Acoustic Noise Measurements Research Articles

Multimodal partial cavity shedding on a two-dimensional hydrofoil and its relation to the presence of bubbly shocks

The shedding dynamics of partial cavitation forming on a NACA0015 hydrofoil is known from the previous studies to be multimodal, exhibiting abrupt changes in Strouhal number as the cavitation number is reduced (Arndt et al. in Instability of partial cavitation: a numerical/experimental approach. National Academies Press, Washington, D.C., Retrieved from the University of Minnesota Digital Conservancy. http://hdl.handle.net/11299/49781 , 2000). The present study aims to understand the underlying shedding mechanisms responsible for this abrupt change in dynamics. As was observed in Ganesh et al. (J Fluid Mech 802:37–78, 2016), the shedding process can result from both re-entrant liquid flow and the formation of propagating bubbly shock waves within the cavity. Time-resolved X-ray densitometry and high-speed videography are combined with time synchronous measurements of acoustic noise produced by the cavity to examine the formation and shedding of the partial cavitation on a NACA0015 hydrofoil. From the experiments on the hydrofoil, it was observed that the mean cavity length increased with decreasing cavitation number, as expected. At higher pressures, stable partial cavities formed that shed smaller vapor clouds mainly due to the re-entrant liquid flow in the cavity closure. With a reduction in cavitation number, the partial cavity grew in length, and shedding often occurred when the cavity was pinched-off from its leading edge. Once a large region of vapor was shed, the pressure pulse caused by its subsequent collapse could suppress the growth of the newly forming partial cavity. This feedback led to complex, multistep cavity shedding dynamics. At still lower cavitation numbers, bubbly shocks were found to be responsible for the strongest periodic shedding of vapor clouds. Spectral analysis of the acoustic signal revealed a multimodal nature, which was more pronounced at lower cavitation numbers, and similar dynamic behavior was also found for the time varying local void fraction measurements in the vicinity of cavity closure. The occurrence of strongest multimodal behavior was also characterized by the presence of bubbly shock waves as the dominant mechanism of shedding. From the measurements and analysis, it is concluded that at least four different types of shedding modes occurred over a range of cavitation numbers at a fixed attack angle (10°).

Measurement and Characterization of Acoustic Noise in Water Pipeline Channels

Measurement and characterization of acoustic noise in water pipeline systems across the 10 Hz to 100 kHz acoustic frequency band are reported in this paper. The motivation for the noise characterization is to support the development of acoustic signal processing and communication systems for water pipeline systems. Measurements are reported from an urban water supply system in the city of Christchurch, New Zealand, using hydrophones inserted into the pipelines. These measurements are some of the first that have been reported for an operational water supply system. In addition, for comparison to a controlled baseline reference, measurements have also been performed in a 160 m pipeline laboratory testbed at The Hong Kong University of Science and Technology. A key finding is that the noise amplitude probability density function (PDF) is found to follow a heavy-tailed distribution and the α-stable distribution is proposed for its characterization. The resultant acoustic noise measurements have also been analyzed to determine their power spectral density (PSD) characteristics. The measured noise PSD is not flat and characteristic frequencies and power density levels are utilized to capture the features of the noise PSD. These findings could have a significant impact on the design of signal processing systems for water pipeline systems due to their deviation from the conventional added white Gaussian noise model. In addition, three different acoustic noise sources with individual power and distribution characteristics are also postulated. These postulated sources are used to propose a straightforward acoustic noise model for supporting the simulation of acoustic noise for signal processing system design in water pipelines.

A high-performance gradient insert for rapid and short-T2 imaging at full duty cycle.

The goal of this study was to devise a gradient system for MRI in humans that reconciles cutting-edge gradient strength with rapid switching and brings up the duty cycle to 100% at full continuous amplitude. Aiming to advance neuroimaging and short-T2 techniques, the hardware design focused on the head and the extremities as target anatomies. A boundary element method with minimization of power dissipation and stored magnetic energy was used to design anatomy-targeted gradient coils with maximally relaxed geometry constraints. The design relies on hollow conductors for high-performance cooling and split coils to enable dual-mode gradient amplifier operation. With this approach, strength and slew rate specifications of either 100 mT/m with 1200 mT/m/ms or 200 mT/m with 600 mT/m/ms were reached at 100% duty cycle, assuming a standard gradient amplifier and cooling unit. After manufacturing, the specified values for maximum gradient strength, maximum switching rate, and field geometry were verified experimentally. In temperature measurements, maximum local values of 63°C were observed, confirming that the device can be operated continuously at full amplitude. Testing for peripheral nerve stimulation showed nearly unrestricted applicability in humans at full gradient performance. In measurements of acoustic noise, a maximum average sound pressure level of 132 dB(A) was determined. In vivo capability was demonstrated by head and knee imaging. Full gradient performance was employed with echo planar and zero echo time readouts. Combining extreme gradient strength and switching speed without duty cycle limitations, the described system offers unprecedented options for rapid and short-T2 imaging. Magn Reson Med 79:3256-3266, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Silent navigator-triggered silent MRI of the abdomen.

To develop and demonstrate the feasibility of a silent respiratory navigator technique for prospective triggering, which was incorporated into a three-dimensional radial zero-echo-time sequence for respiratory navigated silent abdominal imaging. A nonselective hard excitation radiofrequency pulse was used for the navigator sequence with a derated readout gradient, to avoid generation of high levels of acoustic noise. The acquired navigator signals were processed in real time and used for prospective triggering of the zero-echo-time sequence. Ten healthy volunteers were scanned using the proposed and conventional techniques at 1.5 T. An acoustic noise measurement with A-weighted continuous equivalent sound pressure level was also performed. The sound pressure-level values of the background noise, zero-echo-time imaging, conventional, and silent navigators were 68.3, 68.4, 102.5, and 69.4 dBA, respectively. Excellent correlation with correlation coefficients greater than 0.9 was observed between the bellows signals and displacement values calculated from the navigators. Sharpness of the portal vein of both conventional and silent navigator-triggered images was significantly higher than those of nontriggered images. The silent navigator-triggered zero-echo-time technique is feasible and might improve image quality and workflow of abdominal MRI of patients who are prone to acoustic noise. Magn Reson Med 79:2170-2175, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Broadband acoustic energy confinement in hierarchical sonic crystals composed of rotated square inclusions

The propagation of acoustic waves in hierarchical sonic crystals is studied computationally and experimentally. These sonic crystals are composed of a hierarchical order of square inclusions rotated 45° with respect to the square lattice structure. It is shown that these hierarchical sonic crystals are capable of confining acoustic energy over a broad frequency range and at multiple lattice points inside the sonic crystal based on Bragg's scattering effect. Fused deposition modeling additive manufacturing is applied to prepare a finite-sized sample of the hierarchical sonic crystal. Acoustic measurements are conducted on the hierarchical sonic crystal sample in a direct and closely plane-wave field inside an anechoic room. The experimental measurements are in good agreement with the band structure calculated using the finite element method. Potential applications of the hierarchical sonic crystals for acoustic energy harvesting and noise measurements are discussed.

FPGA implementation of hardware efficient adaptive filter robust to impulsive noise

Adaptive filters are prevalent in many real-time signal processing applications. Many adaptive algorithms already exist, but most of them assume white Gaussian noise as disturbance. However, for many applications such as electrocardiogram, foetus heart rate measurement, low frequency atmospheric noise, underwater acoustic noise and signal measurement in instrumentation, the impulsive noise is more common. This study presents a modified robust mixed norm (MRMN) adaptive filter algorithm robust to impulsive noise with higher convergence rate and lower steady state error (SSE). MRMN adaptive filter algorithm has been simulated using Matlab and Xilinx system generator high level synthesis tool and a significant improvement in SSE and convergence speed is obtained compared with the existing adaptive filter algorithms for similar specifications. The proposed algorithm is also described using VHDL and synthesised using Xilinx synthesiser tool in order to implement on field-programmable gate array (FPGA). The FPGA post route and place implementation results show nearly 90% reduction in resource utilisation and nearly 2.6 times improvement in clock frequency as compared with the existing FPGA based implementation for similar specifications.

Acoustic and seismic ambient noise measurements in urban and rural areas

Using portable sound level meters, simultaneous one-third octave spectral measurements of acoustic and seismic ambient noise as a function of frequency were conducted in different urban terrains and at two rural locations. The acoustic measurement frequency band of 6Hz–20kHz includes frequencies lower than are commonly measured in acoustic surveys, while the seismic band of 6Hz–1kHz extends to higher frequencies than most past seismic measurements. The urban acoustic measurements are dominated by traffic noise. The seismic signatures also are heavily influenced by traffic noise but exhibit more variation with respect to urban location due to the influence of subsurface conditions. For most urban measurement sites, the acoustic peak frequency was usually around 60Hz, while the peak seismic frequencies were usually less than 25Hz. The measured urban seismic spectra also tended to have more structure and variation in spectral shape from one location to another compared to the acoustic spectra, and a more rapid decline with increasing frequency, usually decreasing by about 30dB from 30Hz to 600Hz.

Spatial diversity of ambient noise in the new Arctic

The Arctic Ocean is undergoing dramatic changes, the most apparent being the rapidly reducing extent and thickness of the summer ice cover. As has been well established over prior decades, the environmental acoustics of the ice-covered Arctic is dominated by two major effects: the highly inhomogeneous ice cover, and the monotonically upward refracting sound speed profile, the combination of which forces all sound paths to be exposed to strong scattering loss and the associated loss of coherence. In some portions of the Arctic Ocean, however, inflow of warm Pacific water has created the so-called “Beaufort Lens,” a neutrally buoyant high sound velocity layer at 70-80 meter depth, which has dramatically altered the acoustic environment, creating a strong acoustic duct between approximately 100 and 300 m depth. This duct has the potential of trapping sound out to significant ranges (80-100 km) without interacting with the ice cover, resulting in much higher coherence and signal preservation. Acoustic noise measurement results collected with a vertically suspended array during ICEX 2016 illustrate the spatial and temporal noise properties in the presence of this acoustic duct at different depths. Comparisons of the ICEX 2016 data are also made with modeled Arctic noise data. [Work supported by ONR and DARPA.]

Prediction of Horizontal Axis Wind Turbine Acoustics

Wind power industry has been a growing market and maturing technology for several decades. Increasing wind power generation necessitates closer installation of wind turbines to people and their residences. For that reason, wind turbine noise becomes a serious and controversial phenomenon and it is anticipated to become more stringent issue while wind power generation is increased recently. The aim of this study was to propose a methodology to predict the wind turbine blade noise by using two dimensional blade section flows and noise analysis/simulation and combining the noise sources to evaluate the total blade noise with reasonable accuracy. Within the scope of this work, the purpose was to perform a two dimensional flow and noise simulation for the two bladed NREL Phase VI wind turbine with the aid of a commercially available Computational Fluid Dynamics (CFD) software ANSYS-FLUENT by using User Defined Functions after applying some corrections under certain assumptions. Analysis results were compared with the 12% scaled model of NREL Phase VI wind turbine acoustic noise measurements conducted in In Korea Aerospace Research Institute (KARI) at the low speed wind tunnel. Blade noise of the measurements were compared with the analysis results at different wind speeds of 5.4 m/s, 7.4 m/s, 12.3 m/s, and 13.3 m/s. For the tip region and inboard area of the blade, reasonable agreement was achieved at certain wind speeds. Additionally, the summation of the contribution from the each blade section was used to predict the total noise emission.

Noise and Vibration Issues of Wind Turbines and Their Impact – A Review

Wind power is a rapidly growing technology, with an estimated 35% of national end-use electricity demand to be met from wind by 2050 in the US. With such a projected rapid growth, it is necessary to improve and innovate relevant technological areas with due considerations of possible impacts of wind energy harnessing through wind turbines. This paper presents a systematic review of current literature on the issues of noise and vibration of wind turbines and their impact on human health and wild life. The paper reviews the literature on the issues of noise and vibration in wind turbines, the generation mechanisms, the propagation, the impact on human health and wild life. The current status of technology and future developments to mitigate the health and environmental impacts of wind turbine noise and vibration are also reviewed. The paper includes a review of current standards on measurement of acoustic noise of wind turbines and data analysis.

Estimates of acoustic noise generated by supply vessels working with oil-drilling platforms

The paper presents results on spatial measurements of acoustic noise generated by two types of tugs during their movement near the Molikpaq platform and in a dynamic positioning mode during operation with the PA-B platform. Based on the results of these measurements with the aid of simulation and preliminary research of the loss function conducted on acoustic profiles spanning from the platforms to the nearshore Piltun gray whale summer—fall feeding area, the spectra of equivalent point sources are constructed, which make it possible to construct the 1/3-octave spectra of anthropogenic noise at any point of the western profile and estimate the value of their level in a given frequency band with an accuracy of up to 2 dB. Field measurements have shown that in the dynamic positioning mode, the tugs generate 10 dB more noise than during movement; in fact, a diesel electric tug in both modes produced approximately 5 dB less noise than a diesel tug.

Predicting short-period, wind-wave-generated seismic noise in coastal regions

Substantial effort has recently been made to predict seismic energy caused by ocean waves in the 4–10 s period range. However, little work has been devoted to predict shorter period seismic waves recorded in coastal regions. Here we present an analytical framework that relates the signature of seismic noise recorded at 0.6–2 s periods (0.5–1.5 Hz frequencies) in coastal regions with deep-ocean wave properties. Constraints on key model parameters such as seismic attenuation and ocean wave directionality are provided by jointly analyzing ocean-floor acoustic noise and seismic noise measurements. We show that 0.6–2 s seismic noise can be consistently predicted over the entire year. The seismic noise recorded in this period range is mostly caused by local wind-waves, i.e. by wind-waves occurring within about 2000 km of the seismic station. Our analysis also shows that the fraction of ocean waves traveling in nearly opposite directions is orders of magnitude smaller than previously suggested for wind-waves, does not depend strongly on wind speed as previously proposed, and instead may depend weakly on the heterogeneity of the wind field. This study suggests that wind-wave conditions can be studied in detail from seismic observations, including under specific conditions such as in the presence of sea ice.

Acoustic-noise-optimized diffusion-weighted imaging.

This work was aimed at reducing acoustic noise in diffusion-weighted MR imaging (DWI) that might reach acoustic noise levels of over 100 dB(A) in clinical practice. A diffusion-weighted readout-segmented echo-planar imaging (EPI) sequence was optimized for acoustic noise by utilizing small readout segment widths to obtain low gradient slew rates and amplitudes instead of faster k-space coverage. In addition, all other gradients were optimized for low slew rates. Volunteer and patient imaging experiments were conducted to demonstrate the feasibility of the method. Acoustic noise measurements were performed and analyzed for four different DWI measurement protocols at 1.5T and 3T. An acoustic noise reduction of up to 20 dB(A) was achieved, which corresponds to a fourfold reduction in acoustic perception. The image quality was preserved at the level of a standard single-shot (ss)-EPI sequence, with a 27-54% increase in scan time. The diffusion-weighted imaging technique proposed in this study allowed a substantial reduction in the level of acoustic noise compared to standard single-shot diffusion-weighted EPI. This is expected to afford considerably more patient comfort, but a larger study would be necessary to fully characterize the subjective changes in patient experience.

A PERCEPÇÃO DOS PRATICANTES DE ATIVIDADE FÍSICA SOBRE A QUALIDADE AMBIENTAL SONORA DOS PARQUES PÚBLICOS DE CURITIBA-PARANÁ

O objetivo desta pesquisa foi analisar a percepção dos praticantes de atividade física sobre a qualidade ambiental sonora dos parques públicos de Curitiba – Paraná. Foram selecionados quatro parques: três destes localizados muito próximos e/ou cercados por vias de intenso tráfego de veículos e um situado em região mais tranquila, o qual foi considerado como área controle. Foram realizadas 328 entrevistas com os usuários destas áreas (82 em cada parque), medições acústicas e mapeamento acústico. Confirmou-se a grande influência do ruído do tráfego intenso de veículos no ambiente sonoro dos parques, onde a maioria dos pontos e regiões que apresentam níveis sonoros que excederam o limite imposto pela lei municipal de Curitiba (55 dB[A]) estão no perímetro dos parques, próximos das vias de grande movimentação. Como esperado apenas a área controle apresentou valor médio de nível de pressão sonora de acordo com a legislação local e uma boa qualidade do ambiente sonoro conforme a percepção dos usuários. Também foram identificadas outras condições no ambiente, além dos níveis de pressão sonora e forma urbana do entorno, que interferem conjuntamente na paisagem sonora dos parques e na sua percepção pelos frequentadores, tais como: o grau de incômodo ao ruído de trânsito, os tipos de sons que integram a paisagem sonora, a paisagem visual, as expectativas dos usuários aos sons, e os locais das estruturas dos equipamentos de lazer e atividade física.

TU-F-CAMPUS-I-04: Head-Only Asymmetric Gradient System Evaluation: ACR Image Quality and Acoustic Noise

Purpose: A high-performance head-only magnetic resonance imaging gradient system with an acquisition volume of 26 cm employing an asymmetric design for the transverse coils has been developed. It is able to reach a magnitude of 85 mT/m at a slew rate of 700 T/m/s, but operated at 80 mT/m and 500 T/m/s for this test. A challenge resulting from this asymmetric design is that the gradient nonlinearly exhibits both odd- and even-ordered terms, and as the full imaging field of view is often used, the nonlinearity is pronounced. The purpose of this work is to show the system can produce clinically useful images after an on-site gradient nonlinearity calibration and correction, and show that acoustic noise levels fall within non-significant risk (NSR) limits for standard clinical pulse sequences. Methods: The head-only gradient system was inserted into a standard 3T wide-bore scanner without acoustic damping. The ACR phantom was scanned in an 8-channel receive-only head coil and the standard American College of Radiology (ACR) MRI quality control (QC) test was performed. Acoustic noise levels were measured for several standard pulse sequences. Results: Images acquired with the head-only gradient system passed all ACR MR image quality tests; Both even and odd-order gradient distortion correction terms were required for the asymmetric gradients to pass. Acoustic noise measurements were within FDA NSR guidelines of 99 dBA (with assumed 20 dBA hearing protection) A-weighted and 140 dB for peak for all but one sequence. Note the gradient system was installed without any shroud or acoustic batting. We expect final system integration to greatly reduce noise experienced by the patient. Conclusion: A high-performance head-only asymmetric gradient system operating at 80 mT/m and 500 T/m/s conforms to FDA acoustic noise limits in all but one case, and passes all the ACR MR image quality control tests. This work was supported in part by the NIH grant 5R01EB010065

Parallel imaging-based reduction of acoustic noise for clinical magnetic resonance imaging.

The objective of this study was to demonstrate the feasibility of improving perceived acoustic comfort for a standard clinical magnetic resonance imaging protocol via gradient wave form optimization and validate parallel imaging as a means to achieve a further reduction of acoustic noise. The gradient wave forms of a standard T2 axial turbo spin-echo (TSE) sequence in head examinations were modified for acoustic performance while attempting to keep the total acquisition and inter-echo spacing the same. Parallel imaging was then used to double the inter-echo spacing and allow further wave form optimization. Along with comparative acoustic noise measurements, a statistical analysis of radiologist scoring was conducted on volumes from standard and modified sequences acquired from 10 patients after informed consent was obtained. Compared with TSE, significant improvement of acoustic comfort was measured for modified-sequences quiet TSE and quiet TSE with generalized autocalibrating partially parallel acquisitions (P = 0.0034 and P = 0.0003, respectively), and no statistically significant difference in diagnostic quality was observed without the use of parallel imaging. Standard clinical magnetic resonance imaging protocols can be made quieter through adequate gradient wave form optimization. In scans with high signal-to-noise ratio, parallel imaging can be used to further reduce acoustic noise.

Acoustic Green's function extraction from ambient noise in a coastal ocean environment

AbstractWe report on the results of an underwater acoustic field experiment conducted in December 2012 on the continental shelf off the Florida Keys in water of approximately 100 m depth. Ambient noise was recorded concurrently on three moored near‐bottom instruments with horizontal separations of approximately 5 km, 10 km, and 15 km. We focus in this letter on instrument pairs with 5 km and 10 km separations. Consistent with theoretical predictions, coherent sums of many realizations of cross correlations of ambient noise records at two measurement locations are shown to yield approximations to deterministic acoustic Green's functions that describe propagation between those two locations. The results presented demonstrate the feasibility of extracting information suitable for use in tomographic inversions from measurements of underwater acoustic ambient noise.

The Development of a Cross-Flow Fan for Electronic Devices Cooling with a Focus on Laptop Computers

The directive is to develop a new type of cross-flow fan for cooling electronic devices. First, the module is based on the 60X60X6mm fan and attempts to use several parameters to enhance the fan’s performance. The parameters considered here include blade number, angle of the blade, impeller diameter, divergent structure, and outlet area. Both numerical and experimental analyses are carried out on the cross-flow fan which was oriented horizontally. The ANSYS Fluent software is used in a 2D simulation to predict the heat transfer coefficient and the flow fields. Next, the flow and acoustic noise measurements are carried out at different parameters with the aid of AMCA and semi-anechoic chambers. The results show that when the outlet vent size is 70% of fan outside diameter the outcome is an enhanced fan performance of 12%. The optimal number of blades is 37. A block is inserted below the fan blade to improve the fan performance by 8%. The numerical and experimental values of air flow were plotted and found to be consistent. In summary, this study provides a systematic process to develop a cross-flow fan that can meet the requirements for thin and light laptop computers.

소음 신호의 웨이블렛 변환 및 상호상관 함수를 이용한 고장 검출 및 위치 판별

This paper presents a method of fault detection and fault localization from acoustic noise measurements. In order to detect the presence of noise sources wavelet transform is applied to acoustic signal. In addition, a cross correlation based method is proposed to calculate the exact location of the noise allowing the user to quickly diagnose and resolve the source of the noise. The fault detection system is implemented using two microphones and a computer system. Experimental results show that the system can detect faults due to artifacts accidentally inserted during the manufacturing process and estimate the location of the fault with approximately 1 cm precision.

Experimental Identification and Reduction of Acoustic Noise in Small Brushed DC Motors

The smart actuators and components designed for automotive applications have to satisfy the standards and specifications of costumers, who are becoming more and more demanding, concerning the acoustic-noise emissions. Starting from an accurate experimental test campaign, which identifies the cogging torque as the main cause of the acoustic-noise source in the considered small brushed dc motors for radiator cooling fan applications, the authors discuss and propose some alternative redesign solutions of the electrical machine that are able to mitigate the problem. The feasibilities of the proposed solutions are compatible with the competitive automotive component market and the actual automatic production lines. Several prototypes have been manufactured and tested in the same semi-anechoic chamber similar to the original motor set in order to comparatively evaluate the goodness of the potential electrical machine redesigns in terms of acoustic-noise reduction. The reported acoustic-noise measurements prove that, for the considered application, the acoustic-noise emissions can be reduced.