The purpose of the work described in this paper is twofold: (i) to present the results of an experimental investigation of the sound attenuation in different types of forest, and (ii) to validate a part of the Nord 2000 model. A number of measurements have been carried out in regular and irregular forests with trees with deciduous and evergreen leaves, different tree density, different trunk diameter, etc. The experimental results indicate that trees have a noticeable effect on sound propagation at medium and high frequencies at distances longer than 40m. The Nord 2000 model uses a simple algorithm to predict scattering effects when sound propagates in outdoor spaces with obstacles. The comparison of experimental results and predictions shows that the Nord 2000 model predicts the ground effect dip in forests with acceptable accuracy in about 60% of the cases if the flow resistivity of the ground is allowed to vary with distance. The high frequency effect of trees is in general not predicted very well.