기어박스의 경량화 및 소음저감을 위한 재질변경 및 리브 효과

This paper presents a new active control method of sound radiated from a double-walled structure. The purpose of this study is to control of sound radiation power level using both of active noise control and active vibration control. First, modal coupling method is introduced which is required to model a vibro-acoustic coupling system. Then, optimal control laws of active noise control and active vibration control for minimizing the sound radiation power are derived in a vector form. Next, some numerical simulations are carried out to demonstrate the validity of the proposed method. As a result of the analysis, reduction of the sound power level is succeeded by feedforward control. Some numerical results show that active noise control is more effective than active vibration control for minimizing the sound power level. In addition, it is shown that by combination of the both control methods, improvement of a control performance is achieved. Furthermore, it is found that control effect rely on the relationship between placement of control sound sources and the disturbance location, while placement of control vibration source does not have significant impact on the control performance.

This paper presents the experimental characterization of the vibroacoustic fields and the evaluation of noise performances of hydraulic pumps. Research on hydraulic pump noise has traditionally focused on the fluid-borne noise sources, and very often the analyses of vibration and noise have been performed focusing on a few local points. This trend results in the lack of investigation on the overall behaviors of vibroacoustic fields of hydraulic pumps, and it has been one of the obstacles to understand the complete mechanisms of noise generation. Moreover, despite the existence of the ISO standards for the determination of noise levels, diverse metrics have been used for the evaluation of noise performances of the pumps, but the adequacy of these metrics has not been carefully examined. In this respect, this paper aims at introducing a way to characterize and interpret the measured vibroacoustic field and providing proper methods which are also capable of applying the ISO standards for the fair assessment of pump noise performances. For the characterization of the vibroacoustic field, operational deflection shapes (ODS) and corresponding radiated sound fields are visualized at harmonics of the pumping frequency by using a spectral analysis. Observations are made regarding the motions of the pump and its mounting plate and the resultant radiated noise, depending on the frequency, as well as their correlation. A numerical analysis using the Rayleigh integral equation is also performed to further investigate the contribution of the mounting plate motion on the noise radiation. For the evaluation of noise performance, two different units are tested at multiple operating conditions, and comparisons are made based on their measured sound power levels (SWLs) and sound pressure levels (SPLs). The results emphasize the importance of SWL measurement for the fair noise performance evaluation, and the two methods are proposed as practices to determine the minimum number of measurement points for practicability and to have reliable sound power determination for hydraulic pumps.

The angle error inherently biases the calculated sound power level when determined using free-field sound pressure level measurements over an enveloping surface. A regression equation was previously created to predict the angle error associated with omnidirectional sources using just a few easily obtainable input variables related to the geometry of the source and the enveloping measurement surface. This paper assesses the accuracy of that predictive regression equation through an empirical study. A dodecahedron loudspeaker was used as a noise source over a range of positions within the reference box. The modeled angle error correction was applied to the computed sound power levels using measurement data from multiple cylindrical enveloping surfaces in accordance with ISO 3744 and ISO 7779. The corrected sound power levels were compared to the measured sound power levels of the source per ISO 3741 direct and comparison methods.

This brief paper does not try to settle the current dispute over the question whether the sound output of mechanical devices is better expressed in terms of sound power level or sound pressure level. Instead, it considers the relationship between these quantities in spaces which have not been specifically designed to be reverberation rooms, and concludes that, once we have measured the noise of a piece of equipment, no matter whether in terms of the sound power level or the sound pressure level, we had better be cautious in how we use the measured data.

Little research was carried out in the field of acoustic casings for HVAC equipment. In this study, we experimentally analyzed the influence of the casing geometry with regard to the acoustic environment. Two casings (cuboid shape and semi-cylindrical shape) were compared according to their experimentally assessed sound power level. The parameters calculated were the sound pressure and power levels. The experimental study proves that the shape of the casing can indeed have an influence on the sound energy radiated outside. Generally, we obtained that for low frequencies (16Hz–250Hz) the semi-cylindrical casing has a better acoustic behavior, for median frequencies (500Hz–1000Hz) the cuboid casing has better acoustic performance and for high frequencies (2000Hz–16000Hz) both casings have similar performance. Further, in-depth research regarding the influence of the mass, joinery and other specifications upon the acoustic indoor or outdoor environment is needed in order to understand the “optimal” shape for equipment casing.

A broad-band noise prediction code based on the work of Carolus et al was written and incorporated into the low-pressure axial fan design program. For the convenience of design calculation, the turbulence statistics were determined by a semi-rational formula for fully-developed duct flow instead of the experiment. The code predicts 3 different forms of broad-band noise that were superimposed to calculate the total noise emission. The outputs include noise spectra, overall sound power and overall pressure level. In addition, Fukano's model was also utilized for indicative comparison. The influence of the variation of the design parameters, including twisted power, hub ratio and solidity, on the fan noise characteristics was calculated and analyzed. It is indicated that the total sound pressure level is most sensitive to the flow relative velocity, while the chord length can be considered as a secondary cause.

This paper analyses acoustic response of thin plate with discrete patches with variable thickness in water medium. Finite-element method and Rayleigh integral method are used for sound power calculation in water medium. For low-frequency range up to 50 Hz, the peak sound power level (SPL) is achieved at higher taper ratio of 0.9 with parabolically increasing–decreasing thickness variation with one patch. In the frequency range of 70–180 Hz and 260–290 Hz, a taper ratio of 0.6 with four patches on the plate with parabolically increasing–decreasing variation is preferred for minimum SPL. It is observed that SPL is influenced by redistribution of patches, thickness variations, and taper ratios.

<div class="htmlview paragraph">A reactive type of muffler of a 4-stroke Auto-Rickshaw was modeled using a commercially available Acoustic Finite Element Analysis (FEA) software like Sysnoise. Design sensitivity of different geometry parameters of mufflers to its Insertion Loss over a wide frequency range was studied. The analytical results were validated by the Laboratory Speaker test. But in the presence of a hot gas flow, the Insertion Loss characteristics of the muffler were far different. As the flow speed increased, negative attenuation was found in some frequency bands.</div> <div class="htmlview paragraph">The second part of the paper studies the software prediction of structure borne noise from a transmission cover of a Motorcycle. The sound power level was predicted by uncoupled fluid-structure analysis of Boundary Element Method (BEM). To reduce the sound power radiated from its top surface, a ribbed pattern and increase in thickness were proposed. The software output regarding the sound radiation efficiency of the cover was only partly validated.</div>

This report describes investigations of the accuracy and precision of various measurement methodologies for determining^the estimated sound power output of "large" machines in the free field over a reflecting plane.One purpose of this investigation is to place empirical error bounds on many of the free field measurement procedures currently proposed or in use; and in particular, compare the results of "nearfield" and "far-field" measurements.The sources used for the investigation included IT portable air compressors of various types (powered by internal combustion engines), a "reference" sound source, and a loudspeaker driven by a pure tone source.The data recorded include sound pressure level (A-weighted, linear, and 1/3-octave band) on an 81+ point hemispherical array of seven metre radius, and "near-field" measurements, sampled every square metre, on a rectangular surface one metre from the machine surface.These data were reduced to provide information on the deviation of "near field" sound power determinations from "far-field" power level (using subsets of the data as appropriate to various methodologies).The measured data for seventeen sources suggests that the value of a sound power estimate based on "near-field" sound pressure level measurements may be an upper bound to the sound power level estimated from far field measurements, subject to the limitations of sampling error.Estimates of total achievable measurement error of A-weighted so\ind power level of near field determinations relative to far field determinations are made for several measurement methodologies, based on the experimental data. 1 .2.

Advanced hydrostatic transmissions and hydraulic hybrids show potential in new market segments such as commercial vehicles and passenger cars. Such new applications regard low noise generation as a high priority, thus, demanding new quiet hydrostatic transmission designs. The aim of this paper is to investigate noise sources on a series hybrid transmission through simulation and measurements. A model has been developed to capture the interaction of a pump and motor working in a hydrostatic transmission and to predict overall noise sources. The model describes dynamics of the system by coupling lumped parameter pump and motor models with a one-dimensional unsteady compressible transmission line model including a dynamic model of an accumulator. A semi-anechoic chamber has been designed and constructed for sound intensity measurements that can be used to derive sound power. Sound power measurements were conducted on a series hybrid transmission test bench inside the semi-anechoic chamber in order to study the relationship between sound power and two types of noise sources, fluid and structure borne. The focus of these measurements was to investigate the impact of an accumulator in the high pressure line as well as the influence of varying high pressure line length. Results show a strong influence of changing line length and the addition of an accumulator on pressure ripple, but with little impact on sound power. A high correlation was found between sound power levels and control moment amplitudes on the swash plate. This study demonstrates the usefulness of predicting transmission noise sources, and how this information is beneficial in the design process of a transmission.

The central quantity in regulations concerning acoustics is the sound power emitted by a sound source. Building up from the current state of the art of sound power determination, this work will show that there is potential to build a metrological traceability chain for this measurand. Firstly, the lack thereof will be discussed and the problems that arise from this situation will be detailed (Chp. 2). This will argue the need for a proper traceability chain including a primary, secondary and transfer standards. This work will focus on the level of the primary standard, which provides a foundation to the desired traceability chain. The task requires a significant shift in measurement protocol used. All currently standardised procedures require measurements in the sound field - while also sampling the sound field quantities sound pressure or intensity. The newly introduced procedure places the measurement equipment outside of the sound field and measures the motion of the primary standard’s surface directly. The measurement protocols and environments as well as the used physical realisations of primary sources will be described (Chp. 3). Certainly, other types or designs of primary standards are possible. The structural as well as material choices for the realisations described in this work are based on analytical and numerical studies on the characteristics of the chosen circular piston design. They will be described in detail (Chp. 4). Besides radiation characteristics, the design choices for the primary standards used are obviously governed by the suitability to the measurement method. This methodology builds on the Rayleigh integral in its discretised form. By measuring surface velocities as well as phase relations at pre-defined points of the measurement surface, the emitted sound power is calculated. The boundary condition that is imposed by this choice of measurement method is that of the need of a planar radiator which has to be embedded into the floor of the measurement environments. The discretised Rayleigh integral method is a validated methods so that the main question of interest for this work is whether its convergence can be guaranteed and, if so, how many sampling points are required to approximate this convergent value with a pre-set level of uncertainty. Both of these aspects will be discussed (Chp. 5). With the theoretical framework in place, measurement data will be evaluated. As expected, these introduce a new aspects into the uncertainty calculations: noise. The evaluation of the uncertainties associated with calculated sound power levels is a central topic determining the usability of the described primary standards. Based on Monte Carlo simulations, strategies to determine uncertainties will be developed. They focus on the aspects of filtration of noisy data and usability of measurement surfaces - piston versus baffle. It will be shown that in terms of precision, the proposed discretised Rayleigh integral method has the potential to yield satisfactory results (Chp. 6). In terms of accuracy, however, shortcomings of the tested primary standards will be reported. Differences in sound power levels between the ones obtained from the discretised Rayleigh integral and those gathered using standardised methods will be shown and explanations attempted (Chp. 7). The results open the window for further studies particularly on the design of primary standards. However, the suitability and validity of the proposed measurement method to form the basis of a traceability chain for the measurand sound power in air will result from this work.

Acoustic topology optimization of sound power using mapped acoustic radiation modes

The paper is aimed at investigating the number and location of microphones required to accurately calculate the sound power level of a harmonic noise source. The measurement procedure performed is based on the IEC standard for the determination of transformer sound power level. The current procedure defines numerous measurement points around the tested transformer. Reducing the number of measurement points could significantly speed up the measurement procedure when a transformer is tested during the manufacturing process. Within the research work, estimations on the number and location of measurement points are identified to accurately calculate sound power level. The approach is tested and validated using specially developed software in LabView, that allows inclusion or exclusion of measurement points from the sound power level calculations. To prevent harmonic wave interference, acoustic wave reflections and background noise, the measurement procedure took place in the anechoic chamber.

It is well established that there are three principal jet noise components for imperfectly expanded supersonic jets. However, to this date there has been no reliable and practical method for identifying the individual components. First, new scaling laws for the turbulent mixing noise component are developed from a comprehensive experimental database generated by the author. The scaling laws are based on the explicit recognition that a) the variation of the overall sound power level with jet velocity has a weak dependence on jet stagnation temperature ratio; b) the variation of the overall sound pressure level with velocity at every radiation angle is a function of jet stagnation temperature ratio. Therefore, the behavior of the turbulent mixing noise at each radiation angle can be characterized by the two independent parameters: the velocity ratio and the stagnation temperature ratio. These two findings set this study apart from past approaches and form the basis for the methodology developed here. It is demonstrated clearly that there is excellent collapse of the mixing noise spectra over the entire frequency range. Once the normalized or master spectra for the mixing noise are established, it is a trivial matter to subtract these from the total measured spectra to obtain the shock-associated noise. For moderately imperfectly expanded heated supersonic jets, the mixing noise component has the same spectral level as the shock-associated noise, over a wide range of higher frequencies. At the lower radiation angles in the forward quadrant, there is a substantial decrease in the values of the velocity exponents as the stagnation temperature ratio is increased. Proceeding aft, the values start to rise and in the peak sector of noise radiation, the velocity exponent becomes less sensitive to jet stagnation temperature ratio unlike at lower angles, and stays close to the values for the unheated jet.

The effect of rotor blade installation angle on the structure-borne noise of adjustable-blade axial-flow fans is analyzed based on the fluid–solid coupling method. The co-simulation environment ANSYS Workbench is adopted to perform one-way fluid–solid coupling analysis. Following this, the properties of the flow field and noise field with different installation angles are simulated. The flow field simulation results reported significant vorticity near the rotor and stator, and a larger installation angle may cause higher pressure fluctuation. The sound field results showed that the frequency spectrum characteristics for the sound pressure level and the sound power level are almost the same while the installation angle changes from −8° to 8°, and the peaks of frequency spectrum occur at the blade passing frequency and its harmonics. The total sound pressure level (TSPL) and the total sound power level (TPWL) all show increasing trends ranging from −8° to +8°. The maxima of TSPL and TPWL reach 134.1 and 176 dB, while their minima reach 123.1 and 163 dB, respectively. Thus, reduction of the installation angle can reduce the structure-borne noise. Besides, the structure-borne noise generated by adjustable-blade axial-flow fans is low-frequency noise, which lies in the range of 0–500 Hz.