T. Van Renterghem

Correlation analysis of noise and ultrafine particle counts in a street canyon

Ultrafine particles (UFP, diameter<100 nm) are very likely to negatively affect human health, as underlined by some epidemiological studies. Unfortunately, further investigation and monitoring are hindered by the high cost involved in measuring these UFP. Therefore we investigated the possibility to correlate UFP counts with data coming from low-cost sensors, most notably noise sensors. Analyses are based on an experiment where UFP counts, noise levels, traffic counts, nitrogen oxide (NO, NO(2) and their combination NO(x)) concentrations, and meteorological data were collected simultaneously in a street canyon with a traffic intensity of 3200 vehicles/day, over a 3-week period during summer. Previous reports that NO(x) concentrations could be used as a proxy to UFP monitoring were verified in our setup. Traffic intensity or noise level data were found to correlate with UFP to a lesser degree than NO(x) did. This can be explained by the important influence of meteorological conditions (mainly wind and humidity), influencing UFP dynamics. Although correlations remain moderate, sound levels are more correlated to UFP in the 20-30 nm range. The particles in this size range have indeed rather short atmospheric residence times, and are thus more closely short-term traffic-related. Finally, the UFP estimates were significantly improved by grouping data with similar relative humidity and wind conditions. By doing this, we were able to devise noise indicators that correlate moderately with total particle counts, reaching a Spearman correlation of R=0.62. Prediction with noise indicators is even comparable to the more-expensive-to-measure NO(x) for the smallest UFP, showing the potential of using microphones to estimate UFP counts.

Comparison of measurements and predictions of sound propagation in a valley-slope configuration in an inhomogeneous atmosphere

Mountainous areas form a very specific context for sound propagation: There is a particular ground effect and meteorological conditions are often extreme. In this paper, detailed sound propagation calculations are compared to noise measurements accompanied by meteorological observations. The sound source considered is road traffic along the center axis of a valley. Noise levels were measured in two cross sections, at three locations each: one on the valley floor and two on the slopes, up to 166 m above the source. For the numerical calculations, the rotated Greens function parabolic equation method is used, taking into account the undulation of the terrain and an inhomogeneous atmosphere. Typical parameters of this method were optimized for computational efficiency. Predictions agree with measurements to within 3 dBA up to propagation distances of 1 km, in windless conditions. The calculations further show that the terrain profile is responsible for an increase in sound pressure level at distant, elevated points up to 30 dBA compared to a flat ground situation. Complex temperature profiles account for level changes between -3 dBA and +10 dBA relative to a homogeneous atmosphere. This study shows that accurate sound level prediction in a valley-slope configuration requires detailed numerical calculations.

Noise measurements as proxies for traffic parameters in monitoring networks.

The present research describes how microphones could be used as proxies for traffic parameter measurements for the estimation of airborne pollutant emissions. We consider two distinct measurement campaigns of 7 and 12 days, at two different locations along the urban ring road in Antwerp, Belgium, where sound pressure levels and traffic parameters were measured simultaneously. Noise indicators are calculated and used to construct models to estimate traffic parameters. It is found that relying on different statistical levels and selecting specific sound frequencies permits an accurate estimation of traffic intensities and mean vehicle speeds, both for light and heavy vehicles. Estimations of R(2) values ranging between 0.81 and 0.92 are obtained, depending on the location and traffic parameters. Furthermore, the usefulness of these estimated traffic parameters in a monitoring strategy is assessed. Carbon monoxide, hydrocarbon and nitrogen oxide emissions are calculated with the airborne pollutant emission model Artemis. The Artemis outputs fed with directly measured and estimated traffic parameters (based on noise measurements) are very similar. Finally, a method is proposed to enable using a model calibrated at one location at another location without the need for new calibration, making it straightforward to include new measurement locations in a monitoring network.

Urban background noise mapping: the general model

Surveys show that inhabitants of dwellings exposed to high noise levels benefit from having access to a quiet side. However, current practice in noise prediction often underestimates the noise levels at a shielded facade. Multiple reflections between facades in street canyons and inner yards are commonly neglected and facades are approximated as perfectly flat surfaces yielding only specular reflection. In addition, sources at distances much larger than normally taken into account in noise maps might still contribute significantly. Since one of the main reasons for this is computational burden, an efficient engineering model for the diffraction of the sound over the roof tops is proposed, which considers multiple reflections, variation in building height, canyon width, facade roughness and different roof shapes. The model is fitted on an extensive set of full-wave numerical calculations of canyon-to-canyon sound propagation with configurations matching the distribution of streets and building geometries in a typical historically grown European city. This model allows calculating the background noise in the shielded areas of a city, which could then efficiently be used to improve existing noise mapping calculations. The model was validated by comparison to long-term measurements at 9 building facades whereof 3 were at inner yards in the city of Ghent, Belgium. At shielded facades, a strong improvement in prediction accuracy is obtained.

Numerical evaluation of tree canopy shape near noise barriers to improve downwind shielding

The screen-induced refraction of sound by wind results in a reduced noise shielding for downwind receivers. Placing a row of trees behind a highway noise barrier modifies the wind field, and this was proven to be an important curing measure in previous studies. In this paper, the wind field modification by the canopy of trees near noise barriers is numerically predicted by using common quantitative tree properties. A realistic range of pressure resistance coefficients are modeled, for two wind speed profiles. As canopy shape influences vertical gradients in the horizontal component of the wind velocity, three typical shapes are simulated. A triangular crown shape, where the pressure resistance coefficient is at maximum at the bottom of the canopy and decreases linearly toward the top, is the most interesting configuration. A canopy with uniform aerodynamic properties with height behaves similarly at low wind speeds. The third crown shape that was modeled is the ellipse form, which has a worse performance than the first two types, but still gives a significant improvement compared to barriers without trees. With increasing wind speed, the optimum pressure resistance coefficient increases. Coniferous trees are more suited than deciduous trees to increase the downwind noise barrier efficiency.

Urban background noise mapping : the multiple-reflection correction term

Mapping of road traffic noise in urban areas according to standardized engineering calculation methods systematically results in an underestimation of noise levels at areas shielded from direct exposure to noise, such as inner yards. In most engineering methods, road traffic lanes are represented by point sources and noise levels are computed utilizing point-to-point propagation paths. For a better prediction of noise levels in shielded urban areas, an extension of engineering methods by an attenuation term Acan has been proposed, including multiple reflections of the urban environment both in the source and in the receiver area. The present work has two main contributions for the ease of computing Acan. Firstly, it is shown by numerical calculations that Acan may be divided into independent source and receiver environment terms, As and Ar. Based on an equivalent free field analogy, the distance dependence of these terms may moreover be expressed analytically. Secondly, an analytical expression is proposed to compute As and Ar for 3D configurations from using 2D configurations only. The expression includes dependence of the street width-to-height ratio, the difference in building heights and the percentage of facade openings in the horizontal plane. For the expression to be valid, the source should be separated from the receiver environment by at least four times the street width.

Airborne sound propagation over sea during offshore wind farm piling.

Offshore piling for wind farm construction has attracted a lot of attention in recent years due to the extremely high noise emission levels associated with such operations. While underwater noise levels were shown to be harmful for the marine biology, the propagation of airborne piling noise over sea has not been studied in detail before. In this study, detailed numerical calculations have been performed with the Greens Function Parabolic Equation (GFPE) method to estimate noise levels up to a distance of 10 km. Measured noise emission levels during piling of pinpiles for a jacket-foundation wind turbine were assessed and used together with combinations of the sea surface state and idealized vertical sound speed profiles (downwind sound propagation). Effective impedances were found and used to represent non-flat sea surfaces at low-wind sea states 2, 3, and 4. Calculations show that scattering by a rough sea surface, which decreases sound pressure levels, exceeds refractive effects, which increase sound pressure levels under downwind conditions. This suggests that the presence of wind, even when blowing downwind to potential receivers, is beneficial to increase the attenuation of piling sound over the sea. A fully flat sea surface therefore represents a worst-case scenario.