On the sources of wayside noise generated by high-speed trains

A linear array of 14 microphones was used to measure radiated noise generated by a four-carriage electric train travelling at speeds between 160 and 250 km/h. Most of the results given in this paper pertain to apparent source locations of wheel/rail interaction noise, although preliminary data collected in a concurrent study of railway aerodynamic noise are briefly mentioned. An analysis of the measurements suggests that apparent sources of wheel/rail interaction noise are located (i) in the rail or substructure at low frequencies, (ii) on the wheel rim just below the axle at intermediate or peak frequencies, and (iii) on the lower part of the wheel and possibly in the rail at high frequencies.     

Numerical analysis of dipole sound source around high speed trains

As the maximum speed of high speed trains increases, the effect of aeroacoustic noise on the sound level on the ground becomes increasingly important. In this paper, the distribution of dipole sound sources at the bogie section of high speed trains is predicted numerically. The three-dimensional unsteady flow around a train is solved by the large eddy simulation technique. The time history of vortices shows that unstable shear layer separation at the leading edge of the bogie section sheds vortices periodically. These vortices travel downstream while growing to finally impinge upon the trailing edge of the section. The wavelength of sound produced by these vortices is large compared to the representative length of the bogie section, so that the source region can be regarded as acoustically compact. Thus a compact Green's function adapted to the shape can be used to determine the sound. By coupling the instantaneous flow properties with the compact Green's function, the distribution of dipole sources is obtained. The results reveal a strong dipole source at the trailing edge of the bogie section where the shape changes greatly and the variation of flow with time is also great. On the other hand, the bottom of the bogie section where the shape does not change, or the leading edge and boundary layer where the variation of flow with time is small, cannot generate a strong dipole source.     

On combustion generated noise from turbulent diffusion gaseous flames

A study is made into the mechanism of noise emission process from an ‘inverse’ open turbulent diffusion gaseous double-concentric jet flame. Correlations are presented between the mean temperatures, mean and fluctuating velocity, and the fluctuating pressure on one hand and the overall noise emission on the other from the natural gas-air flames and the He-air jets. Time mean and temporal component of velocity were measured using a laser doppler anemometry system. The LDA was operated in the fringe mode with a radial rotating diffraction grating. Signal processing was carried out by a counter linked to an on-line mini computer.A comparison is presented between the experimental results and the theoretical predictions based on a rational correlation and shows a good agreement for the peak frequency of the radiated sound power.     

On the metrology of the speed of sound in liquids

Decentralization of the standards base in measuring the speed of sound in liquids by expressing the speed of sound in fractions of the speed of light in vacuum is considered. A method to take into account the diffraction effects by measuring the speed of sound in the case of a constant acoustic wavelength and binding the absolute speed of sound values to the calculated thermodynamic values obtained by the nonacoustical method when the adiabatic compressibility is equal to the isothermal one is proposed. Errors of the reproducibility of the unit of the speed of sound in water using the proposed method are estimated.     

Prediction of environmental noise from fast electric trains

A model is presented for the environmental noise of fast electric trains on continuous welded rails, based on a consideration of the possible modes of vibration of the wheels. Evidence is presented to support the idea that the wheels are the dominant sources of noise and radiate as resonant dipoles. The model is calibrated by the results of measurements and methods of predicting train noise level and noise energy (per unit area) are developed. The noise intensity is found to increase as the fourth power of the speed. Energy (per unit area) at an observer increases as the third power of the speed and is proportional to train length divided by the distance of the observer from the track. The relevance of the model as a basis for the calculation of train noise in terms of some noise indices is then discussed and it is shown that it can be used to predict the parameters required by some indices which may be relevant to the subjective effects of environmental train noise.     

A method of assessing the noise nuisance arising from the channel tunnel high speed rail system

Until recently railway noise has been accepted as a well established but dwindling source of noise nuisance, and has been subject to relatively little sociological study. The proposal for high speed trains through previously quiet areas has however called for the establishment of subjective noise zones. The paper puts forward such zones, based upon sociological data in other countries and on aircraft, road traffic and community data in the United Kingdom. Planning zones are calculated for the expected frequencies of rail services, and based upon the estimated and measured noise levels of high speed trains. Some comments are made upon the generalization of this information, and on the effects of barriers, cuttings and distance; and on vibration and shock effects near to the railway line itself.     

Estimating the sound speed of the surface layer of the seabed using ocean ambient noise in shallow water

To improve the inversion efficiency, a method for quickly and directly estimating the sound speed of the surface layer of the seabed is proposed. According to the energy flux theory, the ocean ambient noise data received by the vertical line array can be used to extract the bottom loss(BL) passively. The BL curve has an effect on the critical angle, which is used to estimate the sound speed of the surface layer of the seabed. Based on the ray model,the difference between the BL calculated from the noise extraction and the theoretical value is derived, while the beamforming performances of the array at different angles and frequencies is discussed. Considering that the sound ray bends in the environment with a non-constant sound-speed gradient, the angle needs to be corrected to improve the universality of the method.Different critical angles of different frequencies correspond to different effective depths. Data processing result of an experiment conducted in the Yellow Sea shows that within the effective depth where the critical angle remains unchanged, the surface layer of the seabed can be regarded as a constant speed layer. The sound speed of the surface layer of the seabed in this sea area is estimated at 1547 m/s within 0.5 m, which is similar to the active inversion result.     

Use of the sound levels of noise for assessing the adequacy of hearing protectors

The dB(A) sound level of a noise is accepted as a measure of the damage risk to unprotected ears but often it is not a reliable guide to the risk to ears fitted with hearing protectors. For any dB(A) level inside a protector, normally there will be substantially higher sound levels outside that protector. This paper shows how, from sequential frequency attenuation bands of the protector, and sound level weightings, external sound levels can be calculated, below which the noise inside the protector does not exceed a chosen dB(A) level. Further valuable information may be obtained by mapping external dB(A) and dB(C) levels to cover all possible noise spectra that give the chosen dB(A) level inside the protector. Thus, from a pair of measured sound levels, use of the method indicates whether the protector is sufficient or not, or whether more detailed measurment of the noise is required. This knowledge enhances the scope of the sound level meter and reduces the need for frequency analysis of industrial noise. Its application should be a helpful addition to the data provided by suppliers of hearing protectors.     

Study on the noise removal processing of TV picture at high speed

This paper proposes a motion estimation method using the position estimation method of TV noise for motion picture restoration. On the basis of the block-matching used in the past, a new method is developed. The motion of the picture between the frames is calculated by the mean vector and the search position, and the search center moved is corrected by the approximated value. The method was verified as useful in this study.     

Causality and the speed of sound

A usual causal requirement on a viable theory of matter is that the speed of sound be at most the speed of light. In view of various recent papers querying this limit, the question is revisited here. We point to various issues confronting theories that violate the usual constraint.     

Calculation of the speed of sound in seawater from the known ion concentrations

It is shown that the method proposed earlier for calculating the speed of sound in seawater from the known ion concentrations, in a strict sense, can be applied for computing the sound speed in seas if, as a model of seawater, one uses a solution containing six ions or a solution containing four ions, namely, Na⁺, Mg²⁺, SO ₄ ^2? , and Cl^?, with the mole concentration of Na⁺ being replaced by the sum of mole concentrations of Na⁺ and K⁺ and the mole concentration of Mg²⁺ being replaced by the sum of mole concentrations of Mg²⁺ and Ca²⁺. An algorithm of calculation is proposed. It is demonstrated that, when the seawater is considered as a solution containing six ions, the computed value of the speed of sound does not depend on the choice of the specific ion whose concentration is determined from the condition of electric neutrality.     

Estimation of the long term average sound level from hourly average sound levels

It is shown how to estimate the long-term average sound level, L_AeqLT (for free flowing road traffic) from measurements of the hourly A-weighted equivalent sound level, L_Aeq1h. To estimate the parameters of the model which describe noise emission and attenuation, concurrent measurements of L_Aeq1h at two distances from the considered road are needed. A semi-empirical formula is derived for L_AeqLT approximation. Also the uncertainty of this approximation is given as a function the distance from the road and receiver height.     

声速测量及声波的波动学规律研究

声速除了可利用共振干涉法和相位比较法测量外,还可以用干涉、衍射及“洛埃镜”反射等方法测量．声速实验的改进,不仅可扩展声速测量的方法,而且也可加深学生对波动学规律和本质的理解．     

Measuring the speed of sound of quintessence

Quintessence, a time-varying energy component that may account for the accelerated expansion of the universe, can be characterized by its equation of state and sound speed. In this paper, we show that if the quintessence density is at least 1% of the critical density at the surface of last scattering the cosmic microwave background anisotropy can distinguish between models whose sound speed is near the speed of light versus near zero, which could be useful in distinguishing competing candidates for dark energy.