Noise Source Identification in an Under-Determined System by Convex Optimization

Previously, it was shown that the iterative optimization procedure Wideband Acoustical Holography (WBH) can efficiently identify noise sources in under-determined systems: i.e., when the number of measurements is less than the number of equivalent source parameters that need to be determined. However, experience suggests that the WBH method also has its limitations. First, when sound sources are closely spaced, WBH has difficulty in separating them, especially at low frequencies. Secondly the solution obtained from WBH is often highly localized: that is, the WBH solution develops around the strongest source, thus tending to under-estimate the level of nearby weaker sources. To alleviate these issues, the equivalent source strength estimation process has been re-formulated as a convex problem, thus making it possible to balance source sparsity and reconstruction accuracy in a systematic way and, at least in ideal circumstances, to guarantee an optimal solution. In the present work, measurements of radiation from a loudspeaker were conducted with a limited number of microphones. The sound field reconstruction results obtained using convex optimization were compared with the WBH results, and it was typically found that convex optimization did allow relatively weak sources to be successfully identified in the presence of nearby stronger sources

A Comparison of Two Equivalent Source Methods for Noise Source Visualization

The various equivalent source methods for noise source visualization can generally be categorized according to two factors: the complexity of the model (order of the sources) and the flexibility of the model (sources at fixed or unfixed location). The models used in the present work comprised: (i) a large number of low-order sources at fixed locations (low model order, low flexibility), and (ii) a high model order series at an unfixed location (high model order, high flexibility). With reference to case (i), a new equivalent source procedure based on a monopole distribution at fixed locations, which is referred to as wideband holography was recently developed by Hald. By using this method the balance between the model accuracy and the sparsity of the underdetermined system can be optimized. For case (ii), a higher order series at unfixed locations was recently proposed by Liu and Bolton. The unfixed location model was found to offer better accuracy of noise source location estimation compared with the more usual fixed-location model. In the present work, these two methods were used to reconstruct the noise sources of a loudspeaker cabinet. The measurement were conducted using an eighteen channel irregular array. A comparison of the reconstruction result from these two method help to highlight the strengths and weakness of each procedure

Diesel Engine Noise Source Visualization with Wideband Acoustical Holography

Wideband Acoustical Holography (WBH), which is a monopole-based, equivalent source procedure (J. Hald, “Wideband Acoustical Holography,” INTER-NOISE 2014), has proven to offer accurate noise source visualization results in experiments with a simple noise source: e.g., a loudspeaker (T. Shi, Y. Liu, J.S. Bolton, ”The Use of Wideband Holography for Noise Source Visualization”, NOISECON 2016). From a previous study, it was found that the advantage of this procedure is the ability to optimize the solution in the case of an under-determined system: i.e., when the number of measurements is much smaller than the number of parameters that must be estimated in the model. In the present work, a diesel engine noise source was measured by using one set of measurements from a thirty-five channel combo-array placed in front of the engine. The noise source distribution was reconstructed at the front face of diesel engine and it was possible to successfully localize and visualize the major noise sources over a broad range of frequencies, even when using a relatively small number of microphones, as in this case

A Large Double-ring Disk around the Taurus M Dwarf J04124068+2438157

Planet formation imprints signatures on the physical structures of disks. In this paper, we present high-resolution ($\sim$50 mas, 8 au) Atacama Large Millimeter/submillimeter Array (ALMA) observations of 1.3 mm dust continuum and CO line emission toward the disk around the M3.5 star 2MASS J04124068+2438157. The dust disk consists only of two narrow rings at radial distances of 0.47 and 0.78 arcsec ($\sim$70 and 116 au), with Gaussian $\sigma$ widths of 5.6 and 8.5 au, respectively. The width of the outer ring is smaller than the estimated pressure scale height by $\sim25\%$, suggesting dust trapping in a radial pressure bump. The dust disk size, set by the location of the outermost ring, is significantly larger (by $3\sigma$) than other disks with similar millimeter luminosity, which can be explained by an early formation of local pressure bump to stop radial drift of millimeter dust grains. After considering the disk's physical structure and accretion properties, we prefer planet--disk interaction over dead zone or photoevaporation models to explain the observed dust disk morphology. We carry out high-contrast imaging at $L'$ band using Keck/NIRC2 to search for potential young planets, but do not identify any source above $5\sigma$. Within the dust gap between the two rings, we reach a contrast level of $\sim$7 mag, constraining the possible planet below $\sim$2--4 $M_{\rm Jup}$. Analyses of the gap/ring properties suggest a $\sim$Saturn mass planet at $\sim$90 au is likely responsible for the formation of the outer ring, which can be potentially revealed with JWST.Comment: 15 pages, 5 figures. Accepted for publication in Ap

The Use of Wideband Acoustical Holography for Noise Source Visualization

Regularization is an important step in the acoustic holography process, since it can help mitigate problems associated with evanescent waves, the ill-posed nature of the problem, etc. and there is no single regularization method that is best for all problems. Recently, a new monopole-based equivalent source procedure referred to as wideband holography was developed by Hald (J. Hald, ”Wideband Acoustical Holography”, INTER-NOISE 2014) as part of which regularization procedures were proposed to optimize the solution in the case of an under-determined system. By using this method it is possible to balance the model accuracy and the sparsity of the system: i.e., the procedure allows the systematic reduction of number of the equivalent sources in the model. In the present work, experiments were conducted by using a loudspeaker cabinet and an eighteen channel irregular array placed in front of the loudspeaker. Low order equivalent source distributions placed in different locations were used during the acoustic field reconstruction. It was found that wideband holography can improve the accuracy of noise source location prediction, and give better acoustic field reconstruction results compared to other methods