The sound field in long enclosures is not diffuse as assumed by the classical theory of room acoustics, because one dimension of the long enclosures is much greater than the other two. The image source method is the
most commonly used approach for predicting the sound field of long enclosures. However, the image source method is unlikely to be satisfactory for using in predicting intricate patterns of interference due to contributions from each
image source. A theoretical model has been developed for the prediction of sound propagation in a rectangular long enclosure. The model is based on the image 2source method, and the effect of interference among the infinite number
of image sources generated by multiple reflections is incorporated by coherently summing the contributions from the image sources. The various impedances of the boundaries are considered in the model. The term excess attenuation
( EA) is used for the presentation of numerical and experimental results. Experiments were carried out to validate the proposed theoretical model, where the enclosure walls are lined with different kinds of sound absorption material
to simulate different impedance boundaries. It is shown that the developed model agrees better with the experimental data than that of the ODEON software, and the trends in the behavior of sound attenuation along the enclosure at
different frequency have been correctly predicted.