Decrease In Sound Pressure Level Research Articles

Aerodynamic noise analysis of tilting rotor in edgewise flow conditions

This paper presents a comprehensive experimental analysis of the noise characteristics of an isolated tilt-rotor system under edgewise flight conditions. The investigation explores the effect of rotor tilt on noise and aerodynamics using flow velocity, thrust, and far-field noise measurements. Flow field results show that as the tilting angles vary, there is a change in speed over the rotor blade surface, which influences vortex formation and wake attributes. The far-field noise spectra revealed a significant decrease in sound pressure level with increasing tilt angle, predominantly evident in both the high-amplitude tonal peaks and the broadband signal within the low-to-mid frequency range. The directivity patterns of the overall sound pressure level also demonstrated a reduction in magnitude with the tilting of the rotor. This reduction was observed consistently across all top-plane observation points, with a monopole directivity trend. Meanwhile, in the side plane array, the decrease was primarily evident above the plane of rotation with a dipole directivity pattern. The radiation direction at which the minimum sound pressure level occurs was identified near the rotation plane, irrespective of the tilt angle. Furthermore, the time-dependent noise analyses revealed dominant and persistent signal characteristics at the blade passing frequency, whereas the mid-frequency range exhibited more intermittent behaviour, and the high-frequency range indicated transient characteristics.

A numerical study on the benefits of passive-arc plates on drag and noise reductions of a cylinder in turbulent flow

In this study, we introduce a novel arrangement consisting of two arc plates around a cylinder with the privilege of improved fluid flow and noise control. The arc plates are placed symmetrically and concentrically at the rear portion of a circular cylinder. The coverage angle (30 °≤β≤75°) of the plates and the normalized radius of arc plates (1.125≤Rd≤1.625) are varied to find the optimum case in terms of drag and noise reductions. The simulations are performed for a turbulent flow with a Reynolds number of 22 000. The numerical analysis is based on an unsteady Reynolds-averaged Navier–Stokes (URANS) solver and Ffowcs Williams and Hawkings (FW–H) acoustic analogy. It is found that by implementing the arc plates, the noise level and drag coefficient decrease dramatically. The results also reveal a strong correlation between the vortex shedding suppression and the noise reduction. It is shown that as the fluctuation of lift force decreases, the performance of flow and noise control enhances simultaneously. Furthermore, the noise assessment indicates that in a specific configuration of the arc plates, the overall sound pressure level decreases by around 51 dB compared to the uncontrolled case with no arc plates. Also, a maximum noise reduction of 27 dB is achieved, in which the drag coefficient reduces by 39% compared to the case with no arc plates. In conclusion, the results provide strong support for the proposed passive method as a beneficial strategy for noise reduction and wake control of cylindrical structures, which have wide applications in industry.

Investigation of the Interaction Tones Produced by Contra-Rotating Unmanned Aerial Vehicle Rotor Systems

This paper investigates the interaction tones produced by the periodic unsteady loading on the blades of two different contra-rotating unmanned aerial vehicle (UAV) rotor systems using computational, analytical, and experimental methods. Computational fluid dynamics (CFD) simulations are used to predict the unsteady loading on the rotor blades and to visualize the flow through the rotors. Semi-analytical models are also presented for predicting the portion of this unsteady loading due to the interaction of each blade with the bound potential field of the adjacent rotor. A comparison of the unsteady loading predicted using the CFD simulations and the semi-analytical models shows that, for the cases considered, the semi-analytical models accurately predict the unsteady loading on the top rotor and that the bound potential field interaction source is a significant contributor to the total unsteady loading on the bottom rotor. A frequency-domain acoustic analogy method is also presented, which is used to calculate the interaction tones produced by the unsteady loading on the rotor blades at various observer locations. Predictions of the sound pressure levels of the prominent interaction tones at different observer locations are presented and compared with experimental measurements showing generally good agreement. The effect of rotor spacing on the amplitude of the interaction tones is also explored, and it is shown that the methods predict a decrease in sound pressure level produced by increasing the rotor spacing and that these predictions are in reasonable agreement with experimental measurements.

Characteristics of auditory brainstem response in wild yellow grouper Epinephelus awoara

Auditory brainstem response (ABR) technology is used to set up a fish auditory threshold detection system, and the auditory threshold of wild yellow grouper Epinephelus awoara caught in the Taiwan Strait is studied. The ABRs of 10 wild yellow groupers (TL150-200 mm) to 12 different frequencies of sound stimulation from 100 to 2000 Hz are recorded and analyzed. The results show that the auditory threshold of wild yellow grouper fish increases with the increase of frequency in the frequency of 200-2000Hz. The fish is sensitive to low frequency sound signal of 100-300Hz with the most sensitive frequency of 200Hz, and the auditory threshold is 90 dB re 1μPa. The ABR waveform shows three periods: response period, real hearing period and extension period. In the range of 100-2000Hz, the response time decreases with the increase of frequency at the same sound pressure level. At the same frequency, the response time increases with the decrease of sound pressure level. The first amplitude sound pressure of real hearing period is between 50-150nv, and the real hearing period contains the greatest amplitude. When the greatest amplitude is about 50nv and the next sound pressure level is below 50nv, the sound pressure level is the auditory threshold. The extension period is proportional to the response period.

Multi-objective optimization design in a centrifugal pump volute based on an RBF neural network and genetic algorithm

The hydraulic and acoustic performance of centrifugal pumps are interrelated and contradictory in hydraulic structure design. An optimization design method of a volute based on a radial basis function (RBF) neural network and genetic algorithm (GA) is proposed to solve this problem. The efficiency and total sound pressure level of the centrifugal pump are used as the optimization objectives. The diameter of the base circle, the width of the base circle, the installation angle of the volute tongue, and the height of the volute diffuser tube are used as the optimization variables. Latin hypercube sampling (LHS) is used to establish the sample space, the RBF neural network is used to build the agent model between the optimization variables and objectives, and the GA is used to multi-objective optimization. For the Pareto solution set obtained, two extremal individuals and initial individuals are selected for comparative analysis of hydraulic and acoustic performance under different working conditions. The result demonstrates that under the rated working condition, compared with the initial individual, the efficiency of the optimal individual of efficiency increases by 3.79%; the internal noise of the optimal individual of sound pressure level decreases by 5.5%, and the external noise decreases by 2.3%.

Occluded insertion loss from intracochlear pressure measurements during acoustic shock wave exposure

IntroductionInjuries to the peripheral auditory system are among the most common results of high intensity impulsive noise exposure. Hearing protection can mitigate this injury, but careful assessment of the insertion loss they provide is necessary. Insertion loss is typically measured using microphone-based acoustic manikins to measure the decrease in sound pressure level transmitted into the ear canal, which precisely measure the change in air conducted sound, but neglect alternate pathways to the inner ear such as bone conduction. In a previous study we reported intracochlear pressures in cadaveric human specimens to acoustic shock waves, which revealed a substantial bone conducted component (Greene, et al., 2018). Here we evaluate insertion loss to several hearing protection devices (HPDs) in those same specimens using intracochlear pressure measurements. MethodsHuman cadaver heads were exposed to impulsive acoustic pressure waves with peak overpressures of 7 and 28 kPa (171 & 183 dB SPL). Ear canal (EAC), middle ear, and intracochlear sound pressure levels were measured bilaterally with fiber-optic pressure sensors. Surface-mounted sensors measured SPL and skull strain near the opening of each EAC and at the forehead. Responses were measured with specimen ears unoccluded, as reported previously, as well as fitted with four types of HPDs. Impulse peak insertion loss (IPIL) and impulse spectrum insertion loss (ISIL) were calculated for each HPD. ResultsFor all HPDs, IPIL generally increases with exposure level, though ISIL tended to be more consistent, and the spectral characteristics across frequency appear to be highly dependent on exposure level. ISIL measured in the ear canal tended to overestimate insertion loss measured in the cochlea, particularly at frequencies > 1 kHz; however, low signal-to-noise in intracochlear pressures limited comparisons. As a proof of concept, 36 low-level unoccluded exposures, were averaged together, and the resulting signal-to-noise ratio was improved by up to 15 dB. ConclusionsInsertion loss measured in the cochlea was lower than in the ear canal, suggesting substantial contributions from transmission pathways in parallel with air conduction (e.g., bone conduction) were present, which will require novel strategies to mitigate. However, high variance was observed, and noise reduction strategies should be utilized in future studies to facilitate more precise insertion loss estimates.

Determination of optimal flight altitude to minimise acoustic drone disturbance to wildlife using species audiograms

Abstract Unmanned aerial vehicles (UAVs) are increasingly important in wildlife data collection but concern over wildlife disturbance has led several countries to ban their use in National Parks. Disturbance is an animal welfare concern and impedes scientific data collection through provoking aberrant behaviour. Dealing with the issue of disturbance will enable wildlife researchers to use UAV technology more effectively and ethically. Here we present a novel method to determine optimal flight altitude for minimising drone disturbance for wildlife using species audiograms. We recorded sound profiles of seven common UAV systems in the horizontal and vertical planes at 5‐m increments up to 120 m. To understand how mammals perceive UAV sound, we used audiograms of 20 species to calculate the loudness of each UAV for each species across the measured distances. These calculations filter the UAV noise based on the sensitivity of species’ hearing over the relevant frequency spectrum. We have devised a method to optimise the trade‐off between image spatial resolution and flight altitude. We calculated the lowest point at which either the UAV sound level decreases below an acceptable threshold, here chosen as 40 dB, weighted according to species’ hearing sensitivity, or disturbance cannot be significantly further minimised by flying higher. The latter is quantified as the point above which each additional 5 m of flight altitude causes on average less than 0.05 dB decrease in sound pressure level. Reliable data on appropriate flight altitudes can guide policy regulations on flying UAVs over wildlife, thus enabling increased use of this technology for scientific data collection and for wildlife conservation purposes. The methodology is readily applicable to other species and UAV systems for which sound recordings and audiograms are available.

Numerical Study on Control of Sunroof Buffeting

Sunroof has become one of the essential features of a luxury car, and it provides natural air circulation and good illumination into the car. But the primary problem associated with it is the buffeting noise which causes discomfort to the passengers. Though adequate studies were carried out on sunroof buffeting, efficient control techniques are needed to be developed from fundamental mechanism. To reduce the buffeting noise, flow modifications at the entrance of the sunroof is considered in this study. The internal portion of the car with sunroof is simplified into a shear driven open cavity, and two-dimensional numerical simulations are carried out using commercial solver, ANSYS Fluent. Reynolds averaged Navier-Stokes equation is used with the realizable k-? turbulence model. The unsteady numerical result obtained in this study is validated with the available experimental results for the dominant frequency. The prediction is good agreement with experiment. Flow modification technique is proposed to control the sunroof buffeting by implementing geometric modifications. A hump has been placed near the leading edge of the cavity which resulted in significant reduction of pressure oscillations. Parametric studies have been performed by varying the height of hump and the distance of hump from the leading edge. There is no prominent difference when the height of the hump is varied. As the distance of the hump from the leading edge is reduced, the sound pressure level decreases.

THE USE OF AN RBF NEURAL NETWORK AND A GENETIC ALGORITHM FOR MULTI-OBJECTIVE OPTIMISATION OF A CENTRIFUGAL PUMP VOLUTE

When it comes to the construction of hydraulic structures, the hydraulic performance and acoustic performance of centrifugal pumps are connected and conflicting. In order to find a solution to this issue, a technique for optimising the design of a volute that is based on a radial basis function (RBF) neural network and a genetic algorithm (GA) has been developed. The effectiveness of the centrifugal pump as well as the total amount of sound pressure level are employed as the targets for optimisation. The factors that are used for optimisation purposes are the installation angle of the volute tongue, the height of the volute diffuser tube, the installation angle of the volute tongue, and the diameter of the base circle. The Latin hyper-cube sampling (LHS) method is used to construct the sample space. The RBF neural network method is used to develop the agent model between the optimisation variables and goals. Finally, the GA method is utilised to do multi-objective optimisation. In order to do a comparative investigation of the hydraulic and acoustic performance of the persons in the Pareto solution set under a variety of different working situations, the initial individuals and two individuals from the set's extremes are chosen. The findings indicate that under the rated working conditions, the efficiency of the optimal individual of efficiency increases by 3.79%, while the internal noise of the optimal individual of sound pressure level decreases by 5.5% and the external noise decreases by 2.3%. The results also show that the initial individual had a lower level of efficiency than the optimal individual.

Impact of Hearing Loss and Universal Face Masking in the COVID-19 Era

Impact of Hearing Loss and Universal Face Masking in the COVID-19 Era

Analysis of the flow noises of the nuclear main pump caused by the high temperature liquid Sodium in the two-circuit main loop liquid Sodium pump system

The paper investigates the flow noises caused by the high temperature liquid Sodium in the nuclear main pump system. Three-dimensional unsteady CFD coupled sound field methods are exploited. Spectrum characteristics of sound source load are obtained. Directional sound pressure level distribution based on sound source load of rotor blades and guide vanes are presented and discussed. Distribution of sound pressure level of inlet and outlet pipes along the flow direction are also further discussed. Results indicate that with the increase of blade passing frequency, the amplitude of sound pressure level decreases. Response of sound field mainly depends on the load characteristics of rotor blades. Research results have remarkable significances for the analysis of flow noises in liquid Sodium pump systems.

Axial Spacing Effects on Rotor-Rotor Interaction Noise and Vibration in a Contra-Rotating Fan

Because of the potential technical advantages, the contra-rotation technology has become a renewed interest in aviation and other applications. Contra-rotation increases efficiency in comparison with the single-rotor design, but this advantage is not fully harnessed. The axial spacing of two-stage contra-rotating blade rows has a significant impact on a contra-rotating fan/compressor. The results show that with a contra-rotation pattern, the strong unsteadiness of two-stage rotors is caused by the rotor-rotor interaction. The unsteadiness of rotor 1 is caused by the potential disturbance, and the upstream wake leads to the strong unsteadiness of rotor 2. With the increase of axial spacing, the rotor-rotor interaction is weakened, while unsteady features of two-stage rotor blades tend to be consistent. The acoustic and vibration effects of axial spacing are studied. It is found that the axial spacing has great influence of aerodynamic noise. The mean value of sound pressure level decreases by 17.2 dB in total when the axial spacing increased to 1.1 chord from 0.3 chord. For the accuracy of calculation, the scattering effect of the casing wall should be considered in the prediction of the noise. The axial spacing does not have obvious effects on the natural frequencies of the two-stage rotor blades but has certain effect on blade deformation.

Prediction of sound transmission in long spaces using ray tracing and experimental Statistical Energy Analysis

Long spaces such as corridors, train carriages and aircraft cabins often have a significant decrease in sound pressure level along their length when a sound source is positioned at one end. However, the more common situation for noise control involves multiple sound sources positioned along their length. In order to use Statistical Energy Analysis (SEA) to predict spatial-average sound pressure levels along a long space when it is subdivided into a series of adjacent subsystems with one or more point sources, this paper combines ray tracing and a general form of experimental SEA (GESEA) to determine the loss factors. Predictive SEA that includes only direct coupling between subsystems (based on the open area) tends to over- or under-predict the decrease in SPL that is determined from ray tracing. This occurs when there is a point source at one end of an elongated cuboid that is either empty, has staggered barriers, or symmetrically-placed barriers. However, SEA using all Coupling Loss Factors (CLFs) determined from GESEA which incorporates indirect coupling and any negative CLFs shows almost exact agreement with ray tracing. This agreement is maintained when the point source is positioned in other subsystems, or point sources are positioned in multiple subsystems. This provides evidence of the importance of indirect coupling in long spaces, as has previously been identified with structure-borne sound transmission along a series of in-line coupled subsystems.

Vibro-acoustic analysis of coupled tyre model of acoustic cavity and tread structure

A coupled model of tyre tread band and acoustic cavity is presented in this paper. Modal and harmonic analyses show that this simplified coupled structure can resemble the noise characteristics of the tyre with great accuracy. Then, we use the coupled model to study the impact of effective structural and fluid properties on acoustic pressure and sound pressure level, as well as tyre tread deformations. Results indicate that the use of helium and hydrogen moves the first resonant frequency to above 250 Hz and displaces the second peak to above 400 Hz, and hydrogen leads to more than 3 dB decrease in sound pressure level.

Vibro-acoustic analysis of coupled tyre model of acoustic cavity and tread structure

A coupled model of tyre tread band and acoustic cavity is presented in this paper. Modal and harmonic analyses show that this simplified coupled structure can resemble the noise characteristics of the tyre with great accuracy. Then, we use the coupled model to study the impact of effective structural and fluid properties on acoustic pressure and sound pressure level, as well as tyre tread deformations. Results indicate that the use of helium and hydrogen moves the first resonant frequency to above 250 Hz and displaces the second peak to above 400 Hz, and hydrogen leads to more than 3 dB decrease in sound pressure level.

Noise Analysis, Calculation, and Reduction of External Rotor Permanent-Magnet Synchronous Motor

A detailed analysis of electromagnetic noise in external rotor permanent-magnet synchronous motors is presented in this paper. First, the spatial distribution and frequency characteristics of the electromagnetic force acting on the surface of the permanent magnet are discussed. Then, calculation models for electromagnetic force, structural vibration, and acoustic radiation are developed to predict noise by taking an external rotor in-wheel motor as example. The uneven distribution of electromagnetic force on the surface of the permanent magnet is taken into account by means of loading nodal force into the structural model which is verified by modal test. Through the mode superposition method, the vibration on the surface of the outer rotor is calculated, and the acoustic boundary element method is used to predict the acoustic radiation. Noise test is conducted to validate the simulated noise. It is shown in both simulation results and noise test that the electromagnetic force due to slotting effects contributes the most remarkable component to the overall noise. Finally, slot opening width is optimized to reduce the amplitude of magnetic force close to resonance frequencies, and the overall sound pressure level decreases by 6 dB(A) after optimization.

Identification of noise generation and flow kinematics in the air gap for two different blade tip designs of an axial fan

Importance of the role of axial fans in our everyday life as well as in industrial sphere is crucial. Their presence dictates indispensable reduction of the emitted noise level. The aerodynamically born noise due to the kinematics of the air-flow in the gap between rotor and stator is one predominant source of emitted noise by most fans and is the most intensive within the human audible spectrum. Two different rotor blade tip shapes A and B were analysed in details and the results presented in this paper. Their influence on the emitted noise was determined by measurements of the integral aerodynamic characteristic and by the level of the sound pressure level of the fan. It was confirmed that the kinematics of the air-flow within the blade tip gap is significantly different for both observed blade tip designs and plays a crucial role when generation and size of the total emitted noise power is concerned. Frequency spectra of the measured noise pressure level for both observed blade designs was also essentially different and served to confirm the well-known fact that intensity of the sound pressure level decreases with the frequency, which helps to assess and identify mechanisms of the noise generation. A typical measured amplitude decrease of the sound pressure level was observed and explained in details for both blade tip versions for the frequency range f>1000 Hz: appropriate scaling for the blade tip A and for the blade tip B was determined as \(\omega^{-2.8}\) and \(\omega^{-4/3}\) respectively. Distinctive differences between the two observed frequency spectra were established. Identification of locations, sources and specificity of the generated noise together with their detailed explanation was made possible by measurements of local pressure, velocity and their variances and is supported by simultaneous experimental flow visualization.

Vibration and sound radiation of an asymmetric laminated plate in thermal environments

Analytical studies on the vibration and sound radiation characteristics of an asymmetric laminated rectangular plate are carried out in this paper. Theoretical formulations, in which the effects of thermal environments are taken into account, are derived for the vibration and sound radiation based on both first-order shear deformation plate theory and Rayleigh integral. It is found that the natural frequencies, the resonant amplitudes of vibration response and the sound pressure level decrease with the temperature rising. The natural frequencies of asymmetric plates are smaller than those of symmetric plates and the velocity responses of asymmetric plates are larger than those of symmetric plates.

Development of thrust force 6 kN class transverse flux linear motor with synchronous control for direct drive applications

This study investigates the experimental validation of transverse flux linear motors (TFLMs) with permanent magnet excitation and the dynamic simulation of the developed TFLM created using the optimal design. Our research challenge is to use the developed direct drive TFLM to substitute the rack and pinion in a rotary motor because the rack and pinion lacks speed and thrust. The electromagnetic field characteristics are calculated using 3D finite element method analyses, and the dynamic characteristics are simulated using mechanical balance equations. After calculating the integral parameters, the dynamic behavior of the machine is simulated. The Matlab/Simulink model for solving motion equations is used. The peak currents obtained from simulations and experiments are very closely matched within 1%, which is the estimation error. The structural stability is evaluated using structural analyses that consider the load and moment of a robot. In order to reduce the structure-borne noise radiated from the main driving frame, damping materials are applied and the sound pressure level decreases by approximately 8.5 dB(A). Finally, the repetitive position accuracy is considered and measured using an API laser tracker T3. The measured repetitive position accuracy is within 0.1 mm at a speed of 3 m/s.

Theoretical Modelling and Effectiveness Study of Slotted Stand-Off Layer Damping Treatment for Rail Vibration and Noise Control

A promising means of reducing railway noise is to increase the damping of the rail, which decreases the vibration of the rail to reduce noise. To achieve this goal, a slotted stand-off layer damping treatment has been developed, and a compound track model with this treatment is developed for investigating the effectiveness of this treatment in terms of the vibration reduction. Through the dynamic analysis of the track undergoing the slotted stand-off layer damping treatment, some guidelines are proposed on the selection of materials and structure parameters for this treatment. In addition, the prototype of the optimal slotted stand-off layer damping treatment has been built and tested in the laboratory. It is found that the slotted stand-off damping treatment shows significant effects in decreasing the amplitude of the accelerance of the rail and a significant reduction of sound emission reflected as the radiation sound pressure level decreases by 8.2 and 9.4 dB at vertical excitation and lateral excitation, respectively, in the frequency range of 0–4000 Hz.