Commercial road vehicle noise

A survey of the characteristics of the noise emitted by commercial vehicles has been made. The most important single parameter determining the noise of a modern diesel-engined vehicle is the engine speed. All of the other parameters such as load, road speed, etc., have only a secondary effect.The sources of noise on the vehicle are reviewed and it is shown how the characteristics of these sources determine the overall noise characteristics of the vehicle. It has been found that a simple model of the vehicle as a number of coincident point sources predicts the overall noise characteristics of the vehicle to ±2 dB(A). It is shown that there are two extremes of behaviour, the rolling noise controlled vehicle and the power unit noise controlled vehicle; the engine is currently the controlling noise source.Tyre noise has been investigated in some detail as comparatively little has been published previously on this source. Empirical relationships between the tyre noise and speed, tyre size and road surface roughness are given. It is concluded that tyre noise is generated by impacting between elements of the tyre tread and elements of the road surface.Modifications have been made to the engine, exhaust, intake and cooling fan of a 9 ton, 6 litre diesel engined truck which have reduced its ISO test noise level from 88 dB(A) to 80 dB(A). However, it is concluded that 80 dB(A) commercial vehicles are not yet feasible for production. In particular insufficient is known about cooling fan design.Finally cab noise has been investigated and it has been found to originate from the same source as the exterior noise, power unit airborne noise. Therefore any modifications to the power unit to reduce exterior noise will have a similar effect on interior noise. This is confirmed by the vehicle modifications mentioned above which reduced the maximum cab noise from 87 dB(A) to 79 dB(A).     

Magnetostatic field noise near metallic surfaces

We develop an effective low-frequency theory of the electromagnetic field in equilibrium with thermal objects. The aim is to compute thermal magnetic noise spectra close to metallic microstructures. We focus on the limit where the material response is characterised by the electric conductivity. At the boundary between empty space and metallic microstructures, a large jump occurs in the dielectric function which leads to a partial screening of low-frequency magnetic fields generated by thermal current fluctuations. We resolve a discrepancy between two approaches used in the past to compute magnetic field noise spectra close to microstructured materials.     

Stationary test method for noise from a single road vehicle

Legislation in several countries restricting noise emitted by individual road vehicles recommends the use of international standard ISO R362, or something very similar, as a standard. This is a pass-by test with strict conditions laid down regarding the test site, weather conditions, and so on. As such it is not well suited for simple routine and quick roadside enforcement of vehicle noise. This paper describes some initial work aimed at clarifying and resolving a number of the difficulties commonly encountered with stationary tests and describes a procedure which gives good correlation with the ISO pass-by test and may lead to a test which could be used for routine service check and roadside enforcement.     

Shot noise measurements in a wide-channel transistor near pinch-off

We study a shot noise of a wide channel gated high-frequency transistor at a temperature of 4.2 K near pinch-off. In this regime, a transition from the metallic to the insulating state is expected to occur, accompanied by the increase in the partition noise. The dependence of the noise spectral density on current is found to be slightly nonlinear. At low currents, the differential Fano factor is enhanced compared to the universal value 1/3 for metallic diffusive conductors. We explain this result by the effect of thermal fluctuations in a nonlinear regime near pinch-off, without calling for the enhanced partition noise.     

Inhomogeneity of low-noise wearing courses evaluated by tire/road noise measurements using the close-proximity method

Performing numerous analyses of tire/road noise measurements on low-noise pavements during the last several years, the authors observed significant inhomogeneity of the wearing course in numerous cases, while similar problems were almost non-existent when dense pavements were measured.     

A comparison of statistically optimized near field acoustic holography using single layer pressure-velocity measurements and using double layer pressure measurements

Statistically optimized near field acoustic holography (SONAH) is usually based on the assumption that all sources are on one side of the measurement plane whereas the other side is source free. An extension of the SONAH procedure based on measurement with an array of pressure-velocity probes has recently been suggested. An alternative method uses a double layer array of pressure transducers. Both methods make it possible to distinguish between sources on the two sides of the array and thus suppress the influence of extraneous noise and reflections coming from the "wrong" side. This letter compares the two methods.     

Statistical tyre/road noise modeling in Hong Kong on friction course

Based on the close proximity (CPX) method specified in ISO/DIS 11819-2, we recorded and analyzed the instantaneous tyre/road sound pressure levels with 9 road sections that are constructed with the same pavement surfacing materials, that is, friction course. A total of 1320 segments were made in urban areas with a pair of SRTT (Standard Road Test Tyre). We tried to relate the tyre/road noise with the instantaneous acceleration, speed, air temperature, road temperature, road gradient, road surface age to develop a multi-variants model. It was subsequently found that a simple tyre/road noise model linking driving speed and acceleration is the best model. The model provides an easy way to estimate the instantaneous tyre/road noise level. As the tyre/road noise is becoming more dominant component of the road traffic noise, our proposed model has the potential to improve the current practice in estimating the road traffic noise.     

Statistical characteristics of some standard reverberant sound field measurements

The space and time fluctuations of $\text{1}\text{3}\text{-}\text{octave}$ band sound pressure level and reverberation time measurements in hard-walled chambers have been investigated experimentally with the aim of determining characteristic probability density functions and typical variances. The basic random variables observed (that is, the mean square pressure and the reverberation time estimates) are acceptably described by a gamma distribution given in a paper by D. Lubman. A specific sampling procedure has been used for the experiments: the samples were taken in randomly chosen discrete positions and a time averaging performed in each position so that the time variances thus obtained were much smaller than the spatial variances. Such a sampling procedure leads to the statistical characteristics of the random variables being independent of any specific equipment used. With the aid of the basic probability density function, statements of precision with regard to time and space fluctuations can be made from the data obtained in this way. Confidence intervals for standard measurements of sound pressure level, reverberation time, sound pressure level difference, sound power level and transmission loss are presented. The typical variances in various possible situations are not always well-known, which means that one has to use estimated values of the variances in the derivation of the respective confidence statements. This fact causes some uncertainty, and one must regard the confidence statements thus obtained as approximate only. From the results obtained in these investigations, it appears that the degree of this uncertainty would be quite acceptable in a normal measurement situation.     

Sampling errors in near field measurements on non-planar surfaces

Techniques of predicting the radiated field of a vibrating structure from measurements of its near field are well known. The effects of spatial sampling and measurements errors on the calculated radiated field have been investigated by several authors for planar measurement surfaces. In this paper an extension of these analyses to non-planar surfaces is presented. Examples of the application of the new sampling criteria and error calculations are shown for a cylindrical near field measurement surface.     

On-board wet road surface identification using tyre/road noise and Support Vector Machines

Changes in weather have a major influence on driving safety. On wet pavement, tyre grip is reduced and drivers must adapt their driving style accordingly. The correct operation of this adaptation mechanism depends on the perception the driver has of the asphalt status. Due to certain effects, this perception does not always correspond with reality. To improve this perception, it is essential to inform the driver about the asphalt status, efficiently and as quickly as possible. This could be achieved by installing an asphalt status detection system on the vehicle itself. The system could display asphalt status information in the vehicle’s console, allowing drivers to adapt their driving style accordingly.In this paper we propose an asphalt status classification system based on real-time acoustic analysis of tyre/road noise. The proposed system uses a practical approach that allows it to be integrated into a real vehicle. We present the system architecture used to measure the noise and the signal processing algorithms used for the classification of the asphalt state. A practical implementation of the proposed system has been developed and tested. For this preliminary prototype, only wet and dry asphalt states have been covered. Obtained wet/dry classification results have been reported, showing very high success rates.     

A road traffic noise pattern simulation model that includes distributions of vehicle sound power levels

Annoyance, sleep disturbance and other health effects of road traffic noise exposure may be related to both level and number of noise events caused by traffic, not just to energy equivalent measures of exposure. Dynamic traffic noise prediction models that include instantaneous vehicle noise emissions can be used to estimate either of these measures. However, current state-of-the-art vehicle noise emission models typically consider a single emission law for each vehicle category, whereas measurements show that the variation in noise emission between vehicles within the same category can be considerable. It is essential that the influence of vehicles that are producing significantly more (or less) noise than the average vehicle are taken into account in modeling in order to correctly predict the levels and frequency of occurrence of road traffic noise events, and in particular to calculate indicators that characterize these noise events. Here, an approach for predicting instantaneous sound levels caused by road traffic is presented, which takes into account measured distributions of sound power levels produced by individual vehicles. For the setting of a receiver adjacent to a dual-lane road carrying free flow traffic, the effect of this approach on estimated percentile levels and sound event indicators is investigated.     

Effects of an air spoiler on the wake of a road vehicle by PIV measurements

A particle image velocimetry (PIV) technique has been used to analyze the flow characteristics behind a road vehicle with/without an air spoiler attached on its trunk and also to estimate its effect on the wake. A vehicle model scaled in the ratio of 1/43 is set up in the mid-section of a closed-loop water tunnel. The Reynolds number based on the vehicle length is 10⁵. To investigate the three-dimensional structure of the recirculation zone and vortices, measurements are carried out on the planes both parallel and perpendicular to the free stream, respectively. The results show significantly different vorticity distributions in the recirculation region according to the existence of the air spoiler. The focus and the saddle point, appearing in the wake, are disposed differently along the spanwise direction. Regarding the streamwise vortices, the air spoiler produces large wing tip vortices. They have opposite rotational directions to C-pillar vortices which are commonly observed in the case that an air spoiler is absent. The wing tip vortices generate the down force and as a result, they might make the vehicle more stable in driving.     

Minimizing vehicle noise passing the street bumps using Genetic Algorithm

The purpose of this paper is to optimize noise emission level associated with two types of speed reducers for different speeds of a vehicle (20, 40, 60 km/h) by Genetic Algorithm and Artificial Neural Network. The optimization shows that the maximum level of noise is sensitive to speed reducer dimensions. It is reduced by 24 dB(A) by changing the width from 0.6 m to 0.3 m for the height 0.04 m whereas, it is reduced by 32 dB(A) by changing from the height 0.055 m and the width 0.9 m to the height 0.04 m and the width 0.3 m.     

Sound quality prediction of vehicle interior noise using deep belief networks

The sound quality of vehicle interior noise strongly influences passengers’ psychological and physiological perceptions. To predict the sound quality of interior noise, a vehicle road test with four compact cars has been conducted. All recorded interior noise signals have been denoised via a discrete wavelet transform (DWT) denoising procedure and subsequently evaluated subjectively through the anchor semantic differential (ASD) test by a jury. In addition, a novel prediction method, namely, regression-based deep belief networks (DBNs), which substitute the support vector regression (SVR) layer for the linear softmax classification layer at the top of the general DBN’s structure, has been proposed to predict the interior sound quality. The parameter selection of the DBN model has been compared and studied using a grid search. In addition, four conventional machine-learning-based methods have been introduced to enable a comparison of the performance with the newly developed DBNs. Furthermore, the feature fusion ability of DBNs has been studied by varying the amount of information that the dataset offers. The results show the following: (1) The accuracy and robustness of the proposed DBN-based sound quality prediction approach are better than those of the 4 other referenced methods. (2) The multiple-feature fusing process can strongly affect the prediction performance. (3) Finally, the unsupervised pre-training process of the DBNs can enhance the information fusing ability. Finally, the newly proposed regression-based DBN approach may be extended to address other vehicle noises in the future.     

Nocturnal road traffic noise assessment and sleep research: The usefulness of different timeframes and in- and outdoor noise measurements

In this paper, we investigated the procedure of noise assessment in the study of nocturnal noise exposure during sleep in the home situation. The use of two different timeframes (fixed from 11 PM to 07 AM versus personal “Time in Bed” period) was explored as well as the relation between indoor and outdoor noise levels and between the actual and estimated noise levels. Noise recordings were performed inside and outside the bedroom of 24 subjects living in high density road traffic noise areas in the Brussels’ Capital Region during seven consecutive days. Indoor and outdoor noise indicators L_Aeq, L_Amax and individual noise events were analysed. Subjects completed a daily sleep log in which the Time in Bed period was assessed. The results indicate that, for outdoor noise assessment, the use of an average L_Aeq might not be sufficient to reflect well the noise levels during the sleep period. For indoor measurements, significant differences were found in the comparison between both timeframes (L_Aeq: T = 16; p < .001). Considering the relation between indoor and outdoor measurements, low correlations (r = .49; p < .001) were found even when the location of the bedroom as a mediating factor was accounted for (street side; r = .52; p < .001). Therefore, from our study, caution is needed when relying on outdoor noise measurements for the evaluation of sleep disturbances. Furthermore, one needs to be aware of the weak correspondence between indoor and outdoor noise levels in the discussion of what a harmonized noise indicator for the evaluation of noise exposure and sleep disturbances should consist of.