order functions, reduce the number of measurements, and decrease error. The statistically optimized near-field acoustical holography (SONAH) algorithm is utilized to perform the NAH projection after the formulation of the multisource EWM. The combined process is called multisource statistically optimized near-field acoustical holography (M-SONAH). This method is used to reconstruct simulated sound fields generated by combinations of a vibrating piston in a sphere and linear arrays of monopole sources. It is shown that M-SONAH can reconstruct near-field pressures in multisource environments with lower errors and fewer measurements than a strictly plane or cylindrical-wave formulation using the same simulated measurement. [ABSTRACT FROM AUTHOR]

Difficulties arise in attempting to discern the effects of nonlinearity in near-field jet-noise measurements due to the complicated source structure of high-velocity jets. This article describes a measure that may be used to help quantify the effects of nonlinearity on waveform propagation. This measure, called the average steepening factor (ASF), is the ratio of the average positive slope in a time waveform to the average negative slope. The ASF is the inverse of the wave steepening factor defined originally by Gallagher [AIAA Paper No. 82-0416 (1982)]. An analytical description of the ASF evolution is given for benchmark cases--initially sinusoidal plane waves propagating through lossless and thermoviscous media. The effects of finite sampling rates and measurement noise on ASF estimation from measured waveforms are discussed. The evolution of initially broadband Gaussian noise and signals propagating in media with realistic absorption are described using numerical and experimental methods. The ASF is found to be relatively sensitive to measurement noise but is a relatively robust measure for limited sampling rates. The ASF is found to increase more slowly for initially Gaussian noise signals than for initially sinusoidal signals of the same level, indicating the average distortion within noise waveforms occur more slowly. [ABSTRACT FROM AUTHOR]

Near-field acoustical holography methods are used to predict sound radiation from an engine installed on a high-performance military fighter aircraft. Cylindrical holography techniques are an efficient approach to measure the large and complex sound fields produced by full-scale jets. It is shown that a ground-based, one-dimensional array of microphones can be used in conjunction with a cylindrical wave function field representation to provide a holographic reconstruction of the radiated sound field at low frequencies. In the current work, partial field decomposition methods and numerical extrapolation of data beyond the boundaries of the hologram aperture are required prior to holographic projection. Predicted jet noise source distributions and directionality are shown for four frequencies between 63 and 250 Hz. It is shown that the source distribution narrows and moves upstream, and that radiation directionality shifts toward the forward direction, with increasing frequency. A double-lobe feature of full-scale jet radiation is also demonstrated. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The article describes an activity program for effective tool for engaging students in introductory physics classes.

Calculation of acoustic intensity using the phase and amplitude gradient estimator (PAGE) method has been shown to increase the effective upper frequency limit beyond the traditional p-p method when the source of interest is broadband in frequency [Torrie, Whiting, Gee, Neilsen, and Sommerfeldt, Proc. Mtgs. Acoust. 23, 030005 (2015)]. PAGE processing calculates intensity for narrowband sources without bias error up to the spatial Nyquist frequency [Succo, Sommerfeldt, Gee, and Neilsen, Proc. Mtgs. Acoust. 30, 030015 (2018)]. The present work demonstrates that for narrowband sources with frequency content above the spatial Nyquist frequency, additive low-level broadband noise can improve intensity calculations. To be effective, the angular separation between the source and additive noise source should be less than 30 ° , while using phase unwrapping with a smaller angular separation will increase the usable bandwidth. The upper frequency limit for the bandwidth extension depends on angular separation, sound speed, and probe microphone spacing. Assuming the signal-to-additive-noise ratio ( SNR a ) is larger than 10 dB, the maximum level and angular bias errors incurred by the additive broadband noise beneath the frequency limit—or up until probe scattering effects must be taken into account—are less than 0.5 dB and 2.5 ° , respectively. Smaller angular separation yields smaller bias errors. [ABSTRACT FROM AUTHOR]

Supersonic jet noise reduction efforts benefit from targeted source feature extraction and high-resolution acoustic imaging. Another useful tool for feature extraction is partial field decomposition of sources into independent contributors. Since such decomposition processes are nonunique, care must be taken in the physical interpretation of decomposed partially coherent aeroacoustic fields. The optimized-location virtual reference method (OLVR) is a partial field decomposition designed to extract physically meaningful source and field information through the strategic placement of virtual references within a reconstructed field. The OLVR method is applied here to obtain spatially distinct and ordered partial sources at multiple frequencies of a full-scale, high-performance supersonic jet engine operating at 100% engine power. Partial sources are shown to mimic behaviors of the total source distributions including monotonic growth and decay. Because of finite spatial coherence, multiple partial sources are used to reproduce far-field radiation away from the main lobe, and the number of required sources increases with increasing frequency. An analytical multiwavepacket model is fitted to the partial sources to demonstrate how OLVR partial fields can be leveraged to produce reduced-order models. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]