Conny Larsson

Numerical ray tracing in the atmospheric surface layer

A numerical ray‐tracing model, including calculation of the sound pressure, was developed. It is valid for propagation from a point source in a moving, stratified atmosphere. Numerical integration of the ray equations was performed, and all rays reaching a specific point were found. Several expressions for the height dependency of the wind speed and the temperature were used, e.g., Monin–Obukhov similarity theory functions, the parameters of which were determined by use of a least‐squares method. Measurements of sound propagation from a point source over finite impedance ground were made. Meteorological parameters were monitored simultaneously, wind direction and relative humidity at a single height, wind speed and temperature at five elevations. Comparison with the model was made out to a distance of 150 m. The agreement between the model values and those measured was good. The influence of the directional characteristics of the source was studied, and found to be very important.

Effects of meteorological conditions and source height on sound propagation near the ground

Abstract Simultaneous measurements of sound propagation from a loudspeaker and meteorological variables were carried out for various ground covers and meteorological conditions. The wind and temperature gradients cause refraction of the sound rays, and hence influence the sound level. The curvature of a nearly horizontal sound ray can be calculated by using measurements of wind and temperature. The curvature is found to be closely connected to the sound level, when distinctions between octave band, ground, distance, source and receiver heights are made. A new variable, the sound propagation parameter, W, including refraction, distance, source and receiver heights, is introduced. The parameter is used to examine if a specific set-up of source-receiver can be sensitive to meteorological effects. The sound level for different octave bands and ground cover is given for negative and positive values of W, outside an interval close to W = 0 , where large scattering of the sound level is found.

Atmospheric absorption conditions for horizontal sound propagation

Abstract Atmospheric absorption is important for outdoor sound propagation. Weather changes over the day and the year, and this alters the atmospheric absorption. Thirty years of meteorological measurements from six stations in Sweden were used to determine the conditions for atmospheric absorption. The size of the monthly and daily variances was frequency-dependent. The most accurate information about atmospheric absorption climate is achieved if hourly values over 30 years are used. Errors using 1-year hourly data or only the mean values of temperature and humidity for the calculations were determined. Atmospheric absorptions for various parts of the world were estimated. An increase in the accuracy of calculations of outdoor sound levels can be made by taking the local climate into consideration.

Long-term audible noise and radio noise performance from an operating 400 kV transmission line

Long-term measurements of audible noise, radio interference and meteorological variables were carried out close to a 400 kV transmission line in the southern part of Sweden. The aim was to determine the dependence of noise levels on weather conditions. A data processing procedure was developed to exclude sound measurements influenced by sources other than the line. The results are presented and discussed. >

Measurements of meteorological effects on long‐range sound propagation using m‐sequence correlation

An m‐sequence correlation method was used for measuring sound levels 1‐km distant from a point source 1.25 m above the ground, a crop field and a snow‐covered ground, respectively. Meteorological parameters were monitored simultaneously. Wind speed and temperature were measured at several elevations, together with wind direction, relative humidity, and atmospheric pressure at a single height. The effect of refraction on the sound level variation is interpreted in terms of the curvature for near‐horizontal sound rays. It is found that the sound level increases as the curvature goes from negative to positive values. Comparisons with other investigations were made, and qualitative agreement is found. Calculations with acoustic theory were made, the results of which show large discrepancies with those measured.

A method to estimate meteorological effects on sound propagation near the ground

Abstract A method for calculation of the conditions for sound propagation in the atmospheric surface layer is presented. The wind and temperature gradients cause refraction of the sound rays, and hence influence the sound level. The curvature of a nearly horizontal sound ray can be calculated by using measurements of wind and temperature. Meteorological measurements of wind and temperature carried out over a long period of time can be used for computing the distribution of the curvature for a given localisation of source and receiver. An example of the use of this method is presented. Meteorological measurements, taken over a period of one year at a site in southern Sweden, were used to calculate the cumulative distribution of the curvature for different orientations of the source and the receiver. Modifications on the material were made in order to simulate weather conditions for other sites.

Outdoor sound level variations due to fluctuating meteorological parameters

Abstract Simultaneous measurements of meteorological parameters and octave-band sound levels from a point source, over finite impedance ground, have been carried out. A ray-tracing model was used to predict sound levels. A 5-min measurement period was divided into 10-s intervals, and the effect of the meteorological fluctuations on the measured and predicted sound levels was studied. The meteorological fluctuations were rather large due to the lapse conditions. The results with the ray-tracing model were more sensitive to meteorological variations than was indicated by the measurements. Averages of the predicted sound levels, based on the 10-s intervals, were compared with those predicted from average meteorological conditions, as well as with average measured sound levels. The first of these ray-tracing results was in better agreement with the measurements. It seems that during periods with large meteorological fluctuations, the predicted sound level based on averages can be quite in error. It was also concluded that meteorological fluctuations on a longer time-scale can give similar results to the small-scale turbulence effect, causing decoherence of sound rays.

Atmospheric sound propagation near the ground

The most important meteorological effects on sound propagation are refraction‐ground attenuation, atmospheric absorption, and scattering by turbulence. The wind and temperature gradients cause refraction of the sound rays, and hence influence the sound level. The curvature of a nearly horizontal sound ray can be calculated by using measurements of wind and temperature. Atmospheric absorption is important for outdoor sound propagation. The absorption varies for different parts of the world. Results from some stations in Scandinavia are given. Atmospheric absorption for the high frequencies changes during different times of the year. The absorption for low frequencies varies both over the day and the year. The size of the annual and diurnal variance is frequency dependent. An increase in the accuracy of calculations of outdoor sound levels can be made by considering the local climate. The values of atmospheric absorption need to be calculated for the local climate instead of using some global mean value. Th...