The characteristics of acoustic line array prototypes for scale model experiments

The sound fields created by three different settings of a 5 m long linear loudspeaker array were studied experimentally in the present investigation in an attempt to find out a method to produce a reasonable sound field for scaled down model experiments. The first setting was with all the loudspeakers facing vertically upward, while the second setting was with all the loudspeakers facing the horizontal. The last one resembled the first setting but the loudspeakers were put inside a small partial enclosure whose opening was facing vertically upward. Results show that the last setting can produce a sound field with characteristics close to those of a two-dimensional sound field. The second setting results in a sound directivity pattern relevant to that of the measured A-weighted traffic noise.     

A study of noise reduction by acoustic shock waves

I.IntroductionAcousticshockwavcs(ASW)isanimportantphcnomcnoninnonlinearacoustics.Experimentalrcsultshavcshownthatwhenanaircraftcngincinletopcratesneartheson-iccondition,vcrystrongnoisegcncratedbythcfanscanbcreduccdgreat1yowingtothcformationofASWatthcthroatofthcin1etll].ASWisadiscontinuityofacousticvaria-bles,whichisdifTcrcntfromthcshockwavesoccurringinhighspcedsteadyflowinducts.Theformer'sintensityismuch1cssthanthelattcr's.Furthcrmorc,thepositionandintensityofASWisalwayschangedwithtime.l…     

声激波降噪研究

本文数值研究了声激波产生的条件及其降噪效果,详细讨论了声源强度,频率,管道形状,喉部流动马赫数及多频声源对声激波降噪效果的影响。由于声激波降噪需要附加一段具有高速流动的缩放管道,它会附加流动噪声和压力损失,本文提供了这方面的一组实验数据,以说明应用声激波消声器的现实可能性。     

Active noise control with linear control source and sensor arrays for a noise barrier

A study is conducted on minimizing the sum of the squared acoustic pressures with a linear array of control sources and a perpendicular linear array of error sensors, placed above the top of a noise barrier. Particular angular orientations, with respect to the center of the barrier top, and spacings of the linear arrays of control sources and error sensors result in moderate to significant additional reduction of the acoustic pressure in the shadow zone. Visual inspection of the sound pressure field, with and without active noise control, found that uniform and significant additional insertion loss can be generated near the barrier. Numerical simulations were conducted to test the proposed method. For separations between control sources and error sensors much less than a quarter wavelength of the primary noise disturbance, results show that the angular orientation, of the combined linear control source and sensor arrays, is a weak factor for acoustic pressure reduction in the shadow zone. Weak angle dependence serves as an advantage to the proposed method, which yields uniform performance for any angular orientation. An angular orientation involving the alignment of the furthest error sensor with the first diffracting edge of the barrier and the primary source was observed to perform well for a variety of frequencies, since the spacing between error sensors and between control sources is of the order of a quarter-wavelength. Improved noise control in the shadow zone of a barrier is achieved by the use of two control sources and angular orientation as mentioned above. Further spatial extension of the area of reduced acoustic pressure is possible by utilizing an increased number of control sources.     

The noise reduction of the acoustic paths between two rooms interconnected by a ventilation duct

This paper describes measurements made on a system of two rooms separated by a wall but connected by a ventilation duct. Using statistical energy analysis the noise reduction of each path through the system was determined from the measured data. It was found that the most important path through the system is from the source room, into the duct cavity, along the duct and then out into the receiving room.Experiments on air flow showed that for practical purposes air flow has no effect on the system considered here.     

Radiation from an acoustic line source in the presence of a sphere

By using relations between cylindrical and spherical wave functions the scattering of the radiation from an infinitely long uniform acoustic line source by hard and soft spheres has been investigated. Expressions for the scattered pressure field for both situations have been derived.     

An example of in-plant noise reduction with an acoustical barrier

Existing theories for predicting the distribution of sound intensity in rooms and the performance of acoustical barriers are applied in designing a barrier to reduce noise in an industrial environment. The combination of the theories is found to predict the sound pressure level in the barrier shadow zone reasonably well.     

Measurement of interference-pattern parameters of the near acoustic field of a deep-sea noise source

We present the results of a study of the space-frequency energy distribution of the near acoustic field of noise radiation from a moving source. Dynamic spectrograms of the received signal of noise radiation in the frequency range of 0–2.0 kHz at distances of up to 1300 m are given. It is shown that the variation in the levels of spectral components of the signal due to interference modulation in the near field can attain 12 dB. The interference pattern is shown to occur over the entire range of considered distances of 100–1300m. Within the framework of ray theory, using hydrological data for C(z) in the region of measurements, we calculate the near field for a lumped noise source. The calculation results are in good agreement with the results of field experiments. It is shown that the sound velocity profile C(z) in the region of measurements has a pronounced influence on the interference pattern of the near field. Allowance for C(z) in the calculations of the near acoustic field within the framework of the existing theory allows one to predict the interference pattern and choose optimal distances for measurements under near-field conditions.Institute of Applied Physics, Russian Academy of Sciences, Nizhny Novgorod. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 38, No. 8, pp. 817–831, August, 1995.     

Reflection of an acoustic line source by an impedance surface with uniform flow

An exact analytic solution is derived for the 2D acoustic pressure field generated by a time-harmonic line mass source located above an impedance surface with uniform grazing flow. Closed-form asymptotic solutions in the far field are also provided. The analysis is valid for both locally-reacting and nonlocally-reacting impedances, as is demonstrated by analyzing a nonlocally reacting effective impedance representing the presence of a thin boundary layer over the surface. The analytic solution may be written in a form suggesting a generalization of the method of images to account for the impedance surface. The line source is found to excite surface waves on the impedance surface, some of which may be leaky waves which contradict the assumption of decay away from the surface predicted in previous analyses of surface waves with flow. The surface waves may be treated either (correctly) as unstable waves or (artificially) as stable waves, enabling comparison with previous numerical or mathematical studies which make either of these assumptions.     

Fan interaction noise reduction using a wake generator: experiments and computational aeroacoustics

A control grid (wake generator) aimed at reducing rotor-stator interaction modes in fan engines when mounted upstream of the rotor has been studied here. This device complements other active noise control systems currently proposed. The compressor model of the instrumented ONERA CERF-rig is used to simulate suitable conditions. The design of the grid is drafted out using semi-empirical models for wake and potential flow, and experimentally achieved. Cylindrical rods are able to generate a spinning mode of the same order and similar level as the interaction mode. Mounting the rods on a rotating ring allows for adjusting the phase of the control mode so that an 8 dB sound pressure level (SPL) reduction at the blade passing frequency is achieved when the two modes are out of phase. Experimental results are assessed by a numerical approach using computational fluid dynamics (CFD). A Reynolds averaged Navier-Stokes 2-D solver, developed at ONERA, is used to provide the unsteady force components on blades and vanes required for acoustics. The loading noise source term of the Ffowcs Williams and Hawkings equation is used to model the interaction noise between the sources, and an original coupling to a boundary element method (BEM) code is realized to take account of the inlet geometry effects on acoustic in-duct propagation. Calculations using the classical analytical the Green function of an infinite annular duct are also addressed. Simple formulations written in the frequency domain and expanded into modes are addressed and used to compute an in-duct interaction mode and to compare with the noise reduction obtained during the tests. A fairly good agreement between predicted and measured SPL is found when the inlet geometry effects are part of the solution (by coupling with the BEM). Furthermore, computed aerodynamic penalties due to the rods are found to be negligible. These results partly validate the computation chain and highlight the potential of the wake generator system proposed.     

The simulation of acoustic radiation from a moving line source with variable speed

The purpose of this work is to simulate and investigate the sound field generated by a moving line source with finite length and variable speed. Expect for the variation of the acoustic pressure at the specific field point, the distribution of the surface pressure along the surface of the line source was also considered. For achieving this purpose, a numerical method which combines the Time Domain Boundary Element Method (TDBEM) and moving sound source theory was developed in the present work. After comparing the results with the constant and the variable speed case, it showed that the effect of the variable speed not only influenced the variation rate of the frequency modulation, i.e., Doppler effect, but also the time about the maximum acoustic pressure being observed. In addition, the simulation results also presented that the difference as to the amplitude variation of the acoustic pressure still existed between the moving case and the stationary case even if the length of the line source is very long.     

Insertion loss testing of active noise reduction headsets using acoustic fixture

Active Noise Reduction (ANR) technique has been available for hearing protectors and communication headsets for years. However, no standard testing method for ANR headsets has emerged. In this study, we measured insertion losses of four types of commercially available ANR headsets using an Acoustic Test Fixture (ATF) to examine the feasibility of using ATF for this kind of measurement. The measurement outlined required a minimum of instrumentation and was relatively simple to implement. The results show that this method should be especially useful for testing of prototypes and for quality control as well.     

Investigation of rail noise and bridge noise using a combined 3D dynamic model and 2.5D acoustic model

Bridge noise and rail noise are two major sources of an elevated rail transit bridge in the low and medium frequency range (20–1000 Hz). However, in most of the existing literature, the noise radiated from the bridge and rail was investigated separately or using a simplified source model. In this study, an accurate method is proposed to simulate both the rail noise and bridge noise simultaneously. First, the dynamic responses of the rail and multi-span bridge are obtained using a three-dimensional (3D) vehicle-track-bridge interaction analysis model. Then, the two-dimensional (2D) infinite element model is used to calculate 3D modal acoustic transfer vectors of the rail and bridge based on the wavenumber transformation, in order to overcome the singularity and non-uniqueness of the conventional boundary element method and reduce the computation cost. Third, a field test is conducted, and the accuracy of the proposed simulation procedure is verified. Finally, the contribution of the rail and bridge noise to the total noise level is investigated in the whole space near the bridge. Generally the bridge noise occupies a higher contribution in the space beneath the girder due to the shielding effect of the bridge shape on the rail noise, while the rail noise is dominant in the upper space above the bridge. It is found the presence of the vehicle bodies has considerable effect on the rail noise but little influence on the bridge noise. The slope of the roughness level spectrum has significant influence on the dominant field of bridge noise and rail noise. For the excitation of the assumed ISO roughness level used in this study, the difference between the rail noise and bridge noise is only about 3 dB at field points 15–30 m away from the track center, which indicates both the bridge and rail noise should be included in the noise prediction for an elevated rail transit bridge.     

Hybrid device for acoustic noise reduction and energy harvesting based on a silicon micro-perforated panel structure

A kind of hybrid device for acoustic noise reduction and vibration energy harvesting based on the silicon micro- perforated panel （MPP） resonant structure is investigated in the article. The critical parts of the device include MPP and energy harvesting membranes. They are all fabricated by means of silicon micro-electro-mechanical systems （MEMS） tech- nology. The silicon MPP has dense and accurate micro-holes. This noise reduction structure has the advantages of wide band and higher absorption coefficients. The vibration energy harvesting part is formed by square piezoelectric membranes arranged in rows. ZnO material is used as it has a good compatibility with the fabrication process. The MPP, piezo- electric membranes, and metal bracket are assembled into a hybrid device with multifunctions. The device exhibits good performances of acoustic noise absorption and acoustic-electric conversion. Its maximum open circuit voltage achieves 69.41 mV.     

A two-dimensional Fokker-Planck equation degenerating on a straight line

By an example of a two-dimensional hydrodynamic system, second-order Langevin equations with two correlated noise sources are investigated. It is shown that the asymptotic expression (t) for the stationary distribution functionP depends on the order in which the limiting transitions;t andN ₂₂0 (N ₂₂ is the power of one of the noises) are made. Using the method of local expansions in trigonometric form, approximate expressions are written for the distribution functionP at small but finiteN ₂₂ tending atN ₂₂0 to the known exact solution.     

Field performance of a noise barrier

This paper is based on the results of a series of measurements of the performance of full-scale noise barriers of various heights, carried out under a range of wind conditions. It relates to the effects of a long barrier in reducing the noise from localized sources, such as the peaks of traffic noise.     

Influence of source directivity on noise levels in industrial halls: Simulation and experiments

Noise exposure of workers in industrial halls is mainly induced by noisy machines whose acoustical features are often globally known by Sound Pressure Level. The evaluation of the directivity of these noise sources can help to anticipate specific solutions for noise reduction.This study shows how the directivities of three wood-working machines have been characterized. Some characterisations have been achieved with a simple and fast acoustical intensity mapping which meets the constraints of industrial areas.When source directivity is evaluated, its influence on the noise field in industrial halls can be assessed. Some simulations and some experiments allowed the estimation of the noise field induced in workshops by both directional and omnidirectional sources. Comparison of the fields prove that the noise distribution is influenced by the source directivity if the halls are empty. As soon as the halls contain scattering objects, the directivity effect is reduced a lot and the noise field remains nearly the same far from the source whatever source used. Nevertheless, workers close to a machine are exposed to noise according to their position with respect to the machine. Exposure at the workplace can vary from 4 to 8 dB(A) according to the directivity of machines such as those measured in the trials.     

Interaction between acoustic fields of the ambient noise and point surface source

In measuring acoustic field vector properties either complete or partial cancellation between ambient noise energy flux and the oppositely directed one, radiated, for instance, by tone or noise-shaped point source can be found. In that case there is no matter coherent or incoherent fields interact. The phenomenon of interest is named compensation of opposing energy fluxes. The paper is focused on interaction between vertical component of the surface noise energy flux and that of bottom-reflected noise-shaped interfering signal from on-water source in the deep open ocean. Bottom-reflected weak broadband interfering signal measures the extent of resultant compensation. As the vertical projection of the signal energy flux makes the interference maximum, its power appears to be comparable to that of the surface noise and complete compensation takes place at a given frequency. Once it is the interference minimum, just partial compensation can be found. No compensation was observed for co-directional energy fluxes.