Sound Pressure Level Of Noise Research Articles

Large eddy simulation for jet noise characteristics of liquid rocket engine with guide groove

The study of jet noise characteristics of liquid rocket engines is a prerequisite for evaluating the noise level of rockets and effectively controlling the noise. To obtain the effect of engine operating and structural parameters on jet noise, the Large Eddy Simulation combined with the FW-H integration method is applied for the scaled engine with guide groove under different chamber pressures and nozzle expansion ratios. The simulation results show that the noise field distribution of the jet with a guide groove is highly symmetrical and has obvious directivity. The noise caused by jet impingement on the guide groove is strongly correlated to the distribution of high vorticity regions. Remarkably, higher chamber pressure leads to longer potential jet length, higher noise sound pressure levels, and greater enhancement effect of impact on noise. However, the frequency spectral characteristics are basically invariant. When the chamber pressure is constant, the increment of the nozzle expansion ratio in a certain range results in lower noise peak frequency and weaker impact effect.

Effects of thermal and acoustic environments on human comfort in urban and suburban campuses in the cold region of China.

In this study, we investigated the effects of thermal-acoustic environments on human comfort in a cold region by focusing on Xi'an in China as a representative city. Four typical open spaces were identified within two universities, with one located in an urban area and the other in a suburban area. Surveys were conducted using questionnaires and environmental parameters were measured to assess thermal-acoustic perception. The physiological equivalent temperature (PET) and noise sound pressure levels were employed to measure the thermal-acoustic environments in the four open spaces. The results showed that the neutral PET was 19.1°C in Xi'an from March to May, and the neutral temperature range was 14.9-23.4°C. The preferred temperature was 20.0°C. Acoustic sensation votes were lower in suburban areas than urban areas. Respondents in urban areas exhibited significantly higher sensitivity to the thermal and acoustic environments compared with those in suburban areas. Sensitivity to the thermal environment decreased as the environmental noise levels decreased. The temperature perceived as most suitable increased as the sensitivity to the thermal environment decreased. Acoustic comfort evaluations were generally higher in colder and warmer environments. Warmer environments heightened the sensitivity to the acoustic environment and this effect gradually decreased as the sound pressure levels increased. Thus, based on empirical analyses, we compared the effects of outdoor thermal-acoustic parameters on comfort to provide experimental data support for further outdoor thermal comfort research.

Aerodynamic noise characteristics of a centrifugal fan in high-altitude environments.

In high-altitude areas, the air is thin and the atmospheric pressure is low, which can affect the performance of centrifugal fans and aerodynamic noise. In this paper, steady and unsteady simulations of a centrifugal fan flow field are performed at altitudes of 0, 1000, 2000, 3000, 4000, and 5000 m, and the Ffowcs Williams-Hawkings equation is used to predict the aerodynamic noise of the fan. The results indicate that the tonal and broadband noise generated by the fan decrease with increasing altitude, and the A-weighted sound pressure level of each frequency band of the fan decreases when the air volume is held fixed. The maximum sound power level Lwmax, sound pressure pulsation interval, and total noise sound pressure level Lp decrease linearly with increasing altitude. For every 1000 m increase in altitude, Lwmax and Lp decrease by 0.45 dB and 1.05 dB respectively. The fan noise characteristics, performance parameters, and human auditory perception are the main factors that affect the establishment of fan noise standards in high-altitude areas.

The characteristics of noise propagation and distribution on the ultra-wide cross section of highways: A case study in Guangdong Province

In this paper, the typical 10-lane highway in the reconstruction and expansion project is taken as an example to clarify the noise propagation and distribution characteristics of ultra-wide cross section. With the aid of the highway noise prediction model based on the acoustic environment guidelines and field measurements, the noise propagation and distribution characteristics under ultra-wide cross section were summarized through visualized noise maps and quantitative data analysis. The results show that for the highway with ultra-wide cross section, reducing the speed of small vehicles can improve noise reduction in the horizontal direction by about 1 dB(A) than reducing the speed of large and medium-sized vehicles. In addition, the noise reduction effect of all floors can be improved by 0.2–0.9 dB(A) in the vertical direction, which proves that the noise reduction effect is more obvious by reducing the speed of small vehicles. Besides, the noise sound pressure level variation range on low-rise floors increases more significantly with increased traffic volume than high-rise floors. The variation between different building floors is also similar to the increase in traffic volume in the vertical direction. The result can provide theoretical guidance for the prevention of highway noise under ultra-wide cross section.

Cavitation flow and noise reduction design of bionic hydrofoil based on orthogonal optimization

Hydrofoils hold considerable academic and practical significance in the realms of marine science, energy generation, and water-based engineering. They offer enhanced speed, efficiency, stability, and maneuverability. Bionic structures have emerged as potent tools for reducing energy losses and noise in hydraulic machinery, making bionic hydrofoils a hotbed of research activity. While prominent scholars have historically directed their bionic investigations toward airfoils, operating in compressible flow fields, recent research has shifted its focus to hydrofoils. The hydrofoil's operating environment is characterized by water instead of air, featuring incompressible flow, relatively low Reynolds and Mach numbers, and notably, cavitating flow. This study presents the bionic optimization design of a wavy leading edge for the hydrofoil, employing orthogonal experimental theory. The authors establish rankings for structural parameters of bionic hydrofoils and identify optimal parameter combinations, offering an optimization strategy for selecting bionic configurations. Subsequently, the authors conduct a numerical investigation into cavitating flow, integrating the FW–H (Ffowcs Williams and Hawkings) equation for the analysis of cavitation-induced noise. Notably, this research delves into the underlying mechanisms responsible for the efficacy of bionic structures in enhancing hydrodynamic performance, particularly in the reduction of cavitation-induced noise within cavitating flow, an area scarcely explored in formal publications. The results reveal that the amplitude of the wavy leading edge exerts the most significant influence on the lift-to-drag ratio, as well as the far-field sound pressure level, followed closely by the wavelength. When compared with a baseline hydrofoil, the optimized bionic hydrofoil demonstrates a substantial 45% reduction in maximum cavity volume and a noteworthy 1.3 dB reduction in far-field noise sound pressure level. These findings underscore the capacity of the optimized bionic hydrofoil to effectively suppress cavitation and its associated noise. The established optimization strategy, focused on cavitation suppression and noise reduction, lays a robust foundation for subsequent studies involving complex working conditions.

Flow field analysis and noise characteristics of an automotive cooling fan at different speeds

As the core component of the automotive air conditioning cooling system, the automotive cooling fan directly affects the cooling performance of the entire system. Therefore, it is necessary to study the flow field and noise of the automotive cooling fan. This study focuses on the analysis off low field characteristics and noise generation in an automotive air conditioning systems, specifically related to an automobile cooling fan at different speeds. The flow field properties of the fan are examined through the distribution off low line trajectory, pressure distribution on the blade surface, and velocity distribution characteristics at various speeds. The analysis reveals that the maximum air velocity occurs at the point where the fan blade meets the wind guard, while the maximum pressure is observed at this point and at the edge where the fan blade cuts through the cavity. Furthermore, a monitoring point located 1 m in front of the fan is selected to study the average value of fan pneumatic noise pulsation and generate a noise cloud map, aiming to identify the main noise source of the fan. To better understand the noise distribution and variation at different speeds, various noise metrics are utilized, including noise Sound Pressure Level (SPL) spectrum, 1/3-octave SPL spectrum, and 1/3-octave A-weighted SPL spectrum. The noise characterization results indicate that the primary areas of fan noise generation are located at the tip of the leaf margin of the forward swept wing. This research provides valuable insights for the improvement and optimization of the cooling fan design, as well as subsequent noise reduction efforts.

Dysregulations of metabolites and gut microbes and their associations in rats with noise induced hearing loss.

Noise exposure could lead to hearing loss and disorders of various organs. Recent studies have reported the close relations of environmental noise exposure to the metabolomics dysregulations and gut microbiota disturbance in the exposers. However, the associations between gut microbial homeostasis and the body metabolism during noise-induced hearing loss (NIHL) were unclear. To get a full understanding of their synergy in noise-associated diseases, it is essential to uncover their impacts and associations under exposure conditions. With ten male rats with background noise exposure (≤ 40 dB) as controls (Ctr group), 20 age- and weight-matched male rats were exposed to 95 dB Sound pressure level (SPL) (LN group, n = 10) or 105 dB SPL noise (HN group, n = 10) for 30 days with 4 h/d. The auditory brainstem response (ABR) of the rats and their serum biochemical parameters were detected to investigate their hearing status and the potential effects of noise exposure on other organs. Metabolomics (UPLC/Q-TOF-MS) and microbiome (16S rDNA gene sequencing) analyses were performed on samples from the rats. Multivariate analyses and functional enrichments were applied to identify the dysregulated metabolites and gut microbes as well as their associated pathways. Pearson correlation analysis was performed to investigate the associations of the dysregulations of microbiota and the metabolites. NIHL rat models were constructed. Many biochemical parameters were altered by noise exposure. The gut microbiota constitution and serum metabolic profiles of the noise-exposed rats were also dysregulated. Through metabolomics analysis, 34 and 36 differential metabolites as well as their associated pathways were identified in LN and HN groups, respectively. Comparing with the control rats, six and 14 florae were shown to be significantly dysregulated in the LN group and HN group, respectively. Further association analysis showed significant correlations between differential metabolites and differential microbiota. There were cochlea injuries and abnormalities of biochemical parameters in the rats with NIHL. Noise exposure could also disrupt the metabolic profiles and the homeostatic balance of gut microbes of the host as well as their correlations. The dysregulated metabolites and microbiota might provide new clues for prevention of noise-related disorders.

Experimental and Mechanism Study of Aerodynamic Noise Emission Characteristics from a Turbocharger Compressor of Heavy-Duty Diesel Engine Based on Full Operating Range

Heavy-duty diesel engines equipped with turbochargers is an effective way to alleviate energy shortage and reduce gas emissions, but their compressor aerodynamic noise emissions have become an important issue that needs to be addressed urgently. Therefore, to study the aerodynamic noise emission characteristics of a compressor during the full operating range, experimental and numerical simulation methods were used to analyze the aerodynamic noise emissions. The results showed that aerodynamic noise’s total sound pressure level (SPL) increased with increased speed under the test conditions. At low speeds, the total SPL of aerodynamic noise was affected by the mass flow of the compressor more obviously. The maximum difference of aerodynamic noise total SPL was 1.55 dB at 60,000 r/min under different mass flows. At the same speed, the compressor could achieve lower aerodynamic noise emissions by operating in the high-efficiency region (middle mass flows). In the compressor aerodynamic noises, the blade passing frequency (BPF) noise played a dominant role. The transient acoustic-vibration spectral characteristics and fluctuation pressure analysis indicated that BPF and its harmonic frequency noises were mainly caused by the unsteady fluctuation pressure. As the speed increased, the BPF noise contributed more to the total SPL of the aerodynamic noise, and its percentage was up to 75.35%. The novelty of this study was the analysis of the relationship between compressor aerodynamic noise and internal flow characteristics at full operating conditions. It provided a theoretical basis for reducing the heavy-duty diesel engine turbocharger compressor aerodynamic noise emissions.

Hearing Care: Safe Listening Method and System for Personal Listening Devices.

Excessive use of Personal Listening Devices (PLDs) and prolonged exposure to noise from loud music create many potential risks associated with hearing loss. To this end, the World Health Organization has published Recommendation ITU-T H.870 in 2019, which provides adults and children with a set of recommendations for sound dosage and operating times needed to avoid potential hearing risks. Some studies have investigated noise exposure of related applications for listening safety, resulting in some related recommendations and applications; however, these studies often do not pay attention to measurement error, which is important for human real noise exposure estimation to avoid hearing loss. This paper proposes a method for calculating noise exposure that can accurately calculate the actual noise sound-pressure level (SPL) and PLD dosage based on the WHO-ITU standard. We develop a calculation method and design a listening system that includes (i) a Safe Listening Personal Listening Device (SL-PLD) that can measure the listening dose in real time and control the output volume effectively, (ii) a Safe Listening Application (SL-APP) for assisting the SL-PLD to check the listening status in real time and provide alerts. Our experimental results show that (i) the proposed noise calculation method can reach 0.88 dB deviation under the 76 dB reference SPL and 98.8% accuracy, as compared to the SoundCheck tool measurement, (ii) the proposed SL-PLD controls the SPL output effectively as the dose increases, and (iii) the SL-APP determines the dosage usage and will provide a warning when the dosage exceeds a preset value. Therefore, users can adjust their listening behavior for more secure listening by using our methods and applications.

A Dynamic RCS and Noise Prediction and Reduction Method of Coaxial Tilt-Rotor Aircraft Based on Phase Modulation.

For tilt-rotor aircraft with coaxial rotors (coaxial rotor aircraft), reduction of radar cross section as well as acoustic noise can be essential for stealth design, and the rotation of the coaxial rotors can have an influence on noise and dynamic radar cross section (RCS) characteristics. In this paper, an approach to the prediction of both the sound pressure level (SPL) of noise and the dynamic RCS of coaxial-tilt aircraft is carried out, based on the theories of the FW-H equation, the physics optics method (PO) and the physical theory of diffraction (PTD) method. In order to deal with the rotating parts (mainly including coaxial rotors), a generated rotation matrix (GRM) is raised, aiming at giving a universal formula for the time-domain grid coordinate transformation of all kinds of rotation parts with arbitrary rotation centers and rotation axis directions. Moreover, a compass-scissors model (CSM) reflecting the phase characteristics of coaxial rotors is established, and a method of noise reduction and RCS reduction based on the phase modulation method is put forward in this paper. The simulation results show that with proper CSM parameter combinations, the reduction of noise SPL can reach approximately 3~15 dB and the reduction of dynamic RCS can reach 1.6 dBsm at most. The dynamic RCS and noise prediction and reduction method can be meaningful for the radar-acoustic stealth design of coaxial tilt-rotor aircrafts.

Annoyance due to amplitude modulated low-frequency wind farm noise: A laboratory study.

This study tested for differences in perceived annoyance and loudness between road traffic noise (RTN) and wind farm noise (WFN) with amplitude modulation (AM) and tonality. Twenty-two participants, who were primarily university students with no previous exposure to WFN and aged between 19 and 29 (mean, 22 years old; standard deviation, 2) years old with normal hearing, underwent a laboratory-based listening test. Each participant rated perceived annoyance and loudness of WFN and RTN samples played at sound pressure levels (SPLs) ranging from 33 to 48 dBA. Probability modeling revealed that participants were the largest source of variability in ratings of perceived annoyance and loudness while noise type and SPL were relatively minor sources. Overall, no differences were found between WFN and RTN perceived annoyance or loudness ratings. On the other hand, no substantial differences in annoyance were found between low-frequency tonal AM and mid-to-high-frequency AM or "swish" WFN.

Experimental investigation on two-phase flow noise characteristics of R410A through electronic expansion valve of multi-split air conditioner

The refrigerant flowing through an electronic expansion valve (EEV) of a multi-split air conditioner is usually in two-phase state which may generate more severe flow noise than that in sub-cooled state. The purpose of this paper is to experimentally investigate the influences of the two-phase R410A operation conditions on the flow noise characteristics in the EEV, including listening sense forms, frequency distributions and sound pressure levels (SPLs). The experimental conditions cover the refrigerant mass flow rate ranging from 30 to 90 kg h–1 and the refrigerant quality ranging from 0.02 to 0.4. The results show that low mass flow rate facilitates the reduction of noise frequency, and the peak noise frequency was reduced by up to 32% as the mass flow rate decreased. Low refrigerant quality helps to reduce the SPL of flow noise, and the average SPL decreased by 6 dB as the refrigerant quality decreased. The guidance of inlet flow pattern by transforming the churn flow to the annular flow helps to simultaneously reduce the noise frequency and SPL, and the peak noise frequency as well as the averaged SPL decreased by 47% and 4 dB, respectively. Moreover, the hissing noise and the gurgling noise are the only two kinds of listening sense forms of flow noise detected in the present experiment, and the peak frequency and the average SPL of hissing noise are both higher than those of gurgling noise.

Experimental Investigation of Task Performance and Human Vigilance in Different Noise Environments

Twelve healthy male college-age students were recruited to investigate the effects of different noise exposure conditions on complex task performance and vigilance. During each noise exposure, the Multi-Attribute Task Battery (MATB) with low, medium, and high mental workloads were conducted in the order designated by the Latin square method. Meanwhile, a psychomotor vigilance test (PVT) was used to evaluate human vigilance. Heart rate variability (HRV) signals were also collected while participants performed the tasks. The generalized additive mixed-effect model (GAMM) results showed that the increased mental workload had an inverted U-shaped effect on MATB task performance. Noise exposure had no significant impact on the overall performance of MATB tasks. However, when exposed to increased noise sharpness at low mental workloads, Tracking Task (TRA) performance significantly decreased, whereas the System Monitoring Task (SYS) performance was significantly improved. In addition, higher noise sound pressure level and sharpness would impair human vigilance, which was reflected in a lower mean sample entropy of HRV and worse performance on the PVT. The results indicated that noise control in the workplace should consider both sound pressure level and sharpness.

Developing multivariate models for predicting the level of dissatisfaction due to a specific metro noise masked with four specific water sounds

Incorporating pleasant sounds into traffic noises is an important design strategy to optimize noise perception. However, as the previous studies based on the auditory masking method mainly focused on road traffic or aircraft noises, there is a lack of research on alleviating the negative perceptions due to metro noise by adding other sounds. Accordingly, this study analyzed the regulation mechanisms of four specific water sounds (fountain, stream, water-drop and waterfall sounds) on the dissatisfaction due to a specific metro noise and established the multivariate models to predict the dissatisfaction level due to combined metro-water sounds. Seventy-nine subjects participated in the experiment and were exposed to a series of acoustic samples before being asked to cast their negative perception votes. The results suggested that the dissatisfaction with metro noise could be improved by adding the four types of water sounds, among which the capability of water-drop sound was the strongest, followed by the fountain and stream sounds, and the waterfall sound was the least effective. Besides, in order to reduce the dissatisfaction level at the global sound pressure levels (SPLs) of 60 and 65dBA, the SPL of metro noise should be no more than 3 dBA higher than that of water sound, and a water signal-to-noise ratio (WSNR) of 6 dBA was most desirable. Furthermore, among the considered models (energy summation model, independent effects model and energy difference model), the independent effects model, which fitted the dissatisfaction level data moderately, yielded the relatively highest coefficients of determination after one relevant acoustic factor (WSNR Threshold) and personality traits (gender and sensitivity) were added. It is worth noting that a single metro noise and a single sample for each type of water sound were selected and their combined sounds were studied, thus, this study could be considered as a preliminary work and the results and applicability have limitations.

Investigation of Flow-Induced Noise Characteristics in the Oblique Flow Pump

An oblique flow pump is widely used in farmland irrigation and drainage, water transfer projects, thermal power generation, and other fields. However, unstable flow factors in the oblique flow pump easily lead to pump vibration and noise. To improve the stability of pump operation and optimize the operating environment, it is necessary to study the flow-induced noise characteristics of oblique flow pumps. In this paper, CFD and noise simulation software are used to calculate the flow field and sound field of the oblique flow pump. The internal flow characteristics and flow-induced noise characteristics of the oblique flow pump were studied. The results show that when the flow rate of the oblique flow pump deviates from the optimal operating point, especially in the small flow rate, due to the phenomena of backflow and flow separation, more high-energy vortices are produced in the flow channel, and the vortices are distributed in a wide area, which will cause greater flow-induced noise. It is found that with the increase in flow rate, the sound pressure level of flow noise in the inlet channel gradually decreases, the sound pressure level in the guide vane region first decreases and then increases, and the sound pressure level is the lowest at 1.0Q. Generally speaking, the noise sound pressure level in the outlet channel region of the pump also gradually decreases. In addition, at the blade frequency, the radiated noise of the oblique flow pump can show dipole characteristics under various flow conditions, and the radiation level of sound pressure increases with the increase in flow rate. This paper can provide a theoretical basis for the mechanism research and control strategy of flow-induced noise in oblique flow pumps.

Evaluations of the Annoyance Effects of Noise

Noise is defined as audible sound that disrupts silence and causes annoyance. Such annoyance is traditionally assessed by the A-weighted sound pressure level of noise, roughly corresponding to the level of perceived loudness. However, the A-weighted scale is inapplicable for analyzing tonal, pulsed, and predominantly low-frequency noise; therefore, methods have been developed for calculating noise loudness in linear units, sones, which take into account not only auditory sensitivity, but also masking properties and auditory temporal effects. The existence of noise reduction limits and their informational significance have led to other methods for assessing noise annoyance. Annoyance, in addition to loudness, is caused by such subjective noise qualities as sharpness, roughness, fluctuation strength, tonality, etc. Units of measurements have been defined for these and calculation methods developed. Taking such qualities into account, a metric of short-term psychoacoustic annoyance (PAA) has been proposed, which is valid for many types of noise. Another method for evaluating PAA includes conducting an auditory examination and constructing a mathematical model that relates the rank of auditory annoyance to the measured subjective qualities of the noise. The resulting model helps to identify the causes of annoyance (subjective annoying qualities); develop work plans aimed at noise suppression, the formation of pleasant noise by machines and mechanisms; and control changes in the rank of annoyance in the implementation of such plans. This paper compares different methods for assessing the annoyance caused by noise recorded in Moscow subway cars.

Numerical Simulation Study of Aerodynamic Noise in High-Rise Buildings

In order to study the aerodynamic noise on the surfaces of high-rise buildings under the action of strong winds, this paper numerically simulated the sound pressure field on the surface of a high-rise building using the large-eddy simulation method combined with the acoustic analog method of FW-H (Ffowcs Williams–Hawkings) equation and obtained the intensity radiation distribution of sound pressure on the surface of the building to further identify the area with the maximum sound pressure intensity of the noise radiation and thus achieve the purpose of locating noise source. The accuracy of the numerical simulation results for aerodynamic noise obtained in this paper was then verified by comparing with the acoustic wind tunnel experimental results. The locations of noise source obtained by numerical simulation and acoustic wind tunnel experiment were in good agreement. The sound pressure intensity pulsation time course was measured by the acoustic wind tunnel experiment, and the noise sound pressure level spectrum of each part of the building surface was obtained by fast Fourier transform (FFT). Furthermore, the spectral characteristics of the noise sound pressure level were analyzed. The results of the sound pressure level spectrum of aerodynamic noise obtained from the numerical simulation were compared with the acoustic wind tunnel experimental results, which were found to be very similar. The analysis of the sound pressure level spectrum of aerodynamic noise on the building surface reveals that the numerical simulation results in the middle- and high-frequency bands of the spectrum are in good agreement with the acoustic wind tunnel experimental results, but there is a difference between those in the low-frequency bands and the acoustic wind tunnel experimental results. The microphone array used to locate the noise source in the acoustic wind tunnel was found to suffer non-eliminable measurement errors, which might be a potential reason for a reasonably slight difference between the experimental and numerical simulation results. The background noise in the low-frequency band of the acoustic wind tunnel sound pressure level spectrum was relatively large, while there was basically no background noise in the numerical simulation. This paper shows that the numerical simulation method combined with large-eddy simulation and acoustic analogy (FW-H) can calculate the aerodynamic noise intensity at various points on the surfaces of high-rise buildings and reasonably predict the location of sound source. In addition, the numerical simulation results are similar to the acoustic wind tunnel experimental results in most frequency bands.

A Rattle Noise Severity Index for Multi-Mesh Gear Trains Subjected to Torque Fluctuations

AbstractExternal fluctuation torques acting on gear trains having clearances often cause vibro-impact motions with resultant rattle noise issues. In this study, an impact velocity-based rattle severity parameter that correlates with the resultant rattling noise is proposed for multimesh gear trains. An experimental setup is employed to measure torsional vibro-impact motions and the corresponding sound pressure levels of a three-axis gear train under various torque fluctuation conditions. A discrete torsional model of the experimental setup is developed and validated through comparisons to the vibration measurements. An impact velocity-based rattle severity index defined from the predicted response is proposed and shown to correlate well with the measured rattle noise sound pressure levels within a wide range of operating conditions. The demonstrated ability of the proposed rattle severity index in tracking rattle noise allows for the assessment of rattle consequences of a multimesh drivetrain solely from its predicted torsional response.

Numerical simulation of aerodynamic noise for the two-stage adjustable-blade axial-flow fan

In this paper, a three-dimensional calculation model of a two-stage adjustable-blade axial-flow fan is established and verified by grid independence and numerical accuracy. The pressure distribution and sound power-level distribution characteristics of the blade surface are explored with variable blade installation angles. Based on the Q-criterion, the study reveals the spatial distribution of the channel and trailing-edge shedding and channel vortexes in the flow field. Then, the evolution laws of the fan's aerodynamic noise sound pressure level are also investigated, and its frequency domain characteristics with variable blade installation angles are obtained. The results show that when the rotor blade installation angle is −5°, the front-guide vane matches the installation angle of the first-stage impeller. The upper limit of sound power level is the smallest with variable blade installation angles, which is 123.56 dB. Meanwhile, the number and size of vortex structures in the front-guide vane area are the smallest, and the turbulent flow in the flow field is moderate. As the moving blade installation angle is deflected from −10° to 10°, the total sound pressure level of aerodynamic noise at each component of the fan first decreases and then increases. The minimum value is 121.40 and 128.40 dB at the inlet and outlet when the blade installation angle is −5°. In addition, the number of eddies periodically shed in the fan flow field is the least. This research can supply technical support for the noise reduction of the two-stage adjustable blade axial fan.

Research on design method of metamaterial sound field control barrier based on transformer vibration and noise

In order to analyze the propagation path of vibration noise, and carry out finite element numerical calculation research based on electro-magnetic–solid-acoustic multiphysics coupling, a single-phase two-column dry-type transformer is used as the noise source in this paper. According to the distribution characteristics of the spatial sound field and the vibration noise amplitude–frequency characteristics of the transformer, a topology design method of the space coiled acoustic metamaterial (SCAMM) unit is proposed and the analytical calculation methods of acoustic parameters such as sound pressure reflection/transmission coefficient of various SCAMM units are deduced. Besides, the effects of parameters such as the longitudinal width a, the waveguide width b, the ratio of waveguide width to longitudinal width b/a and transverse width t on the SCAMM unit sound transmission loss (STL) are studied, and the anti-resonance frequencies are adjusted by parameter optimization method to achieve the optimal sound insulation effect. In the end, the sound field control barrier based on the SCAMM for transformer vibration and noise suppression is designed, and relevant experiments are carried out. The experiments show that the sound field control barrier based on the SCAMM can reduce the transformer noise sound pressure level (SPL) of the main noise frequencies by up to 25 dB with a thickness of only 4 cm and ventilation.