Shadowing of directional noise sources by finite noise barriers

The present study investigates the shadowing effect of barriers of infinite or finite length in the presence of directional noise sources. The diffraction model termed [Directive Line Source Model (DLSM) J. Acoust. Soc. Am. 107 (2000) 2973-2986] is employed. DLSM is appropriately modified and extended to include the effects of ground reflection, diffraction by the side edges of a finite length barrier, and diffraction by directional noise sources. Results obtained are shown to be in good agreement with available experimental data and known analytical solutions. An application of the enhanced DLSM is illustrated using helicopter type noise, which is highly directive. The noise source is modeled as a directional point source with far field directivity data and the enhanced DLSM is employed to compare the noise field with and without the barrier present for three different directivity patterns, various source locations and orientations, as well as, for various barrier lengths.     

Imaging of directional distributed noise sources

This study relates to the acoustic imaging of noise sources that are distributed and strongly directional, such as in turbulent jets. The goal is to generate high-resolution noise source maps with self-consistency, i.e., their integration over the extent of the noise source region gives the far-field pressure auto-spectrum for a particular emission direction. Self-consistency is possible by including a directivity factor in the formulation of the source cross-spectral density. The resulting source distribution is based on the complex coherence, rather than the cross-spectrum, of the measured acoustic field. For jet noise, whose spectral nature changes with emission angle, it is necessary to conduct the measurements with a narrow-aperture array. Three coherence-based imaging methods were applied to a Mach 0.9 turbulent jet: delay-and-sum beamforming; deconvolution of the beamformer output; and direct spectral estimation that relies on minimizing the difference between the measured and modeled coherences of the acoustic field. The delay-and-sum beamforming generates noise source maps with strong spatial distortions and sidelobes. Deconvolution leads to a five-fold improvement in spatial resolution and significantly reduces the intensity of the sidelobes. The direct spectral estimation produces maps very similar to those obtained by deconvolution. The coherence-based noise source maps, obtained by deconvolution or direct spectral estimation, are similar at small and large observation angles relative to the jet axis.     

Characterisation of tram noise emission and contribution of the noise sources

The recent expansion of French tram networks and the related local residential complaints mean that a better knowledge of the situations leading to negative reactions from the local inhabitants is required. Hence a research project has been conducted to evaluate and describe noise and vibration emission of trams as well as the perception by the local residents. This paper investigates tram noise emission on common straight track sections, involving two vehicle scales. First the acoustic power and the mean vertical directivity of the total tramset is assessed using an arc of microphones. Then the localisation and the analysis of the main noise sources are performed by means of a cross array during the tram pass-by. Two tram types representing two generations of French rolling stock, both running on two sites with distinct track characteristics, have been investigated considering the effect of speed, tram type, and track type on the noise source contributions and spectral features. Most sources are located in the lower part of the trams, mainly related to rolling noise, with a strong dependence on speed and track type. The tram type dependency, although globally of second importance, influences greatly the noise spectral distribution and behaviour. The HVAC was the only roof-mounted source which could be detected; its contribution towards building storeys becomes significant in configurations of low rolling noise. A tram noise emission model based on the various noise sources has been developed.     

Relationship between exposure to multiple noise sources and noise annoyance

Relationships between exposure to noise [metric: day-night level (DNL) or day-evening-night level (DENL)] from a single source (aircraft, road traffic, or railways) and annoyance based on a large international dataset have been published earlier. Also for stationary sources relationships have been assessed. Here the annoyance equivalents model concerning noise annoyance from combined sources and the underlying assumptions are presented. The model first translates the noise from the individual sources into the equally annoying sound levels of a reference source, road traffic, and then sums these levels giving total level L. The annoyance from the combined sources is found by substituting exposure L in the road traffic exposure-annoyance relationship. The most important assumption, independence of the contributions of the sources, is discussed. It appears that independence will be violated substantially only due to the effect of the presence or absence of a quiet side of building which is not incorporated in the model. For use in practice the application of the model is broken down in five steps. The step by step procedure can be used for the assessment of the total noise level and the associated total annoyance on the basis of the DNL or DENL values of the individual sources.     

Modelling and characterization of airborne noise sources

A comprehensive model of an arbitrary airborne noise source is developed in this paper. It can be used for noise prediction in complex environment conditions. The model, described by a limited number of parameters—the source impedance and blocked pressure—is invariant with respect to the surrounding acoustical environment. Procedures for experimental identification of model parameters in an arbitrary enclosure are presented. These procedures require only conventional equipment and facilities, and thus can be applied widely. Two analytical examples confirm the validity of the approach.     

Fundamental sources of noise in sensory transduction

Summary The contribution of chemical steps in trasduction chains to the output noise of a receptor cell (as opposed to the noise inherent in the stimulus,e.g., a dim light flash) has been evaluated both analytically and by a computer simulation study on a simple model, using a Monte Carlo technique. The noise deriving from chemical reactions (chemical shot noise, CSN) is shown to add to the source noise thereby increasing significantly the output variation coefficient. It is proposed that this noise contribution should be taken into account in interpreting the sensitivity of a sensory system and in particular the dose-response curves at low levels of the stimulus. Paper presented at the International Workshop ?Fluctuations in Physics and Biology: Stochastic Resonance, Signal Processing and Related Phenomena?, Elba, 5–10 June 1994.     

Symmetry breakdown of stochastic potential by noise cross-correlations among colored noise sources

A nontrivial phenomenon in stochastic zero-dimensional systems, namely, the symmetry breakdown of stationary probability function due to the correlation between the noises is studied. As a model system to study this effect, we consider a generalized synergetic system of Lorenz type with Gaussian colored noise of each mode. In the frameworks of theoretical approximation and numerical simulations it is shown the fluctuation cross-correlations break the symmetry of bistable synergetic potential, producing an asymmetric effective potential. At that, cross-correlations play the twofold role: cross-correlations are the reason of symmetry breaking at small cross-correlation times on the one hand, and the reason of symmetry restoring at large values of cross-correlation times on the other hand. Moreover, it is shown that symmetry breakdown occurs only if one of multiplicative function is odd. Any other combinations of noises restore the symmetric form of potential.     

Laboratory annoyance and different traffic noise sources

The acute annoyance reaction to different noise sources (lorries, aircraft, mopeds and trains) was investigated in a laboratory experiment. Students were exposed to different noise climates at noise levels 70 and 80 dB(A) for 25 minutes, and their reactions were subsequently assessed by using a questionnaire. Their general sensitivity to noise was also evaluated. The results demonstrated that L_eq gave the best correlation with annoyance. However, lorry noise was found to be less disturbing than aircraft noise at the same L_eq value. This was more pronounced if the different noises were compared at equal peak dB(A) levels. The results suggest that other factors such as the irregularity of the noise or the individual experience of the noise are of importance for the annoyance reaction. A relationship was found between the general annoyance score and annoyance reactions in the laboratory. Questionnaires could thus be a suitable tool for identifying noise sensitive persons.     

An active noise barrier with unidirectional secondary sources

An active noise barrier with unidirectional secondary sources is investigated in this paper, where the unidirectional secondary source consists of two closely located loudspeakers with pre-adjusted phase difference. The secondary sound field of the unidirectional sources is adjusted to maximally match the primary sound field in the shadow zone behind the barrier. It is shown both numerically and experimentally that the noise reduction performance of the active noise barrier can be improved remarkably by replacing monopoles with the unidirectional sources. The mechanism for the improvement is also investigated.     

Time reversal of noise sources in a reverberation room

Usually, time reversal is studied with pulsed emissions. Here, the properties of time reversal of the acoustic field emitted by noise sources in a reverberation room are studied numerically, theoretically, and experimentally. A time domain numerical simulation of a two-dimensional enclosure shows that the intensity of a time-reversed noise is strongly enhanced right on the initial source position. A theory based on the link that exists between time reversal of noise and the "well-known" time reversal of short pulse is developed. One infers that the focal spot size equals half a wavelength and the signal to noise ratio only depends on the number of transceivers in the time reversal mirror. This last property is characteristic of the time reversal of noise. Experimental results are obtained in a 5 X 3 X 3 m3 reverberation room. The working frequency range varies from 300 Hz to 2 kHz. The ability of the time reversal process to physically reconstruct the image of two noise sources is studied. To this end, care is given to the technique to separate two close random sources, and also to the influence of temperature fluctuations on the focusing quality.     

Convective amplification of gas turbine engine internal noise sources

The acoustic field of a noise source is altered when the source is in motion. The change in the acoustic field introduced by the source motion, caused by source alteration and propagation effects, is defined as convective amplification. Previous studies of this phenomenon have been based on analytical models that did not incorporate the physical features necessary for calculation of the convective amplification factor for the internal noise sources of a gas turbine engine, which is required to predict in-flight noise levels from static engine noise measurements. An improved theoretical model was developed. At low frequencies, this model resulted in a convective amplification factor of (1?M₀ cos θ_e)^?4, which is identical with the factor established in earlier studies. At high frequencies, however, convective amplification is a function of flight speed, radiation angle, and source geometry.     

The combination of noise from separate time varying sources

Two methods of combining time varying noise sources are proposed and the applications of these procedures to practical situations discussed. A design method is suggested that enables both L₁₀ and L₉₀ values to be determined when two or more Gaussian traffic noise distributions are combined. It is indicated that the combination technique may be applied to various noise sources, e.g. the interaction of aircraft fly-past and surface traffic noise.     

A three-dimensional physical model of MRI noise based on current noise sources in a conductor

Magnetic flux noise is generated by any conductor in equilibrium with a bath as a result of random fluctuating currents. A physical model of this flux noise is proposed, based on allowable current patterns in the conductor, which we describe as natural current modes. This model gives insight into the spatial characteristics of the magnetic noise which is encountered in a variety of magnetic measurements and imaging modalities such as magnetic resonance imaging (MRI).     

噪声源非均匀分布海洋环境噪声水平声能流理论分析

研究水平非均匀分布噪声源所产生的各向异性海洋环境噪声场声能流。提出一种混合型非均匀分布噪声源模型,理论分析并数值计算了此模型情况下的环境噪声场水平声能流。结果表明:非均匀分布噪声源引起的海洋环境噪声场具有显著非零平均水平声能流;不同接收点的水平声能流明显不同;其幅度和方向取决于各个局部海域不等强度声源产生的合成噪声声能流矢量和。研究了两接收点间噪声的声压和振速水平分量、振速水平分量归一化相关系数随两接收点间距的关系,各量之间表现出较强相关性,为分析水下矢量声场目标探测技术性能提供理论依据。