Acoustic Noise Spectrum Research Articles

Experimental investigation into the acoustical superiority of ogee serrations in reducing leading edge noise

An experiment is conducted to study the leading-edge noise reduction characteristics of ogee serrations and their performance compared to conventional serrations under various operating conditions in an aeroacoustic wind tunnel. Both the acoustic source contours and far-field noise spectra are examined. It is shown that ogee serrations are superior to the conventional sawtooth serrations within the entire frequency range of interest under all the Reynolds numbers tested in the experiment. It is found that the noise from the turbulence grid used to generate the required inflow turbulence becomes important when the frequency is close to and above 2 kHz, and therefore must be excluded in the examination of the leading-edge noise spectra. In addition to their effect on turbulence interaction noise, it is shown that the use of ogee serrations may result in weaker trailing-edge noise than that of sawtooth serrations at certain frequencies. The additional noise reduction due to the use of ogee serrations generally increases as λ, the wavelength of the serration, decreases. Furthermore, like sawtooth serrations, the total noise reduction also increases as the λ decreases. Narrow ogee serrations therefore prove to be more desirable when used for reducing leading-edge noise.

Desorption of Cs from vermiculite by ultrasound assisted ion exchange.

Nuclear power plant accidents typically lead to the contamination of large volumes of soils with radioactive cesium. This element is hard to desorb from soil, especially when it is bound to mica minerals, and aggressive and energy-consuming techniques are often required. In this study, we investigated the use of ultrasound with Mg2+ cation exchange for the removal of a133Cs-contaminated vermiculite over a wide range of temperatures (20-200°C). At room temperature, ultrasound was found to significantly accelerate Cs desorption but only reversibly adsorbed Cs species were removed. Under hydrothermal conditions and ultrasonic irradiation in contrast, the removal efficiency after 1h was 50% at 100°C and more than 95% at 200°C, compared with only 50% without ultrasonication at 200°C. Cs contamination can therefore be nearly totally removed, even from collapsed vermiculite sites where sorption is considered irreversible. Ultrasound waves and high temperatures both make trapped Cs more accessible by spreading the sheets and improving mass transfer. Acoustic noise spectra show that even at high pressure and temperature, cavitation bubbles form, oscillate and collapse, with the desired physical effects. These results demonstrate the potential of synergistic ultrasound and hydrothermal treatment for soil remediation.

A Coupled LES-Synthetic Turbulence Method for Jet Noise Prediction

Large-eddy simulation (LES)-based jet noise predictions do not resolve the entire broadband noise spectra, often under-predicting high frequencies that correspond to un-resolved small-scale turbulence. The coupled LES-synthetic turbulence (CLST) model is presented which aims to model the missing high frequencies. The CLST method resolves large-scale turbulent fluctuations from coarse-grid large-eddy simulations (CLES) and models small-scale fluctuations generated by a synthetic eddy method (SEM). Noise is predicted using a formulation of the linearized Euler equations (LEE), where the acoustic waves are generated by source terms from the combined fluctuations of the CLES and the stochastic fields. Sweeping and straining of the synthetic eddies are accounted for by convecting eddies with the large turbulent scales from the CLES flow field. The near-field noise of a Mach 0.9 jet at a Reynolds number of 100,000 is predicted with LES. A high-order numerical algorithm, involving a dispersion relation preserving scheme for spatial discretization and an Adams–Bashforth scheme for time marching, is used for both LES and LEE solvers. Near-field noise spectra from the LES solver are compared to published results. Filtering is applied to the LES flow field to produce an under-resolved CLES flow field, and a comparison to the un-filtered LES spectra reveals the missing noise for this case. The CLST method recovers the filtered high-frequency content, agreeing well with the spectra from LES and showing promise at modeling the high-frequency range in the acoustic noise spectrum at a reasonable expense.

Influence of end plate placement on the reduction of airfoil tip vortex formation noise

The tonal noise generated at the tip is a major contribution to the aerodynamic noise generated by three-dimensional airfoils. One way to reduce this noise is the use of end plates, which was investigated in the present study for three different circular end plate geometries applied to a cambered NACA 4412 airfoil with aspect ratio 2 and forced boundary layer transition. Microphone array measurements were performed in a small aeroacoustic wind tunnel for chord based Reynolds numbers between 75,000 and 200,000 and geometric angles of attack between 0° and 20°. The acoustic far-field noise spectra obtained for the baseline configuration show a broadband hump centered at a chord based Strouhal number of 13 that is associated with the noise generation at the tip and scales with the third power of the flow velocity. The application of end plates reduces this peak and is most effective for end plates which bound the flow on the suction side of the airfoil. Hot-wire measurements taken for one configuration show that the end plates diffuse the turbulence intensity in the relevant frequency range and reduce the fluid transfer along the pressure gradient at the tip. The suction side end plate additionally prevents the interaction of turbulent structures with the trailing edge and is therefore more effective in reducing noise. Surface flow visualizations for this configuration reveal a separation line extending along the full span up to the tip while the surface flow of the baseline and pressure configuration is strongly affected by the flow swept around the tip. Therefore, the placement of the end plate on the suction side is more effective in reducing tip noise.

Association of Fear and Annoyance with Acoustic Levels of Dental Equipment Among Adolescents Attending Rural Dental Health Center: A Cross-sectional Study

Introduction: Noise is increasingly becoming pervasive in occupational health concerning practicing dentists as well as patients. A stimulus such as noise from dental equipment compared to anxiety is often multifactorial and can result from a combination of fear of pain, dental instruments noise, or even upcoming negative events and future threats. Aim: This study aimed to assess fear and annoyance levels among adolescents attending a rural dental health centre and compare the acoustic noise levels of dental equipment and its association with fear and annoyance levels among adolescents. Methods: The acoustic noise spectra originating from different tools/equipment in a rural dental health centre is assessed using an application named sound meter of a standard mobile android phone (Samsung galaxy grand 2). Following a 15-minute treatment, a questionnaire was filled by a professional investigator through personal interviews with all subjects. The data is subjected to analysis by Statistical Package for Social Sciences version 22.0. Results: 61.5% of the subjects felt anxious about the anticipatory dental visit, and there was a significant difference observed between genders. Chi-square analysis revealed no significant differences in the fear and annoyance with increasing noise levels in various treatment groups. (p> 0.05). Conclusion: In the present study, fear and annoyance levels increased with increasing noise levels of equipment. Annoyance was found to be significantly higher in females.

Acoustic noise spectra under hydrothermal conditions.

Acoustic noise spectra were studied for the first time in overheated water using sonohydrothermal reactor operating at 20kHz ultrasound in the temperature range from 25 to 200°C at the autogenic pressure of 1-14bar. The obtained results highlighted a dominating role of stable cavitation during ultrasonic treatment of hot water. Heating of sonicated water results in the formation of large number of nonlinearly oscillating bubbles synchronous with the driving frequency. At 200°C, the acoustic spectra also display strong subharmonic and multiple ultraharmonic bands. Moreover, cavitation bubbles formed at 200°C exhibit chaotic and random motions. It has been shown that the addition of TiO2 nanoparticles to hydrothermal water heated at 200°C allows to eliminate subharmonic/ultraharmonic bands and stochastic oscillations as well. This effect was assigned to Pickering-like bubble stabilization due to the particle accumulation at the bubble surface.

A Noise Tolerant Spread Spectrum Sound-Based Local Positioning System for Operating a Quadcopter in a Greenhouse.

Quadcopters are beginning to play an important role in precision agriculture. In order to localize and operate the quadcopter automatically in complex agricultural settings, such as a greenhouse, a robust positioning system is needed. In previous research, we developed a spread spectrum sound-based local positioning system (SSSLPS) with a 20 mm accuracy within a 30 × 30 m greenhouse area. In this research, a noise tolerant SSSLPS was developed and evaluated. First, the acoustic noise spectrum emitted by the quadcopter was documented, and then the noise tolerance properties of SSSounds were examined and tested. This was done in a greenhouse with a fixed quadcopter (9.75 N thrust) with the positioning system mounted on it. The recorded quadcopter noise had a broadband noise compared to the SSSound. Taking these SSSound properties into account, the noise tolerance of the SSSLPS was improved, achieving a positioning accuracy of 23.2 mm and 31.6 mm accuracy within 12 × 6 m for both Time-division Multiple Access (TDMA) and Frequency-division Multiple Access (FDMA) modulation. The results demonstrate that the SSSLPS is an accurate, robust positioning system that is noise tolerant and can used for quadcopter operation even within a small greenhouse.

Calculation of Infrasonic Noise Characteristics by Measuring the Current Values of a Two-Dimensional Wind Wave Field

The paper presents a scheme for calculating the current sound pressure values excited by wind waves in the infrasonic frequency range (0.1–5 Hz). The calculation is based on measuring the time dependence (variability) of the two-dimensional profile of the excited surface $$z(x,y,t)$$ on the plane XY with characteristic dimensions exceeding several surface wave lengths. The paper provides a scheme for processing a dataset $$z(x,y,t),$$ which measures the level of standing wind waves and then directly calculates the time dependence and the wave-generated acoustic noise spectrum. The authors present the results of calculating the averaged infrasonic noise spectrum and the spatiotemporal wave spectrum for the specific water area where the wave characteristics $$z(x,y,t)$$ were measured.

FPGA Based Random Pulse Width Modulation for Three Phase VSI

One of the inexpensive notions in present day power electronics control equipment is the objective of random pulse width modulation (RPWM) for control of semiconductor based power converters, accelerated by the progressively increasing anxiety with or regulations regarding electromagnetic fields, vibrations produced due to magnetic field and the induction of acoustic noise. The disperse or spread of acoustic noise spectra is obtained on induction motor by a differentcosimulation of RPWM generation for three phase inverter drive. This pattern is randomized by selecting the triangle randomly among the two triangles. The 8-bit linear shift register is used as feedback for arbitration selection. To generate the pulses sine reference wave is compared with east bit of the register output decide the winning triangle. The outcomes of cosimulation are presented for both RPWM and SPWM and compared Fundamental, Harmonic Spread Factor, Total Harmonic Distortion for various modulation index. Furthermore, Xilinx XC3S500E FPGA processor synthesis results are provided. The experimental validation of SPWM and RPWM are presented at the end and compared.

Analysis of the Acoustic Noise Spectrum of Domestic Induction Heating Systems Controlled by Phase-Accumulator Modulators

In domestic induction heating applications, the modulation technique applied to the inverter has a high influence on the acoustic noise emissions. These noise emissions must be avoided, since they may be audible and annoying to the final user. This paper analyzes the acoustic noise emissions that appear when a series half-bridge resonant inverter is operated with a phase-accumulator-based modulator. This modulation technique has the advantage of operating in the frequency domain, and it is compared with the classical pulsewidth modulator regarding the audible noise generated. The frequencies of the tones in the acoustic noise spectrum are theoretically calculated from the parameters of the phase-accumulator-based modulator. The spectral flatness measure is used to quantify the number of cases in which tones are generated by the modulation. Two techniques are applied to the phase-accumulator-based modulator, and their effect is tested. Theoretical results are experimentally verified.

Determining the place of articulation of Arabic voiceless dorsal fricatives

Previous, mostly impressionistic, examinations of Arabic varieties provide varied descriptions of the dorsal fricative /χ/ place of articulation, ranging from velar to uvular. While some descriptions attribute this variation to dialectal differences, descriptions of the same dialect also vary. 3D ultrasound data were collected from six native speakers of different dialects of Arabic, one each from Syria, Palestine, Faifi (in Saudi Arabia), Egypt, Algeria, and Morocco. Apart from the dorsal fricatives, the corpus included productions of palatal, pharyngeal, and contrasting velar and uvular stops to provide comparative standards for various points of articulation. The Syrian, speaker showed very similar articulations for /χ/ with uvular stops, while the other speakers variably showed a more anterior articulation between the velar and uvular stops. The most anterior articulations were apparent in the Moroccan and Algerian speakers, giving some suggestion of a dialectal difference; however, point of articulation of the dorsal fricative was variable for most of the speakers and not obviously restricted by dialect. The effect of these articulatory observations on the acoustic noise spectra are ongoing.

Rotor Notching for Electromagnetic Noise Reduction of Induction Motors

This paper presents a new technique, notching the induction motor rotor core, so as to reduce the electromagnetic noise by suppressing the vibration due to the rotor slot permeance harmonics. First, the principle of such technique is presented along with theoretical analysis. Then, finite element method is used to calculate the Maxwell force harmonics and the vibration spectra of a 5.5 kW 3000 r/m commercial induction motor and a modified motor. Finally, experimental results are given to validate the proposed technique by testing the vibration and acoustic noise spectra of the two induction motors. It is shown that the modified motor has about 5 dB(A) reduction of the overall sound pressure level compared with the commercial one.

Performance Characterization of Random Pulse Width Modulation Algorithms in Industrial and Commercial Adjustable-Speed Drives

One of the problems of the pulse-width-modulation (PWM)-controlled ac machines is the acoustic noise that could become unacceptable when used in sensitive environments. Random PWM (RPWM) in industrial and commercial adjustable-speed drives (ASD) results in the partial transfer of power from the discrete spectrum (narrowband noise) of the output voltage to the continuous spectrum (wideband noise), with advantageous effects on reducing the acoustic noise in the motor drive system. In this paper, the theoretical power spectrum analysis as the basis for RPWM is presented. Five state-of-the-art RPWM strategies, their voltage, current, and acoustic noise spectra characteristics are quantitatively evaluated. The PWM schemes and theoretical analysis are validated through a 2.2-kW 380-V 50-Hz ASD induction machine experimental setup. The results provide valuable data for practicing engineering community to choose the best option in real-world applications.

Aerodynamics and Aeroacoustics Investigation of a Low Speed Subsonic Jet

Low and high speed subsonic jets have been studied in the last 50 years mainly due its large application in industry, such as the discharge of turbojets and turbofan engines. The purpose of this work is to investigate the aerodynamics and the acoustic noise generated by a single stream jet flow operating at low Mach number 0.25 and Reynolds number of 2,1  105. The main focus is the flow and acoustics characterization of this low speed jet by appling different experimental techniques for evaluating the velocity field via measurements with pitot tube and hot-wire anemometry and farfield noise acquisition by free field microphones. In order to verify the validity of aeroacoustics prediction for such low speed jet, a Computational Fluid Dynamics by means of RANS simulations via k- SST model have been employed coupled with a statistically low-cost Lighthill-Ray-Tracing method in order to numerically predict the acoustic noise spectrum. Sound pressure level as a function of frequency is contructed from the experiments and compared with the noise calculations from the acoustic modeling. The numerical results for the acoustic and flow fields were well compared with the experimental data, showing that this low-cost flow-acoustic methodology can be used to predict acoustic noise of subsonic jet flows, even at low speeds.

Influence of the Human Body Mass in the Open-air MRI on Acoustic Noise Spectrum

The paper analyses changes in spectral properties of the acoustic noise when the examined person lies in the scanning area of the open-air magnetic resonance imager (MRI) and consequently the holder of the lower gradient coils is loaded with the mechanical mass represented by the person’s weight. The acoustic noise pressure level is mapped in the MRI neighborhood, too. Obtained results of spectral analysis will be used for design of a correction filter to suppress the noise in the simultaneously recorded speech signal for 3D modelling of the human vocal tract.

Discussion of "A Variable Switching Frequency PWM Technique for Induction Motor Drive to Spread Acoustic Noise Spectrum With Reduced Current Ripple"

Discussion of "A Variable Switching Frequency PWM Technique for Induction Motor Drive to Spread Acoustic Noise Spectrum With Reduced Current Ripple"

Response to Discussion on “A Variable Switching Frequency PWM Technique for Induction Motor Drive to Spread Acoustic Noise Spectrum With Reduced Current Ripple”

In random pulse width modulation (RPWM) category, a wide variety of schemes is seen as described in the paper. The discussion part is mainly focused on the randomized carrier frequency approach, where the switching periods are selected randomly, irrespective of the ripple voltage. For prediction of rms current ripple over a switching cycle as well as rms current ripple over the line cycle, precise knowledge of the switching period is required; this is not possible with random PWM techniques, though these methods are following a probability distribution function. However, accurate prediction of current total harmonic distortion (THD) is possible with the variable frequency PWM (VFPWM) method, as the switching period is a function of the fundamental angle. The procedure followed for current THD prediction is explained in [1].

Localized Surface Vibration and Acoustic Noise Emitted From Laboratory-Scale Transformer Cores Assembled From Grain-Oriented Electrical Steel

Magnetostriction of grain-oriented 3% SiFe sheets was measured prior to assembly into model transformer cores. Core vibration was measured using a laser scanning vibrometer and harmonic spectra of acoustic noise were evaluated from the microphone outputs. Explanations show why no correlation exists between vibration harmonics profiles and A-weighted acoustic noise spectra. High localized vibration did not cause high noise due to phase differences in surface vibrations, and it is shown that this is the main reason why the A-weighted noise of a three-phase core can be less than that of an equivalent single-phase core. Noise from cores assembled from low magnetostriction materials was not always lowest because of the variable effect of electromagnetic forces.

Experimental Comparison of Conventional and Bus-Clamping PWM Methods Based on Electrical and Acoustic Noise Spectra of Induction Motor Drives

The acoustic noise emitted by an induction motor powered by a voltage source inverter (VSI) is an environmental issue. Harmonics present in the stator current are the main reason for the increased acoustic noise in motor drives. The pulse-width modulation (PWM) technique used to modulate the inverter is a key factor in determining the magnitude of current harmonics. Bus-clamping PWM (BCPWM) techniques have got much attention nowadays due to reduced switching loss, compared with conventional space vector PWM (CSVPWM). In this paper, two BCPWM techniques, namely $60^\circ$ and $30^\circ$ BCPWM methods are compared with CSVPWM on the basis of electrical spectra as well as acoustic noise spectra. Experiments are conducted on a pulse-width-modulated VSI-fed induction motor drives. Harmonic analysis is carried out on the measured line-to-line voltage, stator current, and acoustic noise corresponding to the three methods at different fundamental and carrier frequencies. Based on the magnitude and phase of current harmonics, the frequency and normalized magnitude of dominant magnetic force waves are predicted theoretically. Theoretical predictions and experimental results both show that the magnitudes of dominant acoustic noise components around the carrier frequency are reduced significantly with these BCPWM methods compared with CSVPWM. Furthermore, it is shown that $30^\circ$ BCPWM is better than $60^\circ$ BCPWM in terms of acoustic pleasantness.

Variable-Switching Frequency PWM Technique for Induction Motor Drive to Spread Acoustic Noise Spectrum With Reduced Current Ripple

Pulse-width modulation (PWM) of a motor drive at a fixed switching frequency leads to harmonic currents in narrow bands around integral multiples of the switching frequency resulting in tonal frequency acoustic noise components, which are irritating to the human ear. Random variation of carrier frequency to spread the spectrum poses challenges in device loss calculation, thermal design, and closed-loop controller design. Random pulse positioning is not quite effective at high speeds of the drive, where the problem of acoustic noise is most pronounced. This paper proposes a simple variable-frequency PWM and evaluates the performance of the same experimentally on a 6-kW induction motor drive. The experimental results show that the acoustic noise is spread uniformly over a range of frequencies without introducing low-frequency current harmonics. Compared with fixed-frequency PWM, the dominant acoustic noise is reduced significantly over a wide range of speeds. The variable-frequency PWM is further shown to reduce line current total harmonic distortion at high speeds of the drive. The computational effort required by the variable-frequency PWM is only marginally higher than that required by fixed-frequency PWM.