the role of top surface to performance of reactive t-shape noise barriers

t-shape profile barriers are one of the most successful barriers among many different profiles. it has been shown that using welled diffusers on the top of t-shaped barriers makes a reactive barrier that shows better performance than that of any other used profile barriers compared with their equivalent absorbent barrier. the contribution of the top surface of reactive t-profile barriers to their efficiency in the shadow zone is discussed in this paper. the new multiple impedance discontinuity (nmid) method was used on a few multi-welled surfaces and the application of the findings on the diffuser t-profile barrier along with a descriptive theory of the welled surface effect was presented. an acceptable agreement between the result of the nmid model and bem method for a few welled surfaces were found. the area-averaged impedance model was also used in the nmid model and it was found that this model can be a good performance indicator for a multi-welled surface. in order to explain the contribution of the top surface of a t-profile barrier, it is adequate to use the nmid model on a mixed ground equivalent to the top surface of the barrier where the source and receiver are located at near the ground where the separation of source and receiver is identical with the overall span of the cap. the effect of average admittance of top surface is dominant and the effect of the impedance discontinuity is overshadowed by this effect.