Magnificent Desolation

Density-based representations of atomic environments that are invariant under Euclidean symmetries have become a widely used tool in the machine learning of interatomic potentials, broader data-driven atomistic modeling, and the visualization and analysis of material datasets. The standard mechanism used to incorporate chemical element information is to create separate densities for each element and form tensor products between them. This leads to a steep scaling in the size of the representation as the number of elements increases. Graph neural networks, which do not explicitly use density representations, escape this scaling by mapping the chemical element information into a fixed dimensional space in a learnable way. By exploiting symmetry, we recast this approach as tensor factorization of the standard neighbour-density-based descriptors and, using a new notation, identify connections to existing compression algorithms. In doing so, we form compact tensor-reduced representation of the local atomic environment whose size does not depend on the number of chemical elements, is systematically convergable, and therefore remains applicable to a wide range of data analysis and regression tasks.

Background
Noise exposure in the neonatal intensive care unit (NICU) is consistently higher than current recommendations. This may adversely affect neonatal sleep, weight gain, and overall health. We sought to evaluate the effect of a novel active noise control (ANC) system.

Methods
An ANC device’s noise reduction performance was compared to that of adhesively affixed foam ear covers in response to alarm and voice sounds in a simulated NICU environment. The zone of noise reduction of the ANC device was quantified with the same set of alarm and voice sounds.

Results
The ANC device provided greater noise reduction than the ear covers in seven of the eight sound sequences tested in which a noise reduction greater than the just noticeable difference was achieved. For noise in the 500 Hz octave band, the ANC device exhibited consistent noise reduction throughout expected patient positions. It provided better performance for noise below 1000 Hz than above 1000 Hz.

Conclusions
The ANC device provided generally superior noise reduction to the ear covers and provided a zone of noise reduction throughout the range where an infant would be placed within an incubator. Implications for patient sleep and weight gain are discussed.

Classical jet noise theory indicates that radiated sound power is proportional to the jet velocity raised to the eighth and third powers for subsonic and supersonic jets, respectively. To connect full-scale measurements with classical jet noise theory, this letter presents sound power and acoustic efficiency values for an installed GE-F404 engine. When subsonic, the change in sound power follows the eighth-power law, and the sound power change approximately follows the third-power law at supersonic conditions, with an acoustic efficiency of ∼0.5-0.6%. However, the OAPWL increase from subsonic to supersonic jet velocities is greater than would be predicted.

Time reversal (TR) is a signal processing technique that can be used to focus high amplitude sound or vibration at a desired location. TR focusing can be done with sources placed far from the desired focal location and the technique excels in complex environments. The impulse response between each source and the desired focal location must be obtained prior to the focusing and the environment must remain relatively unchanged for successful focusing. Multiple scattering or reverberation of waves off of many reflecting surfaces in the environment can actually be used advantageously by exploiting those reflections as additional image sources. This talk will provide an introduction to TR and then focus on the use of TR to provide high amplitude focusing of sound and vibration. Applications of high amplitude TR include lithotripsy of kidney stones, histotripsy of lesions, and locating cracks and defects in structures such as in human teeth, spent nuclear fuel storage casks, airplane wings, and automotive bearing caps. Recently, high amplitude TR of airborne sound has been studied in a reverberation chamber to generate a focused difference frequency and to study the nonlinear acoustics of the peak sound levels of 200 dB that have been attained.

The population density of dwarf galaxies in low-density voids is likely determined by the dark matter halo mass function and how galaxy formation proceeds in smaller halos. This depends on the nature of dark matter itself, making the dwarf galaxy population a tracer of its properties. While dwarfs have been found in smaller, closer voids, they have proven difficult to find in larger, more distant voids through magnitude-limited spectroscopic surveys. This is because these surveys detect an overwhelmingly large number of objects behind the voids that must be verified spectroscopically, making void surveys prohibitively inefficient and expensive in terms of large-telescope time. Narrowband imaging for emission lines such as Hα reduces the number of background objects, although the overall number remains large. If imaging is done through a filter set with overlapping transmission wings, then object redshift can be estimated from photometry alone. The precision possible is an order of magnitude greater than single-band photometry, with the caveat that the captured line must be identified through other means. Broadband photometry can be used to reject enough objects with emission of an unwanted type to make obtaining spectra of the remaining objects feasible. In this study, we present an Hα survey for dwarf galaxies with Mr' fainter than −14 mag through the center 4.3 square degrees of the void FN8. Using Sloan $g^{\prime} ,r^{\prime} ,i^{\prime} $ photometry, we exclude enough [O ii] and [O iii] emitters that follow-up spectra of only a few dozen objects are required to statistically estimate the void population density.

In a recent study of noise from a T-7A-installed GE F404 engine, microphones along a 76 m (250 ft) arc were mounted 1.8 m (5 ft) above the ground to quantify human impacts. While helpful for this purpose, the resulting multipath effects pose challenges for other acoustical analyses. For jet noise runup measurements, these effects are complicated by the fact that the noise source is extended and partially correlated, and its spatial properties are frequency dependent. Furthermore, a finite-impedance ground surface and atmospheric turbulence affect interference nulls. This study applies a ground-reflection method developed previously [Gee et al., Proc. Mtgs. Acoust. 22, 040001 (2014)] for rocket noise measurements. The model accounts for finite ground impedance, atmospheric turbulence, and extended source models that are treated as coherent and incoherent arrays of monopoles. Application to the ground runup data to correct the 76 m spectra at a range of angles suggests the incoherent line source model is more appropriate at upstream and sideline angles whereas the coherent source model is more appropriate for downstream propagation. Comparisons with near-field data and similarity spectra show that, while imperfect, this method represents an advancement in correcting jet noise spectra for ground reflection effects.